CONSTRAINT Keyword
The Constraints are the rules that we can apply on the type of data in a table. like not null, unique, check the limit, primary, etc.
Add Constraint
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
ALTER TABLE <TABLE_NAME>
ADD CONSTRAINT <CONSTRAINT_NAME> <SPECIFIC_CONTRAINT> (<COLUMN_NAME1>,<COLUMN_NAME2>);
Example 1
ALTER TABLE Employee ADD CONSTRAINT UniqueName UNIQUE(Emp_Name);
Example 2
ALTER TABLE Employee ADD CONSTRAINT PK_Employee PRIMARY KEY (Emp_ID);
Drop Constraint
The DROP CONSTRAINT keyword or command is used to delete a constraint to an existing column.
The DROP CONSTRAINT is a combination of two keywords DROP and CONSTRAINT. they both are different keywords.
But whenever we have to create a constraint to an existing column we use DROP CONSTRAINT keyword.
Syntax
ALTER TABLE <TABLE_NAME> DROP CONSTRAINT <CONSTRAINT_NAME>;
Example
ALTER TABLE Employee DROP CONSTRAINT PK_Employee;
Summary
The Constraints are the rules that we can apply on the type of data in a table.