Description
--PRIMARY KEY constraint (Primary key constraint): The data of the primary key column is required to be unique and is not allowed to be empty.
--Unique Constraint: Requires the column to be unique, allowed to be empty, but only one null value can occur.
--Check Constraint: A column value range limit, formatting restrictions, and so on, such as age-related constraints.
--Default Constraint: The default value of a column, such as our male classmates more, gender default is male.
--FOREIGN KEY constraint (Foreign key): The column that references the primary table is required to establish a relationship between the two tables.
Grammar:
Alter Table Add constraint constraint name constraint type specific constraint description
Example:
--Add a PRIMARY KEY constraintAlter TableStuinfoAdd constraintPk_stunoPrimary Key(Stuno)--add unique key constraintAlter TableStuinfoAdd constraintUq_stuidUnique(Stuid)--Add a DEFAULT constraintAlter TableStuinfoAdd constraintDf_stuaddressdefault('Address Unknown') forstuaddress--Add a Check constraintAlter TableStuinfoAdd constraintCk_stuageCheck(Stuagebetween the and +)--Add a FOREIGN KEY constraintAlter TableStuinfoAdd constraintFk_stunoForeign Key(Stuno)Referencesstuinfo (Stuno)Delete ConstraintAlter TableTable nameDrop constraintConstraint name
Accumulate SQL Server---SQL statement action constraints