SQL Constraint in Database in Table

SQL Constraints

Constraints are the rules enforced on data columns on a table. These 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 database.

Constraints can either be column level or table level. Column level constraints are applied only to one column whereas, table level constraints are applied to the entire table.

Following are some of the most commonly used constraints available in SQL:

NOT NULL Constraint

Ensures that a column cannot have a NULL value.

DEFAULT Constraint

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

UNIQUE Constraint

Ensures that all the values in a column are different.

PRIMARY Key

Uniquely identifies each row/record in a database table.

FOREIGN Key

Uniquely identifies a row/record in any another database table.

CHECK Constraint

The CHECK constraint ensures that all values in a column satisfy certain conditions.

INDEX

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

Data Integrity

The following categories of data integrity exist with each RDBMS:

Entity Integrity:

There are no duplicate rows in a table.

Domain Integrity:

Enforces valid entries for a given column by restricting the type, the format, or the range of values.

Referential integrity:

Rows cannot be deleted, which are used by other records.

User-Defined Integrity:

Enforces some specific business rules that do not fall into entity, domain or referential integrity.