Constraint are using to restricted the insertion of unwanted data in any columns. we can create constraints on single or multiple columns of any table. It maintain the data integrity of the table.
Constraints can be specified when the table is created (inside the CREATE TABLE statement) or after the table is created (inside the ALTER TABLE statement).
SQL CREATE TABLE + CONSTRAINT Syntax
column_name1 data_type(size) constraint_name,
column_name2 data_type(size) constraint_name,
column_name3 data_type(size) constraint_name,
There are 6 types of constraints in Sql Server:-
- Primary key constraint.
- Foreign Key constraint.
- Unique Key constraint.
- Not Null constraint.
- Check constraint
- Default constraint
In SQL, we have the following constraints:
- PRIMARY KEY – A combination of a NOT NULL and UNIQUE. Ensures that a column (or combination of two or more columns) have a unique identity which helps to find a particular record in a table more easily and quickly
- FOREIGN KEY – Ensure the referential integrity of the data in one table to match values in another table
- UNIQUE – Ensures that each row for a column must have a unique value
- NOT NULL – Indicates that a column cannot store NULL value
- CHECK – Ensures that the value in a column meets a specific condition
- DEFAULT – Specifies a default value for a column