DESCRIPTION
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. This ensures the accuracy and reliability of the data in the table. If there is any violation between the constraint and the data action, the action is aborted.
Constraints can be column level or table level. Column level constraints apply to a column, and table level constraints apply to the whole table. The following constraints are commonly used in SQL :
NOT NULL
Ensures that a column cannot have a NULL value
UNIQUE
Ensures that all values in a column are different
PRIMARY KEY
A combination of a NOT NULL and UNIQUE. Uniquely identifies each row in a table
FOREIGN KEY
Uniquely identifies a row/record in another table
CHECK
Ensures that all values in a column satisfies a specific condition
DEFAULT
Sets a default value for a column when no value is specified
INDEX
Used to create and retrieve data from the database very quickly