SQL constraints are rules that limit the type of data that can be stored in database tables. There are different types of constraints including primary keys, unique constraints, not null constraints, foreign keys, check constraints, default values, and indexes. Primary keys uniquely identify each row, unique constraints ensure all column values are different, foreign keys link data between tables, and check constraints verify data meets certain conditions.

1. By : Dhirendra Chauahn

2. SQL constraints
• “SQL constraints are used to specify rules for the
data in a table.”
• Constraints are used to limit the type of data
that can go into a table.
• Constraints can be column level or table level.
Column level constraints apply to a column, and table
level constraints apply to the whole table.

3. Show Command
The following constraints are commonly used in SQL:
1.PRIMARY KEY
2.UNIQUE
3.NOT NULL
4.FOREIGN KEY
5.CHECK
6.DEFAULT
7.INDEX

4. PRIMARY KEY
PRIMARY KEY - A combination of a NOT NULL and
UNIQUE. Uniquely identifies each
row in a table

5. UNIQUE
UNIQUE - Ensures that all values in a column are
different

6. NOT NULL
NOT NULL - Ensures that a column cannot have a
NULL value

7. FOREIGN KEY
FOREIGN KEY - Uniquely identifies a row/record in
another table

8. CHECK
CHECK- Ensures that all values in a column satisfies
a specific condition

9. DEFAULT
DEFAULT - Sets a default value for a column when
no value is specified

10. INDEX
INDEX - Used to create and retrieve data from the
database very quickly

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