SQL statement Add constraint

SQL statement Add constraint

--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.

The syntax is as follows

alter table table name

add constraint constraint name constraint type specific constraint description

Example:

--Add a PRIMARY KEY constraint

alter table Stuinfo

Add constraint Pk_stuno primary Key(Stuno)

--Add unique key constraints

alter table Stuinfo

Add constraint Uq_stuid Unique(STUID)

--Add a DEFAULT constraint

alter table Stuinfo

Add constraint Df_stuaddress Default(' Address unknown ') for stuaddress

--Adding CHECK constraints

alter table Stuinfo

Add constraint Ck_stuage Check(stuage between and 40)

--Add a FOREIGN KEY constraint

alter table Stuinfo

Add constraint Fk_stuno foreign Key(Stuno) references stuinfo (Stuno)

Delete Constraint

alter table name drop constraint constraint name

SQL statement Add constraint