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MSc IT UFCE8K-15-M Data ManagementPrakash ChatterjeeRoom [email protected]://www.cems.uwe.ac.uk/~p-chatterjee/modules/dm

Lecture 2 : The Relational Data Model

UCE8K15-M Data Management 2012/13

Origins of the Relational Model

The relational model was developed by EF Codd in the early 1970s. Commercial systems based on the relational model appeared in the

late 1970s. At present there are several hundred relational DBMSs and most

computer vendors support 'relational' software. Examples of well-known products include Oracle, DB2, Sybase,

MySQL, MS.SQL Server and MS Access.

Informally, a relational system is a system in which:1. The data is perceived by the user as tables (and nothing but tables).2. The operators available to the user for (e.g.) retrieval are operators that derive “new” tables from "old" ones. For example, there is one operator, restrict, which extract a subset of the rows of a given table, and another, project, which extracts a subset of columns - and a row subset and a column subset of a table can both be regarded in turn as tables in their own right.


Components and terminology (1)

The model uses terminology taken from mathematics, particularly set theory and predicate logic. Basic terminology used in relational theory includes:

relation - this corresponds to a table or flat file with columns and rows

tuple - a row of a relation

attribute - a named column of a relation

domain - the set of allowable values for one or more attributes degree of a relation - the number of attributes it contains

cardinality of relation - the number of tuples it contains.


Components and terminology (2)


Properties of relations

There is only one data structure in the relational data model - the relation.

Every relation and every attribute within a relation must have a distinct name.

Attribute (column) values of a relation are atomic (i.e. single valued).

All values in an attribute (column) are taken from same domain.

The ordering of columns in a relation is not significant.

Duplicate tuples (rows) are not allowed (e.g. each row in a relation must be distinct).

The ordering of tuples (rows) and attributes (columns) is not significant.


Relational algebra & relational calculus

Relational algebra (ra) and relational calculus (rc) are both formal (mathematically based) languages defined by EF Codd.

ra & rc are logically equivalent languages. ra is “procedural” and rc is “declarative” in nature.

ra and rc are the formal grounding of the relational database model and illustrate the basic operations required by any data manipulation language such as SQL.

Relational algebra is an offshoot of first-order logic, is a set of relations closed under operators. Operators operate on one or more relations to yield a relation.

The “closure” property relates to the fact that from any given relational operation another relation is output - it os often refereed to as the “relations in – relations out” property.


Relational algebra operators (1)

Each relational operator takes one or more relations as its input and produces a new relation as output (closure). Codd originally defined eight operators, in two classes:

Set operators: UNION INTERSECTION

DIFFERENCE DIVIDE

The special relational operators: RESTRICT PROJECT

JOIN Cartesian PRODUCT


Relational algebra operators (2)


Relational algebra operators (3)dept – emp – salgrade example (1)

dept – emp – salgrade example

Department : dept (depno, dname, location)

Employee : emp (empno, ename, mgr, sal, deptno)

Salary Grade : salgrade (grade, losal, hisal)



dept table

deptno dname location

10 Accounting New York

20 Research Dallas

30 Sales Chicago

40 Operations Boston



empno ename mgr sal deptno

7369 SMITH 7902 £800.00 20

7499 ALLEN 7698 £1,600.00 30

7521 WARD 7698 £1,250.00 30

7566 JONES 7839 £2,975.00 20

7654 MARTIN 7698 £1,250.00 30

7698 BLAKE 7839 £2,850.00 30

7782 CLARK 7839 £2,450.00 10

7788 SCOTT 7566 £3,000.00 20

7839 KING £5,000.00 10

7844 TURNER 7698 £1,500.00 30

7876 ADAMS 7788 £1,100.00 20

7900 JAMES 7698 £950.00 30

7902 FORD 7566 £3,000.00 20

7934 MILLER 7782 £1,300.00 10

emp table



salgrade table

grade losal hisal

1 £700.00 £1,200.00

2 £1,201.00 £1,400.00

3 £1,401.00 £2,000.00

4 £2,001.00 £3,000.00

5 £3,001.00 £99,999.00



Restrict Subset of the Rows in a Table

RESTRICT EMP WHERE sal > 2000

empno ename mgr sal deptno

7566 JONES 7839 £2,975.00 20

7698 BLAKE 7839 £2,850.00 30

7782 CLARK 7839 £2,450.00 10

7788 SCOTT 7566 £3,000.00 20

7839 KING £5,000.00 10

7902 FORD 7566 £3,000.00 20



Projectsubset the Columns in a Table

PROJECT EMP [EMPNO, SAL,DEPTNO]empno sal deptno

7369 £800.00 20

7499 £1,600.00 30

7521 £1,250.00 30

7566 £2,975.00 20

7654 £1,250.00 30

7698 £2,850.00 30

7782 £2,450.00 10

7788 £3,000.00 20

7839 £5,000.00 10

7844 £1,500.00 30

7876 £1,100.00 20

7900 £950.00 30

7902 £3,000.00 20

7934 £1,300.00 10



Restrict-ProjectRESTRICT EMP WHERE SAL >2000PROJECT EMP[EMPNO, SAL, DEPTNO]

empno sal deptno

7566 £2,975.00 20

7698 £2,850.00 30

7782 £2,450.00 10

7788 £3,000.00 20

7839 £5,000.00 10

7902 £3,000.00 20

call this EMPX

Could you reverse these operations - always? ( project then restrict?)



Product combine each row of one table with each row of the other

PRODUCT DEPT with EMPXempno sal EMPX.

deptno dept.

Depnodname loc

7566 £2,975.00 20 10 Accounting New York






7566 £2,975.00 20 20 Research Dallas

7698 £2,850.00 30 20 Research Dallas

7782 £2,450.00 10 20 Research Dallas

7788 £3,000.00 20 20 Research Dallas

7839 £5,000.00 10 20 Research Dallas

7902 £3,000.00 20 20 Research Dallas



7566 £2,975.00 20 30 Sales Chicago

7698 £2,850.00 30 30 Sales Chicago

7782 £2,450.00 10 30 Sales Chicago

7788 £3,000.00 20 30 Sales Chicago

7839 £5,000.00 10 30 Sales Chicago

7902 £3,000.00 20 30 Sales Chicago

7566 £2,975.00 20 40 Operations Boston








Product (Cartesian product)

DEPT has 4 recordsEMPX has 6 records

so DEPT x EMPX has 24 records

but not very useful



Equi-Joinproduct restricted to rows which have matching common domain

empno

sal EMPX. deptno

dept. deptno

dname loc

7566 £2,975.00 20 20 Research Dallas

7698 £2,850.00 30 30 Sales Chicago


7788 £3,000.00 20 20 Research Dallas


7902 £3,000.00 20 20 Research Dallas



Natural Joinequi-join projected with the duplicate column removed

empno sal deptno

dname loc

7566 £2,975.00 20 Research Dallas

7698 £2,850.00 30 Sales Chicago

7782 £2,450.00 10 Accounting New York


7839 £5,000.00 10 Accounting New York



Basic SQL

SELECT * FROM EMP WHERE SAL > 2000;

SELECT ENAME,SAL,DEPTNO FROM EMP;

SELECT ENAME,SAL,DEPTNO FROM EMP WHERE SAL > 2000;

SELECT * FROM EMP, DEPT WHERE SAL > 2000;

SELECT * FROM EMP,DEPT WHERE SAL > 2000 AND

EMP.DEPTNO = DEPT.DEPTNO;

SELECT EMPNO, SAL, DEPTNO, DNAME FROM EMP,DEPT

WHERE SAL > 2000 AND EMP.DEPTNO = DEPT.DEPTNO;


Bibliography / Readings / Home based activities

Bibliography- An Introduction to Database Systems (8th ed.), C J Date, Addison Wesley 2004- Database Management Systems, P Ward & G Defoulas, Thomson 2006

Readings- Introduction to SQL’ McGraw-Hill/Osbourne (handout)

Home based activities- Ensure you download xampp and install on home PC or laptop (if you have a

slow home internet connection – download to data key or CD here at UWE)- Copy the SQL Workbook onto your data key or CD.- Import the tables from the SQL Workbook into your home MySQL DB. Begin

working through some of the query examples in the workbook using PHPMyAdmin.

msc it ufce8k-15-m data management prakash chatterjee room 3p16 [email protected]...

Documents

relational model

relational data model

relational theory

relational system

relational dbmss

relational software

relation attribute

relational database