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Lecture 4

Data Integrity: Primary and Foreign Keys

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But first - table aliases

Table name prefixes prevent ambiguity in a query remember

emp.deptno=dept.deptno

But they can be tedious to enter

So we can define temporary labels in the FROM clause and use them elsewhere in the query. These temporary labels are called table aliases

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Example without aliases

To list information about all the employees in Chicago, enter:

SELECT DEPT.DEPTNO, DNAME, LOC, ENAME, JOB

FROM EMP, DEPT WHERE EMP.DEPTNO = DEPT.DEPTNO AND EMP.DEPTNO = 30;

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Abbreviating a Table Name using aliases (Again to list information about all the employees in Chicago)

SELECT D.*, ENAME, JOBFROM EMP E, DEPT DWHERE E.DEPTNO = D.DEPTNO

AND E.DEPTNO = 30;

Alias D.* retrieves all of the columns of the DEPT table

This produces the same results as the last query

Aliases E and D defined in the FROM clause

The ‘join-condition’ defines the relationship

between the two tables

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Joining a table to itself

Aliasing allows you to join a table to itself as though it was two separate tables.

Useful when we want to join one row in a table to another row in the same table.

EMP

MANAGERWORKER

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List employees whose salary exceeds their manager’s

SELECT WORKER.ENAME, WORKER.SAL,MANAGER.ENAME, MANAGER.SAL

FROM EMP WORKER, EMP MANAGERWHEREWORKER.MGR = MANAGER.EMPNOAND WORKER.SAL > MANAGER.SAL;

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Manager

Worker

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Result is

ENAME SAL ENAME SALSCOTT 3000 JONES 2975FORD 3000 JONES 2975

SCOTT’s row is joined to JONES’s because SCOTT’s MGR is 7566 and JONES’s EMPNO is 7566

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Topics

Referential Integrity Table constraints:

Primary KeysForeign KeysOn delete cascadeCheckDefault

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Referential Integrity

Oracle 7 onwards allows the use of table constraints to enforce referential integrity.That is, the data in tables must be consistent. Imagine inserting an employee with department 60! The table constraints allow us to enforce that this cannot happen.

(reading chapter 5 Dowling)

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Definition

Referential Integrity; rules in a database that ensure that data is valid and consistent.

Whereas individual constraints can be used to ensure the validity of data in a single column or a table, referential integrity constraints ensure the validity of data between tables.

– N.Dowling Database Design and Management (1998)

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Primary Key

Column(s) can have the primary key constraint placed on them, this has the following affect: No duplicate values Unique index created for column(s) Cannot have NULL values

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Primary Key

Examples from “our” tables are empno in myemp and deptno in mydept.

Only one Primary key is allowed per table but the key can consist of several columns

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Foreign Key

References a primary key in another table, example deptno in myemp (foreign key) references deptno in mydept (primary key) and is thus used to join the tables.

Foreign Keys can also be specified in table constraints.

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deptno dname loc

empno ename job deptno.

MYDEPT TABLE

primary key

primary keyforeign key

MYEMP TABLE

mgr hiredate

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Referential Integrity

To examine this let us look at an example

Currently (as there are no table constraints) we can insert an employee with department number 60.INSERT INTO myemp (empno, deptno)VALUES (1234, 60);

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Table Constraints

We can use alter table to add constraints:

ALTER TABLE mydept ADD CONSTRAINTtab_mydept_pkPRIMARY KEY (deptno);

This defines deptno as a primary key in table mydept. This means NULLs are no longer allowed and every value must be unique.

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Table Constraints

In the previous example, we named the constraint tab_mydept_pk. The name is arbitrary but using a name like tab_mydept_pk tells us a lot about the constraint.

Oracle will use this name in any error messages it generates (thus helping us to identify the cause of the problem).

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Table Constraints

Now we add the foreign key constraint to the myemp table

ALTER TABLE myemp ADD CONSTRAINT

tab_myemp_fkFOREIGN KEY (deptno)REFERENCES mydept (deptno);

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Table Constraints

Now try to insert an employee with department number 60.

INSERT INTO myemp (empno, deptno)VALUES (1234, 60);

ERROR at line 1:ORA-02291: integrity constraint (SPB001.TAB_MYEMP_FK) violated - parent key not found

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Table Constraints

We can also ensure that if a master or primary key row is deleted all the foreign or detail records get deleted with the ON DELETE CASADE constraint

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Table Constraints

ALTER TABLE myemp ADD CONSTRAINT

tab_emp_fkFOREIGN KEY (deptno)REFERENCES mydept (deptno)ON DELETE CASCADE;

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Table Constraints

Checks can also be applied to data or defaults supplied with table constraints. These are used to define conditions that every row must fulfil.

CONSTRAINT salary_checkCHECK (sal > 2000 and deptno

=10)This condition can ONLY refer to

columns within the same table

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Table Constraints

The default constraint defines a value for a column that will be assigned whenever a row is inserted that does not have an explicit value for the column

CREATE TABLE little_example(city VARCHAR2(20) DEFAULT (‘Stirling’), text VARCHAR2(5) DEFAULT (‘#####’));

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Table Constraints

The unique constraint ensure that one or more columns are unique within that table. Unlike a primary key, NULLs are allowed.

CREATE TABLE another_example(cityVARCHAR(20) UNIQUE);

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Table Constraints

Can dropped, disabled and enabled

ALTER TABLE <table> DROP CONSTRAINT <constraint name>;ALTER TABLE <table> DISABLECONSTRAINT <constraint name>;ALTER TABLE <table> ENABLECONSTRAINT <constraint name>;

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Table Constraints

These can be placed on a table when it is created.

CREATE TABLE new_emp(a_empno NUMBER(4) , …,

CONSTRAINT tab_new_emp_pk PRIMARY KEY(a_empno) ,…);

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Summary

Referential Integrity Table constraints:

Primary KeysForeign KeysOn delete cascadeCheckDefault