[MySQL] multi table connection query

In a database with a huge amount of data , In order to play the role of the database , We have developed a variety of database operating languages to query data , Multi table join query is a common module .

Multi table join query , seeing the name of a thing one thinks of its function , It is to summarize and query the information data of multiple tables , To get the data we want .

Next, let's learn the common query methods .

Cartesian collection ：

Cartesian sets simply associate tables when querying data , The data to m*n A combination of forms , Similar to matrix multiplication , It causes a lot of repeated and illogical data , Lost the original intention of filtering data .

The reason for Descartes set ：

* Generating conditions ：
        1. Omit connection condition
        2. Connection condition invalid
        3. All rows in all tables are connected to each other
    * Solution ：
         Add valid filter criteria

Internal connection ：

    * SQL92 grammar ：
      

  SELECT  Query list 
        FROM  Table name 1  Alias 1 , Table name 2  Alias 2
        WHERE  Connection condition                  
        AND  filter                 
        GROUP BY  Group list             
        HAVING  Filter criteria after grouping           
        ORDER BY  Sort list     

    * SQL99 grammar ：
      

  select  Field list 
        from  Table name 1
        [inner] join  Table name 2 on  Conditions 
        where  filter 
        group by  Grouping conditions 
        having  Screening criteria after grouping 
        order by  Sort field 

* Be careful ：
    1. Use the table name prefix to distinguish the same columns in multiple tables
    2. Columns with the same column name in different tables can be distinguished by the alias of the table
    3. If a table alias is used , It's in select Statement needs to use table alias instead of table name
    4. Table aliases support up to 32 Character length , But the less advice the better

External connection （ Commonly used ）：

  The left outer join

    *  grammar ：
        select  Field list 
        from  surface 1
        left [outer] join  surface 2 on  Conditions 
        ...

    * Be careful ：
         The left outer join queries all the data in the left table and its intersection , surface 1 Query main table for
-------------------------------------------------------------------------
5. Right connection
    * grammar ：
    

    select  Field list 
        from  surface 1
        right [outer] join  surface 2 on  Conditions 
        ...

    * Be careful ：
         The right outer join queries all the data in the right table and its intersection , surface 2 Query main table for

Subquery ： Nested query in query , A nested query is called a subquery

Simply put, it is to split the complex query requirements , The split query results or contents are in brackets .

  characteristic ：
    1、 Subqueries are enclosed in parentheses
    2、 Subqueries can be placed in from Back 、select Back 、where Back 、having Back , But it's usually on the right side of the condition
    3、 Subquery takes precedence over main query execution , The main query uses the execution result of the subquery
    4、 Sub queries are divided into the following two categories according to the number of rows in the query results ：
        ① Single line sub query
            The result set has only one line
            Generally used with single line operators ：> < = <> >= <=
            Illegal use of subqueries ：
                a、 The result of the subquery is a set of values
                b、 The result of the subquery is empty
           ② Multi line sub query
            How many lines does the result set have
            Usually used with multiline operators ：any、all、in、not in
            in： Belong to any one of the sub query results
            any and all You can often use other queries instead of
   

Paging query ： seeing the name of a thing one thinks of its function , It is to adjust the number of pages in the display of query results .

* grammar ：
 

   select  Field | expression ,...
    from  surface 
    where  Conditions 
    group by  Grouping field 
    having  Conditions 
    order by  Sorted fields 
    limit  Starting entry index , Number of entries ;
*  Example ： Each page shows 3 Bar record 
        * SELECT * FROM student LIMIT 0,3; --  The first 1 page 
        
        * SELECT * FROM student LIMIT 3,3; --  The first 2 page 
        
        * SELECT * FROM student LIMIT 6,3; --  The first 3 page 

* characteristic ：
    1. Starting entry index from 0 Start

    2.limit Clause at the end of the query statement

    3. The formula ：select * from  surface limit （page-1）*sizePerPage,sizePerPage
     If :
         Number of entries per page sizePerPage
         Number of pages to display page

The joint query ：

* grammar ：

    select  Field | Constant | expression | function  from  surface  where  Conditions  union 【all】
    select  Field | Constant | expression | function  from  surface  where  Conditions  union 【all】
    select  Field | Constant | expression | function  from  surface  where  Conditions  union 【all】
    .....
    select  Field | Constant | expression | function  from  surface  where  Conditions 

* characteristic ：
    1、 The number of columns in multiple query statements must be consistent
    2、 Multiple query statements query almost the same type of columns
    3、union It means to go heavy ,union all It means not to weigh

A little thought ： How not to use max Function query data , Take the maximum wage as an example ：

Method 1 ： Use sorting and paging queries ：

SELECT employee_id,salary
FROM employees
ORDER BY salary DESC
LIMIT 1;

Method 2 ： Subquery

SELECT
	employee_id,salary 
FROM
	employees e 
WHERE
	e.salary NOT IN (
	SELECT DISTINCT
		a.salary 
	FROM
		employees a
	JOIN employees b ON a.salary < b.salary 
	)