Multi table operation - external connection query

Catalog

The left outer join

Right connection

Example operation

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• External connection query is divided into left external connection query and right external connection query
• Take a table as Lord , Take out all the records in it , Then each one is joined to another table , Whether or not you can match the conditions , In the end, they will keep
• Can match , Keep it right ; Can't match , The fields of other tables are empty （null）, Called external connection

• The left outer join

• Query principle
• Query all the data in the left table , And the left and right tables have the intersection part of the data
• The query syntax
• select Name from Table name 1 left [outer] join Table name 2 on Conditions

• Right connection

• Query principle
• Query all the data in the right table , And the left and right tables have the intersection part of the data
• The query syntax
• select Name from Table name 1 right [outer] join Table name 2 on Conditions

• Example operation

• Please click to view the data preparation of the table
• The left outer join
• Query all user information , And the corresponding order information of the user
• 

• --  The left outer join 
  --  Query all user information , And the corresponding order information of the user 
  SELECT
  		u.*,
  		o.number
  FROM
      user u
  LEFT OUTER JOIN
      orderlist o
  ON
  		o.uid=u.id;

• Right connection
• Query all order information , And the user information of the order
• 

• --  Right connection 
  --  Query all order information , And the user information of the order 
  SELECT
  		o.*,
  		u.name
  FROM
      user u
  RIGHT OUTER JOIN
      orderlist o
  ON
  		o.uid=u.id;