Different constraints in SQL

Constraints in SQL:

Constraints are the rules enforced on data columns on table.
These are used to limit the type of data that can go into a table and It ensures the accuracy and reliability of the data in the database.

Constraints are of two types:

Column level: These are applied only to one column

Table level: These are applied to the whole table

Below are the commonly used constraints in SQL.

NOT NULL: Ensures that a column cannot have NULL value.

DEFAULT: Provides a default value for a column when none is specified.

UNIQUE: Ensures that all values in a column are different and we can have one NULL value in this type of column.

PRIMARY: Uniquely identified each rows/records in a database table and we cannot have NULL value in this type of column.

FOREIGN Key: Uniquely identified a rows/records in any another database table.

CHECK: It ensures that all values in a column satisfy certain conditions.

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

Constraints can be specified when we are creating the table or we can add constraints to the table even after the table is created.

Dropping Constraints:

Any constraint that you have defined can be dropped using the ALTER TABLE command with the DROP CONSTRAINT option.

For example, to drop the primary key constraint on the EMPLOYEE table, you can use the following command:

ALTER TABLE Employee DROP CONSTRAINT Employee_PK

Integrity Constraints:

Integrity constraints are used to ensure accuracy and consistency of data in a relational database.

Data integrity is handled in a relational database through the concept of referential integrity.

There are many types of integrity constraints that play a role in referential integrity (RI). These constraints include Primary Key, Foreign Key, Unique Constraints and other constraints mentioned above.