Data query of MySQL (multi table query)

1、FROM Clause

SELECT The query object of is FROM Clause assignment .

Grammar format ：FROM Table name 1 [ [ AS ] Alias 1] [, Table name 2 [ [ AS ] Alias 2 ] ] | JOIN Clause

        FROM Clause can reference a table in two ways , The first way is to use USE Statement to make a database the current database , In this case , If in FROM Clause to specify the table name , Then the table should belong to the current database . The second method is to specify the name of the database to which the table belongs before the table name .

for example , Suppose the current database is db1, Now I want to display the database db2 The watch in tb The content of , Then use the statement ：

SELECT * FROM db2.tb;

        stay SELECT When specifying the column name after the keyword, you can also bring the name of the database and table before the column name , But in general , If the selected field is unique in each table , There is no need to specify .

example ： from Members Retrieve the information of all customers in the table , And use table aliases Users

SELECT * FROM Members AS Users

 2、 Multiple table joins

        To query data in different tables , Must be in FROM Clause specifies multiple tables . Combining data from different columns into a table is called table join . for example , stay Bookstore You need to find the order in the database “ Web programming ” The name of the book's member and the number of orders , You need to Book、Members and Sell this 3 Connect tables , To find the result

 2.1、 Full connection

        Full join refers to crossing each row of each table with each row of other tables to produce all possible combinations , Column contains all the columns that appear in the table , Cartesian product . for example , surface 1 Yes 3 That's ok , surface 2 Yes 2 That's ok , Then they are all connected to the table 3 Yes 3x2=6 That's ok .

2.2、 Internal connection

        The table obtained by full join produces a very large number of rows , The number of rows obtained is the product of the number of rows in each table , And the data in the table generated by full connection is meaningless in most cases . In this situation , Conditions are usually set to reduce the result set to meaningful tables , Such a connection is called inner connection . If the set condition is equivalent , It's also called equivalent connection . 

 2.3、 External connection

        The outer connection includes the left outer connection （LEFT OUTER JOIN） And right outer connection （RIGHT OUTER JOIN） Two kinds of

         The left outer join ： In addition to matching rows in the result table , It also includes rows in the left table that do not match in the right table , For such a line , The selected column from the right table is set to NULL.

         Right connection ： In addition to matching rows in the result table , It also includes rows in the right table that do not match in the left table , For such a line , The selected column from the left table is set to NULL.

  if FROM Separate the tables with commas in the clause , It specifies the full connection , The table obtained by full join produces a very large number of rows

example ： lookup Bookstore The titles of books ordered by customers in the database 、 Number of copies ordered and order time

SELECT Book. Title ,Sell. Number of copies ordered ,Sell. Order time 
FROM Book,Sell
WHERE Book. Book number =Sell. Book number ;

3、 JOIN Connect

Use JOIN Keyword when establishing multi table connection ,JOIN Clause defines how to use JOIN Keyword connection table

Grammar format ：

Table name 1 INNER JOIN Table name 2

| Table name 1 {LEFT | RIGHT} [ OUTER ] JOIN Table name 2

ON Connection condition | USING（ Name ）

  Appoint INNER The connection of keywords is inner connection . The inner connection is in FROM Clause to produce intermediate results ON The result obtained after the condition . Internal connection is the default of the system , It can be omitted INNER keyword . After using the inner connection ,FROM clause ON Conditions are mainly used to connect tables , Other conditions that do not belong to the join table can be used WHERE Clause to specify .

  example ： lookup Bookstore The titles of books ordered by customers in the database 、 Number of copies ordered and order time

SELECT Book. Title ,Sell. Number of copies ordered ,Sell. Order time 
FROM Book INNER JOIN Sell
ON(Book. Book number =Sell. Book number );

  example ： Look up and buy “ Web programming ” And the order quantity is greater than 5 Book information of this

SELECT  Title , Number of copies ordered 
FROM Book JOIN Sell
ON Book. Book number =Sell. Book number 
WHERE  Title =' Web programming ' AND  Number of copies ordered >5;

example ： Look up and buy “ Web programming ” And the order quantity is greater than 5 Ben's book 、 Member name and subscription number  

SELECT Book. Book number , Membership name , Title , Number of copies ordered 
FROM Sell JOIN Book ON Book. Book number =Sell. Book number 
JOIN Members ON Sell. ID number =Members. ID number 
WHERE  Title =' Web programming ' AND  Number of copies ordered >5;

        As an exception , You can connect a table to itself , It's called self connect . To find a row with the same column value in a table , You can use self connect . When using self join, you need to specify two aliases for the table , And references to all columns must be qualified with aliases .

example ： lookup Bookstore Database Sell The orders in the table are different 、 Order number of books with the same book number 、 Book number and number of ordered copies  

SELECT a. The order number ,a. Book number ,a. Number of copies ordered 
FROM Sell AS a JOIN Sell AS b
ON a. Book number =b. Book number  AND a. The order number ！=b. The order number ;

  If the table to be connected has the same column name , And the connection condition is that the column names are equal , be ON The condition can also be changed into USING Clause .USING(column_list) Clause is used to name a series of columns . These columns must exist in both tables . among column_list For the same column names in the two tables .

  Appoint OUTER The connection of keywords is external connection .

example ： Find the book number of all books 、 Quantity and ID number number of the member who ordered the books , If you have never ordered , We should also include the situation .

SELECT Book. Book number ,Book. Number , ID number 
FROM Book LEFT OUTER JOIN Sell
ON Book. Book number =Sell. Book number ;

  If not used LEFT OUTER JOIN, The result will not include the information of books that have not been ordered . After using the left outer connection , The row returned in the result contains information about books that have not been ordered , The ID number field value of the corresponding line is NULL.

4、 Subquery

        In the query condition , You can use the result of another query as part of the condition . for example , Determine whether the column value is equal to the value in the result set of a query , Queries that are part of the query criteria are called subqueries .SQL The standard allows SELECT Use multiple nesting , Used to represent complex queries . Subqueries can be used in addition to SELECT In the sentence , It can also be used in INSERT、UPDATE And DELETE In the sentence . Subqueries are usually associated with IN、EXIST Predicates and comparison operators are used in combination . 

 4.1、IN Subquery

IN Sub query is used to judge whether a given value is in the sub query result set .

Grammar format ： expression [ NOT ] IN ( Subquery )

  example ： Find in Bookstore Zhang San's order information in the database

Ideas ： Containing order information Sell The table does not contain the name of the customer , Only the customer's ID number . To find Zhang San's order information , First you need to know Zhang San's ID number . Therefore, we must first Members Look up Zhang San's ID number in the table , Then query the order information according to the ID number .

SELECT *
FROM Sell
WHERE  ID number  IN
（SELECT  ID number  FROM Members WHERE  Membership name =' Zhang San '）;

4.2、 Compare subqueries

This subquery can be considered as IN Extension of subquery , It compares the value of the expression with the result of the subquery

Grammar format ： expression {< | <= | = | > | >= | != | <>}{ALL | SOME | ANY}（ Subquery ） 

         If the result set of the subquery returns only one row of data , You can compare directly through comparison operators . If the result set of the subquery returns multiple rows of data , Need to use {ALL | SOME | ANY} To limit

        ALL Specifies that the expression is to be compared with each value in the subquery result set , When each value of the expression satisfies the comparison relationship , To return to true, Otherwise return to false

        SOME And ANY Synonyms , It means that as long as the expression meets the comparison relationship with a value in the sub query result set , Just go back to true, Otherwise return to false

  example ： Find the purchased book number “IS-01” Membership information of books

SELECT * FROM Members WHERE  ID number =ANY
（SELECT  ID number  FROM Sell WHERE  Book number ='IS-01'）;

 4.3、EXISTS Subquery

        EXISTS Predicates are used to test whether the result of a subquery is an empty table , If the result set of the subquery is not empty , be EXISTS return true, Otherwise return to false.EXISTS And NOT Use a combination of , namely NOT EXISTS, Its return value is the same as EXISTS Just the opposite .

Grammar format ：[ NOT ]EXISTS ( Subquery )

example ： Find every order 10 Names of members of the above books  

SELECT  Membership name  FROM Members WHERE EXISTS
(SELECT * FROM Sell WHERE  ID number =Members. ID number  and  Number of copies ordered >10);