Understanding the Constraints in SQL

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. 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

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