Multi table operation - sub query

• Catalog

  Basic concepts

  Three common situations

  The result is a single row, single column

  The result is multi row, single column

  The result is multi row, multi column

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• Basic concepts

• The query statement is nested in the query statement , We will call the nested query sub query

• Three common situations

• Please click to view the data preparation of the table

• The result is a single row, single column

• effect ： The result of the query can be used as the query condition of another statement , Use operators to judge
• The query syntax ：
• select Name from Table name where Name =(select Name from Table name [where Conditions ]);
• example ： Query the oldest user name
• 

• --  Query the oldest user name 
  SELECT name,age FROM user WHERE age=(SELECT MAX(age) FROM user);

• The result is multi row, single column

• effect ： As a condition , Use the operator in or not in Judge
• The query syntax ：
• select Name from Table name where Name [not] in (select Name from Table name [where Conditions ])
• example ： Check the order information of Zhang San and Li Si
• 

• --  Check the order information of Zhang San and Li Si 
  SELECT * FROM orderlist WHERE uid IN (SELECT id FROM user WHERE name IN (' Zhang San ',' Li Si '));

• The result is multi row, multi column

• effect ： The query result can be used as a virtual table to participate in the query
• The query syntax ：
• select Name from Table name [ Alias ],(select Name from Table name [where Conditions ]) [ Alias ] [where Conditions ];
• example ： Query the order table id Greater than 4 Order information and user information of
• 

• --  Query the order table id Greater than 4 Order information and user information of 
  SELECT
  		u.name,
  		o.number
  FROM
      user u,
  		(SELECT * FROM orderlist WHERE id>4) o
  WHERE
      o.uid=u.id;