Constraints in Oracle

Constraints:

Data integrity of the database must be maintained. In order to ensure data has integrity we have to implement certain rules or constraints. As these constraints are used to maintain integrity they are called as integrity constraints.

Constraints can be given at two different levels. If the constraint is related to a single column the constraint is given at the column level otherwise constraint is to be given at the table level. Base on the where a constraint is given, constraint are of two types:

1. Column Constraints
2. Table Constraints

Column Constraint:

A constraint given at the column level is called as Column Constraint. It defines a rule for a single column. It cannot refer to column other than the column at which it is defined. A typical example is PRIMARY KEY constraint when a single column is the primary key of the table.

Table Constraint:

A constraint given at the table level is called as Table Constraint. It may refer to more than one column of the table. A typical example is PRIMARY KEY constraint that is used to define composite primary key. A column level constraint can be given even at the table level, but a constraint that deals with more than one column must be given only at the table level. The following is the syntax of CONSTRAINT clause used with CREATE TABLE and ALTER TABLE commands.

[CONSTRAINT constraint] { [NOT] NULL | {UNIQUE | PRIMARY KEY} | REFERENCES [schema.] table [(column)] [ON DELETE CASCADE] | CHECK (condition) }

The following is the syntax of table constraint.

[CONSTRAINT constraint] { {UNIQUE | PRIMARY KEY} (column [,column] ...) | FOREIGN KEY (column [,column] ...) REFERENCES [schema.] table [(column [,column] ...)] [ON DELETE CASCADE] | CHECK (condition) }

The main difference between column constraint and table constraint is that in table constraint we have to specify the name of the column for which the constraint is defined whereas in column constraint it is not required as constraint is given on immediately after the column.

Creating Integrity Constraints
In the following few sections we will see how to integrity constraints.
NOT NULL Constraint:
Used to prevent any null value from entering into column. This is automatically defined forcolumn with PRIMARY KEY constraint.
CREATE TABLE COURSES( ...,name varchar2(20) CONSTRAINT courses_name_nn NOT NULL,.. );
CONSTRAINT option is used to given a name to constraint. The convention followed here is TABLENAME_COLUMN_TYPE.


PRIMARY KEY Constraint This constraint is used to define the primary key of the table. A primary key is used to uniquely identify rows in a table. There can be only one primary key in a table. It may consist of more than one column. If primary key is consisting of only one column, it can be given as column constraints otherwise it is to be given as table constraint. 
Note: You have to use table constraint to define composite primary key. Oracle does the following for the column that has PRIMARY KEY constraint .Creates a unique index to enforce uniqueness. 
CREATE TABLE COURSES( ccode varchar2(5) CONSTRAINT courses_pk PRIMARY KEY,... );
The following example shows how to define composite primary key using PRIMARY KEY constraint at the table level.
CREATE TABLE COURSE_FACULTY( ...,CONSTRAINT COURSE_FACULTY_PK PRIMARY KEY (ccode,faccode) );


UNIQUE Constraint Enforces uniqueness in the given column(s). Oracle automatically creates a unique index for this column.
CREATE TABLE courses ( ... ,name varchar2(20) CONSTRAINT courses_name_u UNIQUE, ... );
If two or more columns collective should be unique then UNIQUE constraint must be given at the table level.


FOREIGN KEY Constraint A foreign key is used to join the child table with parent table. FOREIGN KEY constraint is used to provide referential integrity, which makes sure that the values of a foreign key are derived from parent key. It can be defined either at the table level or at the column level. If a foreign key is defined on the column in child table then Oracle does not allow the parent row to be deleted, if it contains any child rows. However, if ON DELETE CASCADE option is given at the time of defining foreign key, Oracle deletes all child rows while parent row is being deleted.
CREATE TABLE course_faculty (ccode varchar2(5) CONSTRAINT course_faculty_ccode_fk REFERENCES courses(ccode), ... );
Note: When the name of the column in the referenced table is same as the foreign key then column need not be given after the table name. It means REFERENCES courses in the above example will suffice.
Table level constraint is used when foreign key is a composite foreign key.


ON DELETE CASCADE option As mentioned earlier, after a foreign key is defined, Oracle will NOT allow any parent row to be deleted if it has dependent rows in the child table. For example, if CCODE in COURSE_FACULTY table is defined as foreign key referencing CCODE column of COURSES table then it is NOT possible to delete rows from COURSES table if dependent rows exists in COURSE_FACULTY table. However, by using ON DELETE CASCADE it is possible to delete all child rows while parent row is being deleted.
CREATE TABLE course_faculty (ccode varchar2(5)CONSTRAINT course_faculty_ccode_fk REFERENCES courses(ccode) ON DELETE CASCADE, ..  );


CHECK Constraint Defines the condition that should be satisfied before insertion or updation is done.The condition used in CHECK constraint may NOT contain: A reference to pseudo column SYSDATE Subquery If it is given as column constraint, it can refer only to current column. But if it is given as table constraint, it can refer to more than one column of the table. In neither case it can refer to a column of other tables. The following example shows how to create CHECK constraint to make sure GRADE column of COURSE_FACULTY contains letters A, B and C only.
CREATE TABLE course_faculty( ..., grade char(1) CONSTRAINT course_faculty_grade_chk CHECK ( grade in (‘A’,’B’,’C’) ), ... );
The above CHECK constraint does not allow any other characters other than A, B and C. It must be noted that character comparison is always case sensitive. So to ignore case differences you can convert GRADE to uppercase before comparison made as follows:
CREATE TABLE course_faculty ( ..., grade char(1) CONSTRAINT course_faculty_grade_chk
CHECK ( upper(grade) in (‘A’,’B’,’C’) ), ... );
The following is an example of CHECK constraint at table level. The constraint makes sure the starting date (STDATE) of a batch is less than or equal to ending date (ENDDATE) of the batch.
CREATE TABLE batches ( ..., CONSTRAINT batches_dates_chk CHECK ( stdate <= enddate), );