ADD CONSTRAINT Keyword

The ADD CONSTRAINT keyword or command is used to add a constraint to an existing column.

We create a table and then realise that we have forgotten to add constraint in a specific column then Add Constraint Commnad is used.

The ADD CONSTRAINT is a combination of two keywords ADD and CONSTRAINT. they both are different keywords.

But whenever we have to create a constraint to an existing column we use ADD CONSTRAINT keyword.

Syntax

SQL
ALTER TABLE <TABLE_NAME> 
ADD CONSTRAINT <CONSTRAINT_NAME> <SPECIFIC_CONTRAINT> (<COLUMN_NAME1>,<COLUMN_NAME2>);

Example 1

SQL
ALTER TABLE Employee ADD CONSTRAINT UniqueName UNIQUE(Emp_Name);

Example 2

SQL
ALTER TABLE Employee
ADD CONSTRAINT MainPrimary PRIMARY KEY (Emp_ID);

Example 3

SQL
ALTER TABLE Employee
ADD CONSTRAINT CheckAge CHECK (Emp_Age < 55);

Summary

ADD CONSTRAINT is used to create a constraint after a table is already created.