Add Constraint to Table in PostgreSQL

Use ALTER TABLE ADD CONSTRAINT statement to add constraint to a table.

Syntax: 

ALTER TABLE <table_name>
ADD CONSTRAINT <constraint_name> <constraint_definition>;

Using ALTER TABLE ADD CONSTRAINT, any of bellow constraints can be added to existing table

NOT NULL
CHECK
UNIQUE
PRIMARY KEY
FOREIGN KEY

Consider that you already have the following employee table.

Add UNIQUE Constraint

The UNIQUE constraint ensures that the values of a column must be unique and not a duplicate value. The following will set the unique constraint on the email column of the employee table.

Example: Add Unique Constraint

ALTER TABLE employee
ADD CONSTRAINT employee_unq UNIQUE(email);

The following is the result of the above statement.

Add NOT NULL Constraint

The NOT NULL constraint ensures ensure that a given column of a table is never assigned the null value. To add or remove NOT NULL constraint to a column of table you need to use ALTER COLUMN clause with ALTER TABLE.

The following will set the NOT NULL constraint on the gender column of the employee table.

Example: Add NOT NULL Constraint

ALTER TABLE employee
ALTER COLUMN gender SET NOT NULL;

To remove the NOT NULL constraint, use the DROP clause, as shown below.

Example: Remove NOT NULL Constraint

ALTER TABLE employee
ALTER COLUMN gender DROP NOT NULL;

Add Constraints using pgAdmin

You can add constraints using pgAdmin by right clicking on the table and select 'Properties' in the context menu. This will open a popup where you can add or edit multiple columns definition.

In the popup, go to 'Constraints' tab where you can add or edit Primary key, Foreign Key, Check, and unique constraints, as shown below.