SQL Constraints

Constraints help us to prevent invalid data entry into tables. Using constraints we can:

There are several constraints are as follows:

Constraint Description
Not null Specifies that the column can't contain a null value.
Unique Specifies a column or combination of columns whose values must be unique for all rows in the table.
Primary key Uniquely identifies each rows of the table.
Foreign key Established and enforces a foreign key relationship between the column and a column of the referenced table.
Check Specifies a condition that must be true.

All constraints are stored in data dictionary. Constraints are easy to refer if we give them meaningful names. If we dont give a name to a constraint , the server gives a name by its own.



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