Multi table operation - foreign key constraint

• Catalog

  Why foreign key constraints are required

  When the data between tables is related

  The role of foreign key constraints ：

  Foreign key constraints use

  Add foreign key constraints when creating tables

  Delete foreign key constraint

  Add foreign key constraints separately after creating the table

  Examples demonstrate

  First create 2 surface , And use foreign key constraints

  Conduct error test

  Delete foreign key constraint

  Add a foreign key constraint

  ---

• Why foreign key constraints are required

• When the data between tables is related

• If there are no relevant data constraints , The accuracy of the data cannot be guaranteed

• The role of foreign key constraints ：

• Let tables relate to each other , So as to ensure the accuracy of the data

• Foreign key constraints use

• Add foreign key constraints when creating tables

• create table Table name (
• Name data type constraint
• .....
• constraint Foreign key name foreign key ( Foreign key column name of this table ) references Main table name ( Primary table primary key column name )
• )

• Delete foreign key constraint

• alter table Table name drop foreign Foreign key name

• Add foreign key constraints separately after creating the table

• alter table Table name add
• constraint Foreign key name foreign key ( Foreign key column name of this table ) references Main table name ( Primary key column name )

• Examples demonstrate

• First create 2 surface , And use foreign key constraints

• 
• 

• CREATE TABLE user(
  	id INT PRIMARY KEY auto_increment,
  	name VARCHAR(40) NOT NULL
   );
   INSERT INTO user VALUES (NULL,' Zhang San '),(NULL,' Li Si ');
   CREATE TABLE ouser(
  	id INT PRIMARY KEY auto_increment,
  	number VARCHAR(20) NOT NULL,
  	uid INT, --  Foreign key column 
  	CONSTRAINT ou FOREIGN KEY (uid) REFERENCES user (id)
   );
   INSERT INTO ouser VALUES (NULL,'hm1',1),(NULL,'hm2',1),(NULL,'hm3',2),(NULL,'hm4',2);

• Conduct error test

• 
• 

• --  Add one ouser  But there is no corresponding user, Will fail to add 
   INSERT INTO ouser VALUES (NULL,'hm5',3);
  --  Deleting Lisi user will fail 
  DELETE FROM user WHERE name=' Li Si ';

• Delete foreign key constraint

• 

• Add a foreign key constraint

• 

• --  Delete foreign key constraint 
   ALTER TABLE ouser DROP FOREIGN KEY ou;
  --  Add a foreign key constraint 
   ALTER TABLE ouser ADD CONSTRAINT ou FOREIGN KEY (uid) REFERENCES user(id);