Add Constraint

To add a constraint to an existing table use the alter table statement with the add constraint command. There are four different types of constraints:

1. Primary Key Constraints – Enforces unique values for specified column, can be referenced.
2. Foreign Key Constraints – Enforces a reference to a primary key
3. Unique Constraints – Ensures unique values within a column
4. Check Constraints – Limits values acceptable for a column based on an evaluation

Add unique constraint

ALTER TABLE dbo.Presidents

ADD CONSTRAINT President_unique UNIQUE (President)

Add constraint to test value

ALTER TABLE dbo.Presidents

ADD CONSTRAINT President_unique CHECK (YearsInOffice >= 0 AND YearsInOffice < 13)