Activate foreign keys in SQL Server

When the FOREIGN KEY foreign key in SQL Server has been disabled, you can enable / re-enable with the ALTER TABLE command.

The syntax for activating foreign keys in SQL Server

 ALTER TABLE ten_bang 
 CHECK CONSTRAINT fk_ten; 

Variable name or variable value

ten_bang

The name of the table to which the foreign key has been created.

fk_ten

The name of the foreign key you want to disable.

For example

 CREATE T ABLE sanpham 
 ( id_sanpham INT PRIMARY KEY, 
  ten_sanpham VARCHAR(50) NOT NULL, 
  phan_loai VARCHAR(25) 
 ); 
 
 CREATE TABLE hangtonkho 
 ( id_hangtonkho INT PRIMARY KEY, 
  id_sanpham INT NOT NULL, 
  soluong INT, 
  luong_toithieu INT, 
  luong_toida INT, 
  CONSTRAINT fk_htk_id_sanpham 
  FOREIGN KEY (id_sanpham) 
  REFERENCES sanpham (id_sanpham) 
 ); 

In this example, we created the parent table, sanpham, with the primary key including the information field id_sanpham. Then there is a child table named hangtonkho with a foreign key with deletion constraint. The CREATE TABLE statement creates a foreign key on the hangtonkho table named fk_htk_id_sanpham. The foreign key forms the relationship between the id_sanpham column in the hangtonkho table and id_sanpham in the sanpham table.

1. Foreign Key (Cascade Delete) in SQL Server
2. Foreign Key (Set Null) foreign key in SQL Server

If you want to re-enable the foreign key, run the following command.

 ALTER TABLE hangtonk ho 
 CHECK CONSTRAINT fk _htk_id_sanpham; 

The example above uses the ALTER TABLE command to re-enable the foreign key named fk_htk_id_sanpham in the hangtonkho table.

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