SQL Add Constraint Syntax

Sometimes we may decide to add a new constraint to an existing table (to see what are the different types of constraints that can be placed on a database table, please refer to the CONSTRAINT section).

The syntax for adding a constraint in SQL is,

Let's look at the example. Assuming our starting point is the Customer table created in the CREATE TABLE section:

Table Customer

Assume we want to add a UNIQUE constraint to the "Address" column. To do this, we type in the following:

MySQL:

Oracle:

SQL Server:

where Con_First is the name of the constraint.