Types of Integrity Constraints in Oracle

The integrity constraint in oracle impose restriction to enter column values.
The cost of including an integrity constraint is, at most, the same as executing a SQL statement that evaluates the constraint.

There are various types of integrity constraints in oracle.

1)NOT NULL Integrity Constraints:
---------------------------------
Null means the absence of a value. A NOT NULL constraint requires a column of a table must have a value.
Definition:
Create table a_nn( a number NOT NULL, b number);

2)UNIQUE Key Integrity Constraints:
-------------------------------------
A UNIQUE key integrity constraint requires that every value in a column or set of columns be unique—that is, no two rows of a table have duplicate values in a specified column or set of columns.

Unique key allows NULL is a column value.

Create table a_uk(a number UNIQUE, b number);

3)PRIMARY KEY Integrity Constraints:
---------------------------------------
Each table in the database can have at most one PRIMARY KEY constraint.
The column(s) which is primary key corresponding row in the table must be unique.
The column(s) which is primary key corresponding row must not contain null value.

Definition: Create table a_pk ( a number PRIMARY KEY, b number);
4)Referential Integrity Constraints:
---------------------------------------
Different tables in a relational database can be related by common columns, and the rules that govern the relationship of the columns must be maintained. Referential integrity rules guarantee that these relationships are preserved.


5)CHECK Integrity Constraints:
--------------------------------
It must be a Boolean expression evaluated using the values in the row being inserted or updated.

It cannot contain subqueries; sequences; the SQL functions SYSDATE, UID, USER, or USERENV; or the pseudocolumns LEVEL or ROWNUM.

Definition: Create table a_c( a number check (a>10),b number);

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