11
Including Constraints
Objectivesj i
After completing this lesson, you should be able to do the following:
•• Describeri constraintstr i ts
•• Creater te and maintaini t in constraintstr i ts
11-2
What Are Constraints?i
•• Constraintstr i ts enforcef r rulesr l att thet tablet le levell l..
•• Constraintstr i ts preventr t thet deletionl ti off a tablet le ifif theret re arere dependenciesi ..
•• The followingf ll i constrainttr i t typest arere validlid inin
Oracle:r l :
–NOT NULL
–UNIQUEI Key
–PRIMARYI KEY
–FOREIGNI KEY
–CHECK
11-3
Constrainti Guidelinesi li
•• Name a constrainttr i t orr thet Oracler le Serverr r willill generater te a name by usingi thet SYS_Cn formatf r t..
•• Creater te a constraint:tr i t:
–Att thet same timeti as thet tablet le isis createdr t
–Afterft r thet tablet le has been createdr t
•• Definefi a constrainttr i t att thet columnl orr tablet le levell l..
•• Viewi a constrainttr i t inin thet datata dictionaryi ti ry..
11-4
Definingi i Constraintsi
CREATE TABLE [schema.]table
(column datatype [DEFAULT expr]
[column_constraint],
…
[table_constraint]);
CREATE TABLE emp( |
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empno |
NUMBER(4), |
ename |
VARCHAR2(10), |
… |
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deptno |
NUMBER(7,2) NOT NULL, |
CONSTRAINT emp_empno_pk
PRIMARY KEY (EMPNO));
11-5
Definingi i Constraintsi
•• Columnl constrainttr i t levell l
column [CONSTRAINT constraint_name] constraint_type,
•• Tablele constrainttr i t levell l
column,...
[CONSTRAINT constraint_name] constraint_type
(column, ...),
11-6
The NOT NULL Constrainti
Ensures that null values are not permitted for the column
EMP
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EMPNO |
ENAME |
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JOB |
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COMM |
DEPTNO |
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7839 |
KING |
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PRESIDENT |
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10 |
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7698 |
BLAKE |
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MANAGER |
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30 |
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7782 |
CLARK |
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MANAGER |
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10 |
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7566 |
JONES |
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MANAGER |
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20 |
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Absence of NOT NULL |
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NOT NULL constraint |
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NOT NULL constraint |
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(no row may contain |
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constraint |
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a null value for |
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(any row can contain |
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this column) |
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null for this column) |
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11-7
The NOT NULL Constrainti
Defined at the column level
SQL> CREATE TABLE emp(
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empno |
NUMBER(4), |
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ename |
VARCHAR2(10) NOT NULL, |
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job |
VARCHAR2(9), |
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mgr |
NUMBER(4), |
6hiredate DATE,
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sal |
NUMBER(7,2), |
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comm |
NUMBER(7,2), |
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deptno |
NUMBER(7,2) NOT NULL); |
11-8
The UNIQUEI Key Constrainti
UNIQUE key constraint
DEPT
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DEPTNO |
DNAME |
LOC |
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---------- |
-------- |
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10 |
ACCOUNTING |
NEW YORK |
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20 |
RESEARCH |
DALLAS |
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30 |
SALES |
CHICAGO |
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40 |
OPERATIONS |
BOSTON |
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Insert into |
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50 |
SALES |
DETROIT |
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60 |
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BOSTON |
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Not allowed (DNAME SALES already exists)
Allowed
11-9
The UNIQUEI Key Constrainti
Defined at either the table level or the column level
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SQL> CREATE TABLE |
dept( |
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deptno |
NUMBER(2), |
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dname |
VARCHAR2(14), |
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loc |
VARCHAR2(13), |
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CONSTRAINT dept_dname_uk UNIQUE(dname)); |
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11-10
The PRIMARYI KEY Constrainti
PRIMARY KEY
DEPT 
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DEPTNO |
DNAME |
LOC |
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10 |
ACCOUNTING |
NEW YORK |
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20 |
RESEARCH |
DALLAS |
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30 |
SALES |
CHICAGO |
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40 |
OPERATIONS BOSTON |
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Insert into |
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20 |
MARKETING |
DALLAS |
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FINANCE |
NEW YORK |
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Not allowed (DEPTNO 20 already exists)
Not allowed (DEPTNO is null)
11-11