SQL Constraint document

CREATE TABLE Employees (EmpId int identity (empname), Empgender char (2), empage int,empemail varchar (100), empaddress varchar (+)) gocreate Table Department (depid int identity (), Depname varchar (50))

--Manually delete a column (delete empaddress column) ALTER TABLE Employees drop column empaddress--Manually add a column (add a column empaddr varchar) ALTER TABLE Employees add empaddr varchar (1000)--Manually modify the data type of empemail (varchar) ALTER TABLE employess ALTER COLUMN EMPADDR  VARCHAR (200)--Add a PRIMARY KEY constraint to EmpId ALTER TABLE employess add constraint Pk_employees_empid primary KEY (EMPID)--add yi8ge non-null constraint In fact, even if you modify the column ALTER TABLE employeesalter column EmpName varchar (a) Not null--adds a unique constraint to the EmpName ALTER TABLE Employees add Constrai Ntuq_employees_empname Unique (EmpName)--Add a DEFAULT constraint for gender ALTER TABLE Employees add Constraintdf_employees_empgender Default (' Male ') for empgender--a check constraint for age, age must be between 0-120 years old and 0 and 120 years old ALTER TABLE employess add Constraintck_employees_empage Check (empage>=0 and empage<=120)--Add FOREIGN KEY constraint, table employee has a column Empdeptid references tbldepartment table Deptidalter Add depid int not nullalter table Department add constraint Pk_department_depid primary KEY (DEPID) ALTER TABLE Employees AD D constraintfk_employees_department foreign KEY (depid) references departMnet (depid)-When adding additional constraints, set cascading updates, cascade delete--[on delete{noaction| SET null| DEFAULT}]--[on update{noaction| SET null| Default}]--delete the original foreign key ALTER TABLE Employees DROP constraint fk_employees_department--Set the cascade Delete when setting the foreign key Alte      R Table Employees Add constraintfk_employees_department foreign key (depid) references departmnet (depid) on DELETE cascade  --Delete a constraint on a name ALTER TABLE Employees DROP constraint fk_employees_department--A statement to delete multiple constraints, with the constraint name separated by a comma alter TABLE Employees Drop constraint fk_employees_department,ck_emplpoyees_empage,uq_employees_empname--A statement add multiple constraints ALTER TABLE Employees Add constraint uq_emplyees_empname unique (empname), constraint ck_emplyees_empage check (empage>=0 and empage<= 120)

　　

SQL Constraint document