What are database constraints? [closed]

Answer 1

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

Answer 2

constraints are conditions, that can validate specific condition. Constraints related with database are Domain integrity, Entity integrity, Referential Integrity, User Defined Integrity constraints etc.

Answer 3

1. UNIQUE constraint (of which a PRIMARY KEY constraint is a variant). Checks that all values of a given field are unique across the table. This is X-axis constraint (records)

2. CHECK constraint (of which a NOT NULL constraint is a variant). Checks that a certain condition holds for the expression over the fields of the same record. This is Y-axis constraint (fields)

3. FOREIGN KEY constraint. Checks that a field's value is found among the values of a field in another table. This is Z-axis constraint (tables).