Constraints in SQL/ORACLE

Constraints

Two types of constraints are provided: column constraints and table constraints. Column constraints are associated with a single column but table constraints are associated with more than one column. Any column constraint can also be formulated as a table constraint.

The specification of a (simple) constraint has the following form:

[constraint ] primary key | unique | not null

A constraint can be named. The most important constraints are not null, unique, primary key constraints.The primary key constraint enables a unique identification of each tuple in a table.

With a primary key, the database system ensures that no duplicates appear in a table.

For example, for our EMP table, the specification

create table EMP

(

emp_code number(4) constraint pk emp primary key,

. . .

);

In this case emp_code must be unique in this table.a table must have onlyone primary key, but more than one unique keys. Null values are not allowed for primary keys.