What are database constraints?

Constraints are part of a database schema definition.

A constraint is usually associated with a table and is created with a CREATE CONSTRAINT or CREATE ASSERTION SQL statement.

They define certain properties that data in a database must comply with. They can apply to a column, a whole table, more than one table or an entire schema. A reliable database system ensures that constraints hold at all times (except possibly inside a transaction, for so called deferred constraints).

Common kinds of constraints are:

  • not null – each value in a column must not be NULL
  • unique – value(s) in specified column(s) must be unique for each row in a table
  • primary key – value(s) in specified column(s) must be unique for each row in a table and not be NULL; normally each table in a database should have a primary key – it is used to identify individual records
  • foreign key – value(s) in specified column(s) must reference an existing record in another table (via it’s primary key or some other unique constraint)
  • check – an expression is specified, which must evaluate to true for constraint to be satisfied

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