database concepts · 2020-01-14 · database concepts page 2 of 19 i. alternate key: out of the...

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Q1. What is a Database?

Ans: A Database is an organized collection of related information that supports for easy access, modification and maintenance.

Q2. What is a DBMS?.

Ans: Database is managed by special software packages known as Database Management Systems (DBMSs). The purpose of DBMSs software is to allow the user to create, modify and

administration of database.

Q3. What is a RDBMS? Name some popular RDBMS

Ans: In the Relational data model, database is represented as a collection of related Tables. Database management software used to manage Relational Databases is called a Relational Data Base

Management System. Examples of Relational database management systems are: MS-Access, MySQL, PostgreSQL, SQLite, Microsoft SQL Server, Oracle, SAP, dBase, FoxPro, etc.

Q4. What are the benefits of using a DBMS?

Ans: Benefits of DBMS are:

a. Redundancy (duplicity) can be controlled b. Inconsistency can be avoided

c. Data can be shared d. Security restrictions can be applied.

Q5. Explain the following terms

A. Relation: A relation means a 'Table', in which data is organized in the form of rows and columns. Relation: Student

ADNO NAME CLASS SECTION AVERAGE

101 Anu 12 A 85

105 Balu 12 D 65

203 Leena 11 B 95

205 Madhu 10 B 70

305 Surpreeth 9 C 70

483 Usha 6 A 60

B. Tuple/Record/Row:

Horizontal subset of information in a table is called Tuple. Example:

101 Anu 12 A 85

C. Attribute /Field/Column/Key:

Vertical subset of information in a table is called Attribute. Example: ADNO , NAME , CLASS , SECTION , AVERAGE

D. Domain: A domain is defined as the set of all unique values permitted for a column. A domain

of date column is the set of valid dates, a domain of integer column is whole numbers, a domain of SECTION is ('A','B','C','D')

E. Cardinality: Cardinality is number of rows (tuples) in a table.

Example: Cardinality of Relation Student is 6 (Number of rows).

F. Degree: The degree is the number of attributes (columns) in a table.

Example: Degree of Relation Student is 5 (Number of columns).

G. Candidate Key: Candidate keys are all those columns in a relation which have unique values and hence are eligible to act as a primary key. Example: In the above table, AdNo and Name

both have unique values. Therefore, both AdNo and Name are candidate keys.

H. Primary Key: A column or set of columns that uniquely identify a row within a table is called its

primary key. Example: Out of the candidate keys: AdNo and Name, only AdNo is selected as the primary key.

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I. Alternate Key: Out of the candidate keys, after selecting one primary key, the remaining

candidate keys which are not made primary key are called alternate key. Example: From the candidate keys, AdNo is selected as the primary key, therefore the remaining candidate key is

Name and it is the Alternate key.

J. Foreign Key: A foreign key is a column in one table that uniquely identifies a row of another table. In other words, a foreign key is a column or a combination of columns that is used to

establish a link between two tables.

Q6. What is SQL? What is its use?

Ans: Structured Query Language (SQL) is a standard language used for accessing databases. This is a special purpose programming language used to create a table, manage data and manipulate

data.

Q7. What are the advantages of SQL? 1) SQL is portable running in all servers, mainframes, PCs, laptops, and even mobile phones.

2) High speed SQL queries can retrieve large amounts of information quickly and efficiently.

3) SQL is easy to learn and understand. 4) SQL is used with Relational databases with any vendor to create, manage and secure

databases. 6) SQL acts as both Programming language and Interactive language.

7) It provides Client Server Architecture. 8) Supports Object Based Programming and is highly flexible.

Q8. What are the different types of SQL statements?

1. Data Definition Language (DDL) statements is a set of commands that defines the different

structures in a database. DDL statements are used to create databases and table structures, modify the existing structure of the table and remove the existing table. Some of the DDL

statements are CREATE TABLE, ALTER TABLE and DROP TABLE.

2. Data Manipulation Language (DML) statements are used to access and manipulate data in existing tables. The manipulation includes inserting data into tables, deleting data from the

tables, retrieving data and modifying the existing data. The common DML statements are SELECT, UPDATE, DELETE and INSERT.

3. Data Control Language (DCL): This is a category of SQL commands which are used to control the access to databases and tables. Examples of DCL commands are - GRANT, REVOKE.

MySQL Commands

1. CREATE DATABASE: (DDL) It creates a database with the specified name. CREATE DATABASE databasename;

2. SHOW DATABASES: (DML) It shows a list of databases present in the system.

SHOW DATABASES;

3. USE: (DML) It opens the specified database for use.

USE databasename;

4. SELECT DATABASE() :It Shows the name of the current database (DML) SELECT DATABASE();

5. SHOW TABLES: (DML) It shows a list of tables present in the current database.

SHOW TABLES;

Ques: What are the different datatypes available in MySQL? What is their purpose?

1. CHAR: CHAR is used for storing fix length character strings. String values will be right padded

with spaces to bring it up to some fixed size before they are stored on the disk. Thus it wastes a lot of disk space.

Format: CHAR(n)

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2. VARCHAR: Varchar is used for variable character string. It can have maximum of 2000

characters. Format: VARCHAR (n)

3. DECIMAL or NUMERIC: Decimal and numeric data types have fixed precision and scale.

Format: DECIMAL (p , s) and NUMERIC (p , s) 'p' is the precision or the total number of significant places including one place for decimal point.

's' is the scale or the number of digits after the decimal point.

Example, a column defined with DECIMAL(6,2) would allow the value: 1 2 3 . 4 5 (p=6, s=2) and DECIMAL(5,2) would allow the value: 1 2 . 3 4 (p=5,s=2)

4. INT/INTEGER: The INT data type is the integer data type in SQL. This is used to store whole

number (without any fraction part).

5. DATE is used to store valid date values, which is ranging from January 1, 4712 BC to December 31, 9999 AD. The date values are in the format YYYY-MM-DD

Format: DATE

Ques: What are the different Constraints available in MySQL? What is their purpose?

MySQL provides some rules, called Constraints, which help us to ensure validity of the

data. These constraints are:

PRIMARY KEY : Sets a column or a group of columns as the Primary Key of a table.

Therefore, NULLs and Duplicate values in this column are not accepted.

NOT NULL : Makes sure that NULLs are not accepted in the specified column.

6. CREATE TABLE: (DDL) It creates a table with the specified name. We need to specify the

column names, data types, size and constraints for each column. Table and Column names

cannot start with a digit but can contain both letters and numbers. Spaces and symbols are not

allowed except underscore (_). CREATE TABLE tablename

( columnname1 datatype(size) constraint,

columnname2 datatype(size) constraint, ….

ColumnnameN datatype(size) constraint );

Note: Do not put a comma after last column information

Ques Create a table student with the following structure:

Fields Data Type Constraints admno numeric(5) PRIMARY KEY name varchar(20) NOT NULL class integer(2) section char(1) dob date fees numeric(7,2) create table student ( admno numeric(5) primary key, name varchar(20) not null, class integer(2), section char(1), dob date, fees numeric(7,2) );

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7. DESCRIBE : (DML) It shows the table structure as well as constraints, if any.

DESCRIBE tablename;

Example: desc student;

+---------+--------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+--------------+------+-----+---------+-------+ | admno | decimal(5,0) | NO | PRI | NULL | | | name | varchar(20) | NO | | NULL | | | class | integer(2) | YES | | NULL | | | section | char(1) | YES | | NULL | | | dob | date | YES | | NULL | | | fees | decimal(7,2) | YES | | NULL | | +---------+--------------+------+-----+---------+-------+

8. INSERT : (DML) It inserts a data Row into the table

INSERT INTO tablename (column1, column2, ..., columnN) VALUES (value1, value2, ...valueN);

We can insert only one Row at a time. Char, Varchar and Date values are enclosed in single

quotes ' ' . Numbers are written normally. Dates are specified in the format 'YYYY-MM-DD'. To insert the current date into a table, MySQL's built-in function CURDATE() can be used.

Ques Insert the following records in the table student:

+-------+--------------+-------+---------+------------+---------+ | admno | name | class | section | dob | fees | +-------+--------------+-------+---------+------------+---------+ | 111 | Anu Jain | 12 | A | 2000-12-31 | 2500.00 | | 222 | Mohit Sharma | 11 | B | 2001-12-31 | 4500.00 | | 333 | K.P. Gupta | 12 | B | 2000-01-11 | 3000.00 | | 444 | Ajit Kumar | 10 | A | 2002-03-04 | 2000.00 | | 555 | Nandini | 12 | C | 2000-03-04 | 3000.00 | | 666 | Rohan Sharma | 11 | B | 2001-12-05 | 2500.00 | | 777 | Leena | NULL | NULL | 2000-03-05 | NULL | +-------+--------------+-------+---------+------------+---------+ insert into student values (111, 'Anu Jain', 12, 'A', '2000-12-31',2500); insert into student values (222, 'Mohit Sharma', 11, 'B', '2001-12-31',4500); insert into student values (333, 'K.P. Gupta', 12, 'B', '2000-01-11',3000); insert into student values (444, 'Ajit Kumar', 10, 'A', '2002-03-04',2000); insert into student values (555, 'Nandini', 12, 'C', '2000-03-04',3000); insert into student values (666, 'Rohan Sharma', 11, 'B', '2001-12-05',2500);

Explicitly Inserting NULL Values : If a column can hold NULL values, its value can be omitted from the INSERT INTO statement. You need to specify the corresponding column names whose

values are specified in INSERT INTO statement, NULL value will automatically be inserted in the

rest of the columns.

insert into student (admno, name, dob) values (777, 'Leena ', '2000-03-05');

9. SELECT: (DML) It retrieves data from a table SELECT * / columnnames / expressions as “alias name”

FROM tablename WHERE condition

ORDER BY columnname ASC/DESC;

Retrieving all Columns Ques: Display all information from student table. Select * from student;

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+-------+--------------+-------+---------+------------+---------+ | admno | name | class | section | dob | fees | +-------+--------------+-------+---------+------------+---------+ | 111 | Anu Jain | 12 | A | 2000-12-31 | 2500.00 | | 222 | Mohit Sharma | 11 | B | 2001-12-31 | 4500.00 | | 333 | K.P. Gupta | 12 | B | 2000-01-11 | 3000.00 | | 444 | Ajit Kumar | 10 | A | 2002-03-04 | 2000.00 | | 555 | Nandini | 12 | C | 2000-03-04 | 3000.00 | | 666 | Rohan Sharma | 11 | B | 2001-12-05 | 2500.00 | | 777 | Leena | NULL | NULL | 2000-03-05 | NULL | +-------+--------------+-------+---------+------------+---------+

Retrieving Selected Columns

Ques: Display name and class information from student table. Select name , class from student; +--------------+-------+ | name | class | +--------------+-------+ | Anu Jain | 12 | | Mohit Sharma | 11 | | K.P. Gupta | 12 | | Ajit Kumar | 10 | | Nandini | 12 | | Rohan Sharma | 11 | | Leena | NULL | +--------------+-------+

Using Arithmetic Operators with SELECT

Using arithmetic operators on tables does not create new columns in the tables or change the

actual data values. The results of the calculations appear only in the output.

Ques: Display fees + 500 as increased fees for all students Select admno , name , fees + 500 from student; +-------+--------------+------------+ | admno | name | fees + 500 | +-------+--------------+------------+ | 111 | Anu Jain | 3000.00 | | 222 | Mohit Sharma | 5000.00 | | 333 | K.P. Gupta | 3500.00 | | 444 | Ajit Kumar | 2500.00 | | 555 | Nandini | 3500.00 | | 666 | Rohan Sharma | 3000.00 | | 777 | Leena | NULL | +-------+--------------+------------+

Note: The value of fees stored in the table remains the same.

Using Column Alias

Column alias gives a different heading to appear in output instead of the actual column. Using

Column Alias does not rename a column. It simply displays a different column name in the output. The AS keyword between the column name and alias is optional

+ Addition

- Subtraction

* Multiplication

/ Division

% Modulus ( remainder)

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Ques: Display fees * 12 as annual fees for all students Select admno, name, fees * 12 as "annual fees" from student;

If a Column alias consists of more than one word, then it should be enclosed in quotes " ". +-------+--------------+--------------+ | admno | name | annual fees | +-------+--------------+--------------+ | 111 | Anu Jain | 30000.00 | | 222 | Mohit Sharma | 54000.00 | | 333 | K.P. Gupta | 36000.00 | | 444 | Ajit Kumar | 24000.00 | | 555 | Nandini | 36000.00 | | 666 | Rohan Sharma | 30000.00 | | 777 | Leena | NULL | +-------+--------------+--------------+

Ques: Display fine as 10% of fees for all students Select admno , name , fees*10/100 as fine from student; +-------+--------------+------------+ | admno | name | fine | +-------+--------------+------------+ | 111 | Anu Jain | 250.000000 | | 222 | Mohit Sharma | 450.000000 | | 333 | K.P. Gupta | 300.000000 | | 444 | Ajit Kumar | 200.000000 | | 555 | Nandini | 300.000000 | | 666 | Rohan Sharma | 250.000000 | | 777 | Leena | NULL | +-------+--------------+------------+

Putting text in Query output

Query output can be made more presentable by inserting items such as symbols or text in the query output

Ques: Display details of all students in the format : Name studies in class-Section Select name, 'studies in' , class , '-' , section from student; +--------------+------------+-------+---+---------+ | name | studies in | class | - | section | +--------------+------------+-------+---+---------+ | Anu Jain | studies in | 12 | - | A | | Mohit Sharma | studies in | 11 | - | B | | K.P. Gupta | studies in | 12 | - | B | | Ajit Kumar | studies in | 10 | - | A | | Nandini | studies in | 12 | - | C | | Rohan Sharma | studies in | 11 | - | B | | Leena | studies in | NULL | - | NULL | +--------------+------------+-------+---+---------+

Retrieving specific rows using WHERE clause

WHERE clause is used to filter records. It is used to extract only those records that fulfill a specified criterion. Where condition in the SELECT command can be formed with the following

a) Relational Operators are used to compare two values. The result of the comparison is True or

False

SELECT * / columnnames / expressions as “alias name”

FROM tablename WHERE columnname >= Value;

= Equal to

> Greater than

< Less than

>= Greater than or equal to

<= Less than or equal to

!= or <> Not equal to

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Ques: Display students' name, who are paying below 3000 fees. Select name from student where fees < 3000;

+--------------+ | name | +--------------+ | Anu Jain | | Ajit Kumar | | Rohan Sharma | +--------------+

Ques: Display students' name, who are paying above or equal to 3000 fees. Select name from student where fees >= 3000; +--------------+ | name | +--------------+ | Mohit Sharma | | K.P. Gupta | | Nandini | +--------------+

Ques: Display students' information, who are not in class 10 Select * from student where class != 10; +-------+--------------+-------+---------+------------+---------+ | admno | name | class | section | dob | fees | +-------+--------------+-------+---------+------------+---------+ | 111 | Anu Jain | 12 | A | 2000-12-31 | 2500.00 | | 222 | Mohit Sharma | 11 | B | 2001-12-31 | 4500.00 | | 333 | K.P. Gupta | 12 | B | 2000-01-11 | 3000.00 | | 555 | Nandini | 12 | C | 2000-03-04 | 3000.00 | | 666 | Rohan Sharma | 11 | B | 2001-12-05 | 2500.00 | +-------+--------------+-------+---------+------------+---------+

b) Logical Operators ( NOT, AND, OR) : NOT negates a condition. If a condition returns a True value, adding NOT causes the condition

to return a False value and vice versa. AND and OR are used to connect relational expressions in the WHERE clause. AND requires both conditions to be true in order to return TRUE but OR

returns TRUE If any of the comparisons are true, The symbol || can be used in place of OR, && can be used in place of AND , ! can be used in place of NOT operator.


FROM tablename WHERE NOT (condition);

SELECT * / columnnames / expressions as “alias name” FROM tablename

WHERE condition1 AND/OR condition2;

Ques: Display students' information, who are not in class 11 Select * from student where NOT (class = 11); +-------+------------+-------+---------+------------+---------+ | admno | name | class | section | dob | fees | +-------+------------+-------+---------+------------+---------+ | 111 | Anu Jain | 12 | A | 2000-12-31 | 2500.00 | | 333 | K.P. Gupta | 12 | B | 2000-01-11 | 3000.00 | | 444 | Ajit Kumar | 10 | A | 2002-03-04 | 2000.00 | | 555 | Nandini | 12 | C | 2000-03-04 | 3000.00 | +-------+------------+-------+---------+------------+---------+

Ques: Display information of students in class 11B. Select * from student where class = 11 AND section = ’B’; +-------+--------------+-------+---------+------------+---------+ | admno | name | class | section | dob | fees | +-------+--------------+-------+---------+------------+---------+ | 222 | Mohit Sharma | 11 | B | 2001-12-31 | 4500.00 | | 666 | Rohan Sharma | 11 | B | 2001-12-05 | 2500.00 | +-------+--------------+-------+---------+------------+---------+

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Ques: Display 11th and 12th class students' information. Select * from student where class = 11 OR class = 12; +-------+--------------+-------+---------+------------+---------+ | admno | name | class | section | dob | fees | +-------+--------------+-------+---------+------------+---------+ | 111 | Anu Jain | 12 | A | 2000-12-31 | 2500.00 | | 222 | Mohit Sharma | 11 | B | 2001-12-31 | 4500.00 | | 333 | K.P. Gupta | 12 | B | 2000-01-11 | 3000.00 | | 555 | Nandini | 12 | C | 2000-03-04 | 3000.00 | | 666 | Rohan Sharma | 11 | B | 2001-12-05 | 2500.00 | +-------+--------------+-------+---------+------------+---------+

c) Condition based on Range : BETWEEN/NOT BETWEEN operator is used to define the range

of values within which the column values must fall to make a condition true. Range includes both the upper and the lower values.


FROM tablename WHERE columnname BETWEEN lowerValue AND UpperValue;

Ques: Display students' information, who are paying fees between 2500 and 3500.

Select * from student where fees BETWEEN 2500 AND 3500; +-------+--------------+-------+---------+------------+---------+ | admno | name | class | section | dob | fees | +-------+--------------+-------+---------+------------+---------+ | 111 | Anu Jain | 12 | A | 2000-12-31 | 2500.00 | | 333 | K.P. Gupta | 12 | B | 2000-01-11 | 3000.00 | | 555 | Nandini | 12 | C | 2000-03-04 | 3000.00 | | 666 | Rohan Sharma | 11 | B | 2001-12-05 | 2500.00 | +-------+--------------+-------+---------+------------+---------+

d) Condition based on a List : IN/NOT IN operator is used to select values that match any value

in a list of specified values. SELECT * / columnnames / expressions as “alias name”

FROM tablename

WHERE columnname IN (value1, value2, …..valueN);

Ques: Display students' information, who are in section A, B, D and E. Select * from student where section IN ('A','B','D','E'); +-------+--------------+-------+---------+------------+---------+ | admno | name | class | section | dob | fees | +-------+--------------+-------+---------+------------+---------+ | 111 | Anu Jain | 12 | A | 2000-12-31 | 2500.00 | | 222 | Mohit Sharma | 11 | B | 2001-12-31 | 4500.00 | | 333 | K.P. Gupta | 12 | B | 2000-01-11 | 3000.00 | | 444 | Ajit Kumar | 10 | A | 2002-03-04 | 2000.00 | | 555 | Nandini | 12 | C | 2000-03-04 | 3000.00 | | 666 | Rohan Sharma | 11 | B | 2001-12-05 | 2500.00 | +-------+--------------+-------+---------+------------+---------+

e) Condition based on pattern matches : LIKE/NOT LIKE clause is used for pattern matching

of string data using wildcard characters

%( to match zero or many characters) and _( to match one character) . The symbols can be used in combinations.

Some examples : 'Am%' matches any string starting with Am.

'%Singh%' matches any string containing 'Singh' '%a' matches any string ending with 'a'

'_ _ _' matches any string that is exactly 3 characters long. '_ _ %' matches any string that has at least 2 characters.

'_ _ _ g' matches any string that is 4 characters long with any 3 characters in the

beginning but 'g' as the 4th character.

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FROM tablename WHERE columnname LIKE ‘pattern’;

Ques: Display the names that start with letter "A". select * from student where name like 'A%';

+-------+------------+-------+---------+------------+---------+ | admno | name | class | section | dob | fees | +-------+------------+-------+---------+------------+---------+ | 111 | Anu Jain | 12 | A | 2000-12-31 | 2500.00 | | 444 | Ajit Kumar | 10 | A | 2002-03-04 | 2000.00 | +-------+------------+-------+---------+------------+---------+

Ques: Display names, whose name's second letter is 'o'. select * from student where name like '_o%'; +-------+--------------+-------+---------+------------+---------+ | admno | name | class | section | dob | fees | +-------+--------------+-------+---------+------------+---------+ | 222 | Mohit Sharma | 11 | B | 2001-12-31 | 4500.00 | | 666 | Rohan Sharma | 11 | B | 2001-12-05 | 2500.00 | +-------+--------------+-------+---------+------------+---------+

Ques: Display names, whose name has 7 characters. select * from student where name like '_ _ _ _ _ _ _'; +-------+---------+-------+---------+------------+---------+ | admno | name | class | section | dob | fees | +-------+---------+-------+---------+------------+---------+ | 555 | Nandini | 12 | C | 2000-03-04 | 3000.00 | +-------+---------+-------+---------+------------+---------+

f) Checking missing values : IS NULL / IS NOT NULL is used to select rows in which the

specified column IS NULL or IS NOT NULL


FROM tablename WHERE columnname IS NULL;

NULL represents a value that is unavailable, unassigned, unknown or inapplicable. NULL is not

same as a Zero. Since Zero (0) is an integer value but NULL is nothing. Relational Operators (= and != ) are not allowed with NULL. If any column value involved in an arithmetic expression

is NULL, the result of the arithmetic expression is also NULL.

Ques: Display students' information, whose class is missing. select * from student where class is null;

+-------+--------+-------+---------+------------+------+ | admno | name | class | section | dob | fees | +-------+--------+-------+---------+------------+------+ | 777 | Leena | NULL | NULL | 2000-03-05 | NULL | +-------+--------+-------+---------+------------+------+

Sorting the Results- ORDER BY is used to display the query output in ascending or in

descending order of a single column or multiple columns

SELECT * / columnnames / expressions as “alias name” FROM tablename

WHERE condition ORDER BY columnname1 ASC/DESC , columnname2 ASC/DESC;

Ques: Display students' information, in descending order of fees

Select * from student order by fees desc;

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+-------+--------------+-------+---------+------------+---------+ | admno | name | class | section | dob | fees | +-------+--------------+-------+---------+------------+---------+ | 222 | Mohit Sharma | 11 | B | 2001-12-31 | 4500.00 | | 333 | K.P. Gupta | 12 | B | 2000-01-11 | 3000.00 | | 555 | Nandini | 12 | C | 2000-03-04 | 3000.00 | | 111 | Anu Jain | 12 | A | 2000-12-31 | 2500.00 | | 666 | Rohan Sharma | 11 | B | 2001-12-05 | 2500.00 | | 444 | Ajit Kumar | 10 | A | 2002-03-04 | 2000.00 | | 777 | Leena | NULL | NULL | 2000-03-05 | NULL | +-------+--------------+-------+---------+------------+---------+

Ques: Display students' information, in order of class and section.

Select * from student order by class, section; +-------+--------------+-------+---------+------------+---------+ | admno | name | class | section | dob | fees | +-------+--------------+-------+---------+------------+---------+ | 777 | Leena | NULL | NULL | 2000-03-05 | NULL | | 444 | Ajit Kumar | 10 | A | 2002-03-04 | 2000.00 | | 222 | Mohit Sharma | 11 | B | 2001-12-31 | 4500.00 | | 666 | Rohan Sharma | 11 | B | 2001-12-05 | 2500.00 | | 111 | Anu Jain | 12 | A | 2000-12-31 | 2500.00 | | 333 | K.P. Gupta | 12 | B | 2000-01-11 | 3000.00 | | 555 | Nandini | 12 | C | 2000-03-04 | 3000.00 | +-------+--------------+-------+---------+------------+---------+

Sorting on Column Alias: If a Column alias is defined on a column, we can use it for displaying rows in an ascending or descending order using ORDER BY clause:

SELECT columnname1, columnname2 as alias FROM tablename

WHERE condition ORDER BY alias;

Eliminating duplicate values : DISTINCT clause is used to display unique values from a particular column of a table.

SELECT DISTINCT (column) FROM tablename ;

DISTINCT can be used with only one column at a time. When DISTINCT keyword is specified, only one instance of the duplicated data is shown. If we write the keyword ALL in place of

DISTINCT, then the result of SELECT query displays all the values including duplicate values

Ques: Display different classes from student table.

Select distinct class from student; +-------+ | class | +-------+ | 12 | | 11 | | 10 | | NULL | +-------+

Ques: Display class in student table. Select All class from student; +-------+ | class | +-------+ | 12 | | 11 | | 12 | | 10 | | 12 | | 11 | | NULL | +-------+

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Precedence of Operators

1 !

2 -(unary minus)

3 ^

4 *, /, DIV, %, MOD

5 -, +

6 =, <=>, >=, >, <=, <, <>, !=, IS, LIKE, IN

7 BETWEEN,

8 NOT

9 &&, AND

10 ||, OR

11 = (assignment)

10. UPDATE: (DML) It updates/modifies data values in a table. UPDATE statement can be used to

update one or more columns together. WHERE clause helps in updating of particular rows in a table

UPDATE tablename

SET columnname1 = value1, columnname2 = value2… WHERE condition;

Ques: Increase fees value by 500 for all students Update student set fees = fees + 500;

Ques: Increase the fees value by 100 for admno 222 Update student set fees = fees + 100 where admno = 222;

11. DELETE :(DML)It Deletes a Row from a table. DELETE removes the entire row, not the individual

column values DELETE FROM tablename WHERE condition;

Ques: Remove admno 444 information. Delete from student where admno = 444;

DELETE can be used to delete all rows of the table by skipping the where clause.

Ques: Remove all records. Delete from student;

12. ALTER TABLE :(DDL) It is used to restructure the table.

To Add a new Column in The Table

ALTER TABLE tablename ADD columnname datatype(size);

To Remove an existing Column from A Table

ALTER TABLE tablename DROP columnname;

To Modify the data type or size of a Column

ALTER TABLE tablename MODIFY columnname newdatatype(newsize);

Ques: Add one new column totalfees with numeric(10,2)

Alter table student add totalfees numeric(10,2);

After execution of the above ALTER TABLE statement, the totalfees column is added and a NULL value is automatically assigned to all the existing rows in this column.

Ques: Change totalfees datatype as numeric(12,2) Alter table student modify totalfees numeric(12,2);

Ques: Remove totalfees column. Alter table student drop totalfees;

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13. DROP TABLE :(DDL)It removes the entire structure of the table and information DROP TABLE tablename;

Example: Drop table student;

14. DROP DATABASE :(DDL)It removes the entire database from system. DROP DATABASE databasename;

Ques: Differentiate between Update and Alter commands

Alter Table Update

It is a DDL statement It is a DML statement

Used to restructure the table Used to change the data values in specified

columns of all those records which satisfy the specified condition

Ques: Differentiate between Delete and Drop commands

Drop Table Delete

It is a DDL statement It is a DML statement

Used to remove entire table structure with contents

Used to remove the records which satisfy the specified condition

Ques: Explain Multiple Row Functions / Aggregate Functions used in MySQL.

Multiple Row Function works on multiple values and returns an aggregate result for the entire group. These functions are also called Aggregate Functions or Group

Functions. Some Aggregate functions in MySQL are:

1. MAX(column) Returns the maximum of the non-NULL values of a particular column.

2. MIN(column) Returns the minimum of the non-NULL values of a particular column.

3. AVG(column) Returns the average of the non-NULL values of a particular column. The

AVG(column) function can be applied on numeric (int/decimal) type only. Averages of

String and Date type data are not defined.

4. SUM(column) Returns the sum of the non-NULL values of a particular column. The

SUM(column) function can be applied on numeric (int/decimal) type only. Sums of String

and Date type data are not defined.

5. COUNT(column) Returns the count of the number of non-NULL of a particular column.

If the argument is an *, then it counts the total number of rows satisfying the

condition in the entire table.

Syntax :

SELECT AggregateFunction( distinct ColumnName) FROM TableName ;

Note :

Aggregate functions does not take NULL into consideration and ignores it.

WHERE condition can be specified to restrict the rows from calculations in Aggregate

functions.

If you want to exclude duplicate values from the calculations in Aggregate functions,

use the DISTINCT keyword. The ALL keyword includes even duplicates. If nothing is

specified, then ALL is assumed as the default

count(*) counts the number of records in the whole table and not any particular

column

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Consider the table Student shown below: +-------+--------------+-------+---------+------------+---------+ | admno | name | class | section | dob | fees | +-------+--------------+-------+---------+------------+---------+ | 111 | Anu Jain | 12 | A | 2000-12-31 | 2500.00 | | 222 | Mohit Sharma | 11 | B | 2001-12-31 | 4500.00 | | 333 | K.P. Gupta | 12 | B | 2000-01-11 | 3000.00 | | 444 | Ajit Kumar | 10 | A | 2002-03-04 | 2000.00 | | 555 | Nandini | 12 | C | 2000-03-04 | 3000.00 | | 666 | Rohan Sharma | 11 | B | 2001-12-05 | 2500.00 | | 777 | Leena | NULL | NULL | 2000-03-05 | NULL | +-------+--------------+-------+---------+------------+---------+

Ques: Display the total fees collected from students. select sum(fees) from student;

+-----------+ | sum(fees) | +-----------+ | 17500.00 | +-----------+

Ques: Display the average fees of students select avg(fees) from student;

+-------------+ | avg(fees) | +-------------+ | 2916.666667 | +-------------+

Ques: Display the highest and the lowest fees paid by students select max(fees), min(fees) from student;

+-----------+-----------+ | max(fees) | min(fees) | +-----------+-----------+ | 4500.00 | 2000.00 | +-----------+-----------+

Ques: Count the number of values in fees select count(fees) from student;

+-------------+ | count(fees) | +-------------+ | 6 | +-------------+

Ques: Count the number of records in the table select count(*) from student;

+----------+ | count(*) | +----------+ | 7 | +----------+

Ques: Count the number of different sections select count( distinct section) from student;

+--------------------------+ | count( distinct section) | +--------------------------+ | 3 | +--------------------------+

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Ques: What is the use of Group By clause in MySQL?

GROUP BY clause is used in the SELECT statement to arrange the records of a table

into groups based on unique values in the columns. It group rows with same column

values, so that the aggregate functions can now be applied to separately to each group instead of the whole table. When Group by is used aggregate functions return

one result for each group.

Syntax :

SELECT GroupingColumn , AggregateFunction(columnname)

FROM Tablename GROUP BY GroupingColumn;

Grouping using Multiple Columns: SELECT GroupingColumn1 , GroupingColumn2 , AggregateFunction(columnname) FROM Tablename GROUP BY GroupingColumn1 , GroupingColumn2;

Note: When Grouping is done using multiple columns, The GROUP BY clause operates on both

GroupingColumn1 and GroupingColumn2 to identify unique column combinations and

make groups of those rows.

Ques: Display number of students in each class. select class , count(*) from student group by class; +-------+----------+ | class | count(*) | +-------+----------+ | NULL | 1 | | 10 | 1 | | 11 | 2 | | 12 | 3 | +-------+----------+

Ques: Display number of students in each section of each class. select class , section , count(*) from student group by class, section; +-------+---------+----------+ | class | section | count(*) | +-------+---------+----------+ | NULL | NULL | 1 | | 10 | A | 1 | | 11 | B | 2 | | 12 | A | 1 | | 12 | B | 1 | | 12 | C | 1 | +-------+---------+----------+

Ques: Display sum of fees for each class. select class , sum(fees) from student group by class;

+-------+-----------+ | class | sum(fees) | +-------+-----------+ | NULL | NULL | | 10 | 2000.00 | | 11 | 7000.00 | | 12 | 8500.00 | +-------+-----------+

Ques: What is the use of Having clause in MySQL?

HAVING is used to place condition on groups returned by 'GROUP BY' clause. It is

used when we want to see the output only for those groups which satisfy some condition.

Syntax:

SELECT GroupingColumn , AggregateFunction( columnname )

FROM Tablename GROUP BY GroupingColumn

HAVING Groupcondition;

Note: Having condition can be formed using any columns returned by the Group By clause

or any AggrgateFunction applied on these groups.

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Ques: Display sum of fees which is more than 5000 for each class select class , sum(fees) from student group by class having sum(fees)>5000; +-------+-----------+ | class | sum(fees) | +-------+-----------+ | 11 | 7000.00 | | 12 | 8500.00 | +-------+-----------+

Ques: Display average fees for those classes where number of students is more than 1 select class , avg(fees) from student group by class having count(*)>1; +-------+-------------+ | class | avg(fees) | +-------+-------------+ | 11 | 3500.000000 | | 12 | 2833.333333 | +-------+-------------+

Ques: Display sum of fees for class =10; select class , sum(fees) from student group by class having class != 11; +-------+-----------+ | class | sum(fees) | +-------+-----------+ | 10 | 2000.00 | | 12 | 8500.00 | +-------+-----------+

Ques: What is the Cartesian Product of two tables?

CARTESIAN PRODUCT (OR CROSS JOIN) of two tables is a table obtained by joining every row of first table with each row of the second table. A Cartesian product of two

tables contains all the columns from both the tables. No. of Rows in Cartesian Product = No. of Rows in Table1 X No. of Rows in Table2 (Product of Rows)

No. of columns in Cartesian Product = No. of Cols in Table1 + No. of Cols in Table2 (Sum of Columns)

Syntax:

SELECT * FROM table1 , table2 ;

Cartesian product of two tables is obtained by giving the names of both tables in FROM

clause of the SELECT query.

Ques: Display the Cartesian product of student and Library tables shown below : STUDENT

+-------+--------------+-------+---------+------------+---------+ | admno | name | class | section | dob | fees | +-------+--------------+-------+---------+------------+---------+ | 111 | Anu Jain | 12 | A | 2000-12-31 | 2500.00 | | 222 | Mohit Sharma | 11 | B | 2001-12-31 | 4500.00 | | 333 | K.P. Gupta | 12 | B | 2000-01-11 | 3000.00 | | 444 | Ajit Kumar | 10 | A | 2002-03-04 | 2000.00 | | 555 | Nandini | 12 | C | 2000-03-04 | 3000.00 | | 666 | Rohan Sharma | 11 | B | 2001-12-05 | 2500.00 | | 777 | Leena | NULL | NULL | 2000-03-05 | NULL | +-------+--------------+-------+---------+------------+---------+

LIBRARY +--------+--------------------+-------+-------------+ | BookNo | BookName | Admno | DateofIssue | +--------+--------------------+-------+-------------+ | 1001 | Harry Potter | 555 | 2017-03-01 | | 1002 | A Brave New World | 111 | 2017-03-05 | | 1003 | The Alchemist | 444 | 2017-03-07 | | 1004 | The Da Vinci Code | 222 | 2017-03-11 | | 1005 | Lord of The Rings | 555 | 2017-03-17 | | 1006 | Gone with the Wind | 111 | 2017-03-21 | +--------+--------------------+-------+-------------+

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The Cartesian Product of tables Student and Library can be obtained by the following

command:

select * from student , library;

+-------+--------------+-------+---------+------------+---------+--------+--------------------+-------+-------------+ | admno | name | class | section | dob | fees | BookNo | BookName | Admno | DateofIssue | +-------+--------------+-------+---------+------------+---------+--------+--------------------+-------+-------------+ 1. | 111 | Anu Jain | 12 | A | 2000-12-31 | 2500.00 | 1001 | Harry Potter | 555 | 2017-03-01 | 2. | 111 | Anu Jain | 12 | A | 2000-12-31 | 2500.00 | 1002 | A Brave New World | 111 | 2017-03-05 | 3. | 111 | Anu Jain | 12 | A | 2000-12-31 | 2500.00 | 1003 | The Alchemist | 444 | 2017-03-07 | 4. | 111 | Anu Jain | 12 | A | 2000-12-31 | 2500.00 | 1004 | The Da Vinci Code | 222 | 2017-03-11 | 5. | 111 | Anu Jain | 12 | A | 2000-12-31 | 2500.00 | 1005 | Lord of The Rings | 555 | 2017-03-17 | 6. | 111 | Anu Jain | 12 | A | 2000-12-31 | 2500.00 | 1006 | Gone with the Wind | 111 | 2017-03-21 | 7. | 222 | Mohit Sharma | 11 | B | 2001-12-31 | 4500.00 | 1001 | Harry Potter | 555 | 2017-03-01 | 8. | 222 | Mohit Sharma | 11 | B | 2001-12-31 | 4500.00 | 1002 | A Brave New World | 111 | 2017-03-05 | 9. | 222 | Mohit Sharma | 11 | B | 2001-12-31 | 4500.00 | 1003 | The Alchemist | 444 | 2017-03-07 | 10. | 222 | Mohit Sharma | 11 | B | 2001-12-31 | 4500.00 | 1004 | The Da Vinci Code | 222 | 2017-03-11 | 11. | 222 | Mohit Sharma | 11 | B | 2001-12-31 | 4500.00 | 1005 | Lord of The Rings | 555 | 2017-03-17 | 12. | 222 | Mohit Sharma | 11 | B | 2001-12-31 | 4500.00 | 1006 | Gone with the Wind | 111 | 2017-03-21 | 13. | 333 | K.P. Gupta | 12 | B | 2000-01-11 | 3000.00 | 1001 | Harry Potter | 555 | 2017-03-01 | 14. | 333 | K.P. Gupta | 12 | B | 2000-01-11 | 3000.00 | 1002 | A Brave New World | 111 | 2017-03-05 | 15. | 333 | K.P. Gupta | 12 | B | 2000-01-11 | 3000.00 | 1003 | The Alchemist | 444 | 2017-03-07 | 16. | 333 | K.P. Gupta | 12 | B | 2000-01-11 | 3000.00 | 1004 | The Da Vinci Code | 222 | 2017-03-11 | 17. | 333 | K.P. Gupta | 12 | B | 2000-01-11 | 3000.00 | 1005 | Lord of The Rings | 555 | 2017-03-17 | 18. | 333 | K.P. Gupta | 12 | B | 2000-01-11 | 3000.00 | 1006 | Gone with the Wind | 111 | 2017-03-21 | 19. | 444 | Ajit Kumar | 10 | A | 2002-03-04 | 2000.00 | 1001 | Harry Potter | 555 | 2017-03-01 | 20. | 444 | Ajit Kumar | 10 | A | 2002-03-04 | 2000.00 | 1002 | A Brave New World | 111 | 2017-03-05 | 21. | 444 | Ajit Kumar | 10 | A | 2002-03-04 | 2000.00 | 1003 | The Alchemist | 444 | 2017-03-07 | 22. | 444 | Ajit Kumar | 10 | A | 2002-03-04 | 2000.00 | 1004 | The Da Vinci Code | 222 | 2017-03-11 | 23. | 444 | Ajit Kumar | 10 | A | 2002-03-04 | 2000.00 | 1005 | Lord of The Rings | 555 | 2017-03-17 | 24. | 444 | Ajit Kumar | 10 | A | 2002-03-04 | 2000.00 | 1006 | Gone with the Wind | 111 | 2017-03-21 | 25. | 555 | Nandini | 12 | C | 2000-03-04 | 3000.00 | 1001 | Harry Potter | 555 | 2017-03-01 | 26. | 555 | Nandini | 12 | C | 2000-03-04 | 3000.00 | 1002 | A Brave New World | 111 | 2017-03-05 | 27. | 555 | Nandini | 12 | C | 2000-03-04 | 3000.00 | 1003 | The Alchemist | 444 | 2017-03-07 | 28. | 555 | Nandini | 12 | C | 2000-03-04 | 3000.00 | 1004 | The Da Vinci Code | 222 | 2017-03-11 | 29. | 555 | Nandini | 12 | C | 2000-03-04 | 3000.00 | 1005 | Lord of The Rings | 555 | 2017-03-17 | 30. | 555 | Nandini | 12 | C | 2000-03-04 | 3000.00 | 1006 | Gone with the Wind | 111 | 2017-03-21 | 31. | 666 | Rohan Sharma | 11 | B | 2001-12-05 | 2500.00 | 1001 | Harry Potter | 555 | 2017-03-01 | 32. | 666 | Rohan Sharma | 11 | B | 2001-12-05 | 2500.00 | 1002 | A Brave New World | 111 | 2017-03-05 | 33. | 666 | Rohan Sharma | 11 | B | 2001-12-05 | 2500.00 | 1003 | The Alchemist | 444 | 2017-03-07 | 34. | 666 | Rohan Sharma | 11 | B | 2001-12-05 | 2500.00 | 1004 | The Da Vinci Code | 222 | 2017-03-11 | 35. | 666 | Rohan Sharma | 11 | B | 2001-12-05 | 2500.00 | 1005 | Lord of The Rings | 555 | 2017-03-17 | 36. | 666 | Rohan Sharma | 11 | B | 2001-12-05 | 2500.00 | 1006 | Gone with the Wind | 111 | 2017-03-21 | 37. | 777 | Leena | NULL | NULL | 2000-03-05 | NULL | 1001 | Harry Potter | 555 | 2017-03-01 | 38. | 777 | Leena | NULL | NULL | 2000-03-05 | NULL | 1002 | A Brave New World | 111 | 2017-03-05 | 39. | 777 | Leena | NULL | NULL | 2000-03-05 | NULL | 1003 | The Alchemist | 444 | 2017-03-07 | 40. | 777 | Leena | NULL | NULL | 2000-03-05 | NULL | 1004 | The Da Vinci Code | 222 | 2017-03-11 | 41. | 777 | Leena | NULL | NULL | 2000-03-05 | NULL | 1005 | Lord of The Rings | 555 | 2017-03-17 | 42. | 777 | Leena | NULL | NULL | 2000-03-05 | NULL | 1006 | Gone with the Wind | 111 | 2017-03-21 | +-------+--------------+-------+---------+------------+---------+--------+--------------------+-------+-------------+

Table 1: Student Number of rows (cardinality) = 7

Number of columns (degree) = 6

Table 2: Library

Number of rows (cardinality) = 6

Number of columns (degree) = 4 Cartesian product: Student X Library

Number of rows (cardinality) = 7 * 6 = 42

Number of columns (degree) = 6 + 4 = 10

Ques: What is an Equi-Join of two tables?

SQL Joins are used to relate information in different tables.

An EQUI-JOIN of two tables is obtained by putting an Equality condition on the

Cartesian Product of two tables in order to extract meaningful data from the

underlying tables. This equality condition is put on the common column of the two tables which have similar data and the same data types. This common column is, generally,

the primary key of one table and foreign key of the other table. We specify both

table names in the FROM clause of SELECT command and also give the Join Condition

specifying the equality of matching columns.

Syntax:

SELECT * FROM table1 A , table2 B

WHERE A.PrimaryKey = B.ForeignKey

AND AdditionalCondition;

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STUDENT +-------+--------------+-------+---------+------------+---------+ | admno | name | class | section | dob | fees | +-------+--------------+-------+---------+------------+---------+ | 111 | Anu Jain | 12 | A | 2000-12-31 | 2500.00 | | 222 | Mohit Sharma | 11 | B | 2001-12-31 | 4500.00 | | 333 | K.P. Gupta | 12 | B | 2000-01-11 | 3000.00 | | 444 | Ajit Kumar | 10 | A | 2002-03-04 | 2000.00 | | 555 | Nandini | 12 | C | 2000-03-04 | 3000.00 | | 666 | Rohan Sharma | 11 | B | 2001-12-05 | 2500.00 | | 777 | Leena | NULL | NULL | 2000-03-05 | NULL | +-------+--------------+-------+---------+------------+---------+

LIBRARY +--------+--------------------+-------+-------------+ | BookNo | BookName | Admno | DateofIssue | +--------+--------------------+-------+-------------+ | 1001 | Harry Potter | 555 | 2017-03-01 | | 1002 | A Brave New World | 111 | 2017-03-05 | | 1003 | The Alchemist | 444 | 2017-03-07 | | 1004 | The Da Vinci Code | 222 | 2017-03-11 | | 1005 | Lord of The Rings | 555 | 2017-03-17 | | 1006 | Gone with the Wind | 111 | 2017-03-21 | +--------+--------------------+-------+-------------+

Ques: Display student details with corresponding book details select * from student , library where student.admno = library.admno;

+-------+--------------+-------+---------+------------+---------+--------+--------------------+-------+-------------+ | admno | name | class | section | dob | fees | BookNo | BookName | Admno | DateofIssue | +-------+--------------+-------+---------+------------+---------+--------+--------------------+-------+-------------+ | 555 | Nandini | 12 | C | 2000-03-04 | 3000.00 | 1001 | Harry Potter | 555 | 2017-03-01 | | 111 | Anu Jain | 12 | A | 2000-12-31 | 2500.00 | 1002 | A Brave New World | 111 | 2017-03-05 | | 444 | Ajit Kumar | 10 | A | 2002-03-04 | 2000.00 | 1003 | The Alchemist | 444 | 2017-03-07 | | 222 | Mohit Sharma | 11 | B | 2001-12-31 | 4500.00 | 1004 | The Da Vinci Code | 222 | 2017-03-11 | | 555 | Nandini | 12 | C | 2000-03-04 | 3000.00 | 1005 | Lord of The Rings | 555 | 2017-03-17 | | 111 | Anu Jain | 12 | A | 2000-12-31 | 2500.00 | 1006 | Gone with the Wind | 111 | 2017-03-21 | +-------+--------------+-------+---------+------------+---------+--------+--------------------+-------+-------------+

Ques: Display admno, name of student, bookname and issue date select student.admno,name,bookname,dateofissue from student,library where student.admno = library.admno; +-------+--------------+--------------------+-------------+ | admno | name | bookname | dateofissue | +-------+--------------+--------------------+-------------+ | 555 | Nandini | Harry Potter | 2017-03-01 | | 111 | Anu Jain | A Brave New World | 2017-03-05 | | 444 | Ajit Kumar | The Alchemist | 2017-03-07 | | 222 | Mohit Sharma | The Da Vinci Code | 2017-03-11 | | 555 | Nandini | Lord of The Rings | 2017-03-17 | | 111 | Anu Jain | Gone with the Wind | 2017-03-21 | +-------+--------------+--------------------+-------------+

Note :

If a column with the same name exists in both the tables, then it is mandatory to prefix the column name with the table name from which it is to be selected in order to eliminate

ambiguity.

Ques: Display name, class, section, dateofissue from student and library tables for books

issued after 2017-03-15

Select name , class , section , dateofissue from student,library where student.admno = library.admno and dateofissue > '2017-03-15'; +----------+-------+---------+-------------+ | name | class | section | dateofissue | +----------+-------+---------+-------------+ | Nandini | 12 | C | 2017-03-17 | | Anu Jain | 12 | A | 2017-03-21 | +----------+-------+---------+-------------+

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Note :

Additional conditions can also be specified along with the Joining Condition in the where

clause while making a Join

Ques: Display admno, name, bookname for students of class 12 select S.admno, name , bookname from student S , library L where S.admno = L.admno and class = 12;

+-------+----------+--------------------+ | admno | name | bookname | +-------+----------+--------------------+ | 555 | Nandini | Harry Potter | | 111 | Anu Jain | A Brave New World | | 555 | Nandini | Lord of The Rings | | 111 | Anu Jain | Gone with the Wind | +-------+----------+--------------------+

Note :

Alias names can be given to tables in the From clause while making a Join. You can then

refer to the tables only by their alias.

Ques: Display name, class, section of student who has issued book ‘The Da Vinci Code’ select name , class , section from student S , library L where S.admno = L.admno and bookname = 'The Da Vinci Code';

+--------------+-------+---------+ | name | class | section | +--------------+-------+---------+ | Mohit Sharma | 11 | B | +--------------+-------+---------+

Ques: What is Foreign Key?

Foreign key is a key in a table which derives its values from the Primary Key column

of some other table. It is used to ensure referential integrity and to get Equi-Join of two tables.

Ques: What is Referential Integrity?

Referential Integrity is the property of a relational database which ensures that no

entry in a foreign key column of a table can be made unless it matches a primary key

value in the corresponding related table.

Ques: What is a Union? Union is an operation of combining the output of two SELECT statements. Union of two

SELECT statements can be performed only if their outputs contain same number of

columns and data types of corresponding columns are also the same.

Syntax:

SELECT col1 , col2… FROM table1 UNION ALL SELECT col1 , col2… FROM table2; Note :

Union does not display any duplicate rows unless ALL is specified with it.

Ques: Write a command to create a union of Student1 and Student2? Table: Student1 Table: Student2

Rollno Name

11 Kumar

22 Mohan

33 Rohit

Resultant Union of the Two Tables is: SELECT * FROM Student1 UNION SELECT * FROM Student2;

Rollno Name

22 Mohan

11 Rahul

77 Kavita

Rollno Name

11 Kumar

22 Mohan

33 Rohit

11 Rahul

77 Kavita

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Ques: What is Relational Algebra?

Relational algebra consists of a set of operations that take one or two relations as

input and produces a new relation as output.

Operation in relational algebra are : Selection, Projection, Union, Cartesian Product

Ques: What do you understand by the Selection and Projection operation in Relational

Algebra ? Explain by an Example

Consider the following Relation

Relation: Student ADNO NAME CLASS SECTION AVERAGE

101 Anu 12 A 85

105 Balu 12 D 65

203 Leena 11 B 95

205 Madhu 10 B 75

305 Surpreeth 9 C 70

483 Usha 6 A 60

Selection in relational algebra returns those

tuples(records) in a relation that fulfil a

condition

(Produce a table containing subset of rows).

Syntax: σ condition (relation)

σ class=12 (Student)

101 Anu 12 A 85

105 Balu 12 D 65

Projection in relational algebra returns

those columns in a relation that are given in

the attribute list

(Produce table containing subset of columns).

Syntax: ∏ attribute list (relation)

∏ Adno,Name (Student)

ADNO NAME

101 Anu

105 Balu

203 Leena

205 Madhu

305 Surpreeth

483 Usha

Ques: What is the Union of two tables?

The Union operator is used to combine two or more tables. Each table within the UNION

should have the same number of columns, similar data types and also the columns

must be in the same order. In the union operation, duplicate records will be

automatically removed from the resultant table.

Table: Student1 Table: Student2 Rollno Name

11 Kumar

22 Mohan

33 Rohit

Rollno Name

22 Mohan

11 Rahul

77 Kavita

σ (Students 1)

Union σ (Students 2)

∏ rollno, Name (Students 1)

Union

∏ rollno, Name (Students 2)

Resultant Union

of the Two

Tables is:

Rollno Name

11 Kumar

22 Mohan

33 Rohit

11 Rahul

77 Kavita

In the above resultant table, student1 is

copied as it is, but in student2, roll no 22

Mohan's information is same as student1. So, that is not copied in the resultant table again.

Roll no 11 is same as student1, but name is

different. So, that is copied in the resultant

table. Roll no 77 is not in student1 table, so

that is also copied in the resultant table.

database concepts · 2020-01-14 · database concepts page 2 of 19 i. alternate key: out of the...

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