Multi table operation - Auto Association query

• Catalog

  Basic concepts

  Data preparation

  Example operation

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• Basic concepts

• In the same table, the data is related , We can query this table as multiple tables
• This is achieved by taking different aliases for the table

• Data preparation

• 
• CREATE TABLE employee(
          id INT PRIMARY KEY auto_increment, -- Employee number
          NAME VARCHAR(20),  -- Employee name
          mgr INT, -- Superior number
          salary DOUBLE -- Wages
  );
• -- Add data
• INSERT INTO employee VALUES (1001,' The Monkey King ',1005,9000.00),
• (1002,' Pig eight quit ',1005,8000.00),
• (1003,' The sand monk ',1005,8500.00),
• (1004,' Small white dragon ',1005,7900.00),
• (1005,' Tang's monk ',NULL,15000.00),
• (1006,' Wusong ',1009,7600.00),
• (1007,' Li Kui ',1009,7400.00),
• (1008,' Lin Chong ',1009,8100.00),
• (1009,' Song Jiang ',NULL,16000.00);

• Example operation

• Check the names of all employees and their immediate superiors , Employees without superiors also need to inquire
• adopt mgr Can find the superior
• Then employees without superiors also need to query , This requires external connection query
• 

• --  Check the names of all employees and their immediate superiors , Employees without superiors also need to inquire 
  --  Conditions ：employee.mgr=employee.id
  SELECT
  			e1.id,
  			e1.name,
  			e1.mgr,
  			e2.id,
  			e2.name
  FROM
      employee e1
  LEFT OUTER JOIN
      employee e2
  ON
  		e1.mgr=e2.id;