What is a constraint?

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What is a constraint?

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A constraint enforces the integrity of a database. It defines rules regarding the values allowed in the columns of a table. A constraint is the standard mechanism for enforcing integrity. Using constraints is preferred to using triggers, rules, and defaults. Most of the RDBMS databases support the following five types of constraints:

• NOT NULL constraint: It specifies that the column does not accept NULL values.


• CHECK constraint: It enforces domain integrity by limiting the values that can be placed in a column.


• UNIQUE constraint: It enforces the uniqueness of values in a set of columns.


• PRIMARY KEY constraint: It identifies the column or set of columns whose values uniquely identify a row in a table.


• FOREIGN KEY constraint: It establishes a foreign key relationship between the columns of the same table or different tables.

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