Accumulate SQL Server---SQL statement action constraints

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