--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