SQL Server FOREIGN KEY constraint

Definition of the primary foreign key in SQL Server:

1.

Create Table int Primary Key  not NULL  into Values (Ten, 'IT '), (+,' Finance'), (+, 'engneer ')

Create Table int Primary Key  not NULL int Foreign Key References Dept (DEPT_NO))

The above notation, is the column level definition, defines the column field directly when defining foreign key

2.

create  table  Dept (dept_no int  primary  key , Dept_name nvarchar (+) not  null  Prmary key  (dept_no)) Insert into  dept values  (10, ' IT ' ), (20, ' Finance ' ), (30, Engneer ' ) 

Create Table int Primary Key  not NULL int,constraintforeignkeyreferences dept (DEPT_NO))

This is a table-level definition.

3. You have created a table that does not have a primary foreign key defined, you can use ALTER TABLE to modify

alter  table  employee add  foreign  key  (DEPT_NO) references  Dept (dept_no) alter  table  employeeadd  constraint  dept_no_fk  Foreign  key  (dept_no) references  Dept (dept_no) 

Add primary Key

Alter Table Deptaddconstraintprimarykey(dept_no)

4. Remove the primary key

Alter Table Drop constraint

Remove foreign keys

Alter Table Drop constraint DEPT_NO_FK

The building was deleted, the building was deleted, the hand hurts originally intended to put the MySQL statement also written, the reason is that last night to read about the MySQL video, about check does not work, and their foreign key constraints are also vague, do not know how to write, the original is two ways of writing, which is to see the MySQL video to see. I'm going to have to write MySQL again sometime.