19 July 2014

SQL Constraints

SQL constraints are used to specify rules for the data in a table.
 If there is any violation between the constraint and the data action, the action is aborted by the constraint.

Syntax for SQL Constraints

CREATE TABLE table_name
column_name1 data_type(size) constraint_name,
column_name2 data_type(size) constraint_name,
column_name3 data_type(size) constraint_name,
 In SQL, we have the following constraints:
NOT NULL - Indicates that a column cannot store NULL value
UNIQUE - Ensures that each row for a column must have a unique value
PRIMARY KEY - A combination of a NOT NULL and UNIQUE. Ensures that a column (or combination of two or more columns) have an unique identity which helps to find a particular record in a table more easily and quickly
FOREIGN KEY - Ensure the referential integrity of the data in one table to match values in another table
CHECK - Ensures that the value in a column meets a specific condition
DEFAULT - Specifies a default value when specified none for this column

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