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What are constraints? Explain different types of
constraints?
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Answer Posted / rahul shrivastava
Constraints enable the RDBMS enforce the integrity of the
database automatically, without needing you to create
triggers, rule or defaults.
Types of constraints:
• PRIMARY KEY
• UNIQUE
• FOREIGN KEY
• CHECK
• NOT NULL
A PRIMARY KEY constraint is a unique identifier for a row
within a database table. Every table should have a primary
key constraint to uniquely identify each row and only one
primary key constraint can be created for each table. The
primary key constraints are used to enforce entity
integrity. ( Entity integrity is an integrity rule which
states that every table must have a primary key and that
the column or columns chosen to be the primary key should
be unique and not null)
A UNIQUE constraint enforces the uniqueness of the values
in a set of columns, so no duplicate values are entered.
The unique key constraints are used to enforce entity
integrity as the primary key constraints.
A FOREIGN KEY constraint prevents any actions that would
destroy link between tables with the corresponding data
values. A foreign key in one table points to a primary key
in another table. Foreign keys prevent actions that would
leave rows with foreign key values when there are no
primary keys with that value. The foreign key constraints
are used to enforce referential integrity.
A CHECK constraint is used to limit the values that can be
placed in a column. The check constraints are used to
enforce domain integrity.
A NOT NULL constraint enforces that the column will not
accept null values. The not null constraints are used to
enforce domain integrity, as the check constraints.
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