SQL constraints are set set of rules by which SQL engine enforces the integrity of database. Constraint maintains the standard rules to follow so that integrity in database can be maintained. If there is any data violation then it enforces to abort that activity. You can create the constraint at the time of creating a table or alter any table.
Normally we use following type of constraints, you can click on it and refer it in detail.
– PRIMARY KEY Constraint
– FOREIGN KEY Constraint
– CHECK Constraint
– DEFAULT Constraint
– UNIQUE Constraint
– NOT NULL Constraint
Reference: Manzoor Siddiqui [www.SQLServerLog.com]