f27db introduction to database systems lecture 2: using relational databases

21/04/23 Introduction to Databases 1

F27DB Introduction to Database Systems Lecture 2: Using Relational Databases

Helen Hastie [email protected]

Student hours: Tuesday 2pm-3pmTwitter: @IntroDBHW and #IntroDBHW

Material available on Vision


Recap

• So far you have seen how to:• Create a table• Add some data• Run some queries to extract the data

• From the first lab you should know how to:• Start MySQL• Run commands from a text file and by typing• Extract data from a single table

• Next• Relational databases• => relationships between multiple tables


Extending the Spy database

• We have a new specification from the MySpy agency.

• The agency require to store information about spies, spy masters, and bank accounts• Each spy should have a unique code name, a first

name, last name, date of birth, sex, distinguishing mark, and payment due.

• A spy may also have several skills and several spies may have the same skill.

• A spy master has a unique code name.• A bank account has a unique account number,

income to date, and expenditure to date.• A spy master handles several spies. Each spy and

each spy master has one bank account.


The existing spy table

• Each spy should have a unique code name, a first name, last name, date of birth, sex, distinguishing mark, and payment due.

code name

first name

last name

date of birth sex markamount

due

007 James Bond 12 December 1972

M Mole on chin

5050

bud Fanny Charleston

31 July 1983 F scar on cheek

25.67

freddie John Smith 05 September 1954

M one finger missing

312.5


Alterations to be made

• A spy might have >1 skill• A spy has a bank account• A spy has a spy master• A spy master has a code, a bank account

and supervises several spies

• All this complexity does not fit into a single table

• Start by identifying entities and drawing an Entity Relationship or E-R diagram


E-R diagram

SpycodeNamefirstNamedateOfBirthgendermarkamountDue

SpyMastermCodeNamecontact

SpyAccountaccountNumberIncomeexpenditure

SpySkillListskillCodeskillName

MM

has

M

1 manages

1

has

11

has1

In the E-R diagram, the fact that a spy has other entities - a bank account, a spymaster and skills is ONLY shown by the relationship lines


The final tables

SpycodeName firstNamedateOfBirthgendermarkamountDuespyMaster bankAccount

SpyMastermCodeNamebankAccountcontact



M

1

1

M

1

1

skilled at

has

manages

1

1

SpyWithSkillspyCodeskillCode

1

M

practisedby

Bold : primary keyItalic : foreign keyBold and italic : both

has

Now the relationships are also shown by foreign keys

NB Many to many relationships need an extra ‘linking’ table


Foreign keys

• A foreign key is a field in a relational table that matches the primary key column of another table.

• It establishes the links between the tables.

• Examples• bankAccount in SpyMaster• bankAccount in Spy• spyMaster in Spy


The final tables

SpycodeName firstNamedateOfBirthgendermarkamountDuespyMaster bankAccount

SpyMastermCodeNamebankAccountcontact



M

1

1

M

1

1

skilled at

has

manages

1

1

SpyWithSkillspyCodeskillCode

1

M

practisedby

Bold : primary keyItalic : foreign keyBold and italic : both

has


Spy DB

SpyMaster

mCodeName

bankAccount contact

M 12345 Drop 5

P 56789 PO Box 23

Q 13579 Jimmie's

SpyAccount

account Number

income

expenditure

12345 0 0

13579 23567 1345.89

23456 2000 1345

34567 345 56.34

45678 3579.57 5280.45

56789 12678 10345

SpySkillList

skillCode skillName

1 top shot

2 skilled with a knife

3 fast runner

4 quick thinker

5 can pilot a helicopter

SpyWithSkill

spyCode

skillCode

007 1

007 4

bud 2

bud 3

bud 4

freddie 4

freddie 5

Spy

code Name

first Name

lastName dateOfBirthgender

markamount

Duespy

Masterbank

Account

007 James Bond 12 December 1972

M Mole on chin 5050 Q 23456

bud Fanny Charleston 31 July 1983 F scar on cheek 25.67 Q 34567

freddie John Smith 05 September 1954

M one finger missing

312.5 M 45678

M

1manages

1

Mskilled at

1

M

practisedby

1

has

1

1

1

has


One to many relationships

• One spymaster, several spies• The Spy table contains the code name of the

spy’s spymaster, as a link to the details in the bank account table• The spymaster codename in the Spy table is a

foreign key• It links to a primary key in the SpyMaster table

SpyMaster

code name

bankAccount

contact

M 12345 Drop 5

P 56789 PO Box 23

Q 13579 Jimmie's

Spy

code Name

first Name

lastName etcspy

masterbank

Account

007 James Bond . . . Q 23456

bud Fanny Charleston . . . Q 34567

freddie

John Smith . . . M 45678


One to one relationships

• One bank account, one spymaster• The SpyMaster table contains the bank account

number, as a link to the details in the bank account table• The bank account number in the SpyMaster table is a

foreign key• It links to a primary key in the Bank Account table

Bank Account

account numberIn

comeExpend

iture

12345 0 0

13579 23567 1345.89

23456 2000 1345

34567 345 56.34

45678 3579.57 5280.45

56789 12678 10345

SpyMaster

code name

bankAccount

contact

M 12345 Drop 5

P 56789 PO Box 23

Q 13579 Jimmie's


Many to many relationships

• A separate table has been created for skills• In this way, each skill is listed only once and so

we get consistency with spelling

• A code number has been invented for each skill• Long primary keys waste space

• skillName is not very long, but is just used to demonstrate

SpySkillList

skillCode skillName

1 top shot


3 fast runner

4 quick thinker




• A separate table is created with a list of links – which spy, which skill• This is essential for many-to-many

relationshipsSpyWithSkill

spyCode

skillCode

007 1

007 4

bud 2

bud 3

bud 4

freddie 4

freddie 5



SpySkillList

skillCode skillName

1 top shot


3 fast runner

4 quick thinker


SpyWithSkill

spyCode

skillCode

007 1

007 4

bud 2

bud 3

bud 4

freddie 4

freddie 5

Spycode name

first name

etc

007 James . . .

bud Fanny . . .

freddie John . . .

• Note that the SpyWithSkill table has• 2 foreign keys, linking to 2 different tables• A composite primary key consisting of both

fields• BOTH fields are needed to make a row unique


Creating the tables

• In the Create Table command, or as a separate Alter Table command, you can provide the foreign key information showing that one column refers to another

• We use the InnoDB engine to make sure that foreign key references are supported

CREATE TABLE SpyWithSkill (

spyCode VARCHAR(10),

skillCode INT ,

PRIMARY KEY (spyCode, skillCode),

FOREIGN KEY (skillCode) REFERENCES SpySkillsList (skillCode),

FOREIGN KEY (spyCode) REFERENCES Spy (codeName)

)ENGINE=INNODB;


AUTO-INCREMENT

• In the SpySkillList table, each skill has been given a unique automatically generated sequence number• New entries get the next number

• To allocate the next number, add ‘AUTO-INCREMENT’ to the column in the CREATE TABLE command• E.g.

skillCode INT AUTO-INCREMENT PRIMARY KEY,


INSERTING VALUES WITH AUTO-INCREMENT

• Use a slightly different format of the INSERT command. Because you are not inserting a value for every column, you must specify which ones:• INSERT INTO SpySkillList (skillName)

VALUES (‘explosive expert’);• The skillCode appears magically! SpySkillList

skillCode skillName

1 explosive expert

2 top shot


4 fast runner

5 quick thinker



Foreign key constraints

• You must make sure that the table that you are referencing with ‘Foreign Key’ exists

• Either create all the tables without foreign key constraints, then add in the foreign keys constraints afterwards using the ALTER TABLE command (This way is taken in Brian’s examples)

• Or create tables in suitable order, including foreign key constraints

• Note that you must drop the tables in order so that all the ones containing foreign keys are dropped before the table referenced by the foreign key

• I have done this in the Spy tables• This way works most of the time, but not if 2 tables are

referenced both ways• E.g. Spy has a SpyMaster

• SpyMaster has a second-in-command who is a Spy


Suitable order for creating Spy tables

• Have a look back at the Spy tables• What is wrong with this order for table

creation?• Spy• SpyAccount• SpyMaster• SpyWithSkill• SpySkillList

• What would be a more suitable order?

•SpyWithSkill•SpySkillList•Spy•SpyMaster•SpyAccount


Creating your database from a text file

• We recommend that you create your database from a text file containing commands to create the table and to insert the data

• In this way, you can • Easily correct errors in your commands and rerun them• Retain a record of what you did• Move db from home to uni, if required

• But to rerun the commands, you must start by dropping the tables• DROP TABLE tablename;• Again, order is important. You can’t drop a table which is

being referenced.


Text file - verbose mode

• To see your commands listed on the screen as MySQL runs through them from a text file, you need to be in ‘verbose mode’

• So alter the startup command to be• mysql –u username –D username –h mysql-

server-1 –vp• The last bit has

• v for verbose• p for password


Retrieving data from >1 table

• We’d like details of spy masters and their accounts

• How NOT to do it!• SELECT * required columns

…FROM SpyMaster, SpyAccount from these tables

• This gives you EVERY row from the Spy table joined to EVERY row from the BankAccount table, as shown on the next slide


Problem

• We ONLY want those rows with the matching account numbers• In bold below

Answer

code nameBank

Accountcontact

Account Number

Income

Expenditure

M 12345 Drop 5 12345 0 0

P 56789 PO Box 23 12345 0 0

Q 13579 Jimmie's 12345 0 0

M 12345 Drop 5 13579 23567 1345.89

P 56789 PO Box 23 13579 23567 1345.89

Q 13579 Jimmie's 13579 23567 1345.89

. . . . . . . . . . . . . . . . . .

M 12345 Drop 5 56789 12678 10345

P 56789 PO Box 23 56789 12678 10345

Q 13579 Jimmie's 56789 12678 10345


Solution

• Restrict the rows to those with matching account numbers• SELECT * FROM SpyMaster, SpyAccount

WHERE bankAccount = accountNumber

Answer

code namebank

Accountcontact

account Number

Income

Expenditure

M 12345 Drop 5 12345 0 0

Q 13579 Jimmie's 13579 23567 1345.89

P 56789 PO Box 23 56789 12678 10345


Column name uniqueness

• Restrict the rows to have matching account numbers• SELECT * FROM SpyMaster, SpyAccount

WHERE bankAccount = accountNumber

• In the above query, we know which tables the column names come from, because they are unique within the above named tables• E.g.The bankAccount column only exists in the SpyMaster

table

• BUT, supposing we had called them both accNum?• We need a way of distinguishing them


Specifying table together with column name

• There are 2 ways to specify which table the column belongs to

• You can also use the table name as well• SELECT * FROM SpyMaster, SpyAccount

WHERE SpyMaster.bankAccount = SpyAccount.accountNumber

• Or use an alias for the table name (shorter!)• SELECT * FROM SpyMaster M , SpyAccount A

WHERE M.bankAccount = A.accountNumber

• It’s quite nice to use the alias all the time, to make it very clear which table the column name belongs to. It is only essential• When the same column name occurs in >1 table• When a query involves using the same table

more than once (not covered today)


Less rows, less columns

• As before, you can restrict the number of columns• SELECT mCodeName, income

FROM SpyMaster, SpyAccountWHERE bankAccount = accountNumber

• And restrict the number of rows on other criteria• SELECT income, expenditure

FROM SpyMaster, SpyAccountWHERE bankAccount = accountNumberAND mCodeName = ‘M’;


Restricting rows

• We are now using the WHERE clause for 2 reasons• To specify the foreign key between linked

tables• To specify some criteria on the data e.g.

• income < 100• mCodeName = ‘M’

• The order is not important• The DBMS will work out the most efficient way

to execute your query• However, I recommend that you put all the

foreign key links together, at the start, so they come straight after the table names


Query 1:N

• For each spymaster, list their code name and their contact point, and the code names of each of the spies that they supervise• SELECT mCodeName, contact, codeName

FROM Spy S, SpyMaster MWHERE S.spyMaster = M.mCodeName;


Query 1:N

• There is some duplication in the result • In a real application, you would need to use

scripts or tools to lay the results out nicely.

+-----------+---------+----------+| mCodeName | contact | codeName |+-----------+---------+----------+| M | Drop 5 | freddie | | Q | Jimmy's | 007 | | Q | Jimmy's | 1 | | Q | Jimmy's | bud | +-----------+---------+----------+


Using column aliases

• You can rename output columns using aliases• SELECT mCodeName as masterCode, contact, codeName

as SpyCodeFROM Spy S, SpyMaster MWHERE S.spyMaster = M.mCodeName;

+------------+---------+----------+

| masterCode | contact | spyCode |

+------------+---------+----------+

| M | Drop 5 | freddie |

| Q | Jimmy's | 007 |

| Q | Jimmy's | 1 |

| Q | Jimmy's | bud |

+------------+---------+----------+


Query M:N

• List the names of the spies with the skill ‘top shot’

• SELECT firstName, lastNameFROM Spy S, SpyWithSkill W, SpySkillList LWHERE S.codeName = W.spyCodeAND W.skillCode = L.skillCodeAND L.skillName = ‘top shot’;


Maintaining data integrity

• Some issues. E.g.• What if we give a spy an account number which

doesn’t exist in the Account table?• This won’t be permitted if the Spy table includes a

foreign key referencing the account number in the Account table


SQL Tutorial

• There is a good tutorial on the w3schools site• http://www.w3schools.com/sql/

• Complete reference on the MySQL website


What next?

• Updating and deleting will be covered in Brian’s sql lecture

• There’s a separate handout for Lab2• Try out all the queries and invent your own• Notice the command to log interactions to a

text file

• Brian’s lectures continue with database material

• Helen’s lectures move on to web pages

f27db introduction to database systems lecture 2: using relational databases

Documents

spy mastera spy master

spy masters

skilla spy

existing spy tableeach

madea spy

spy databasewe

bank accounta spy

bank accountseach spy