Interview shock 62: what are the precautions for group by?

Be careful ： The following content of this article is based on  MySQL 5.7 InnoDB  Database engine .

1.group by  Can't add after  where

stay  MySQL  in , be-all  SQL  The query syntax should follow the following syntax order ：

• select

• from

• where

• group by

• having

• order by

• limit

The above grammatical order is not interchangeable , Otherwise, the report will be wrong . For example, we can't  group by  Then add  where  Query statement , Otherwise, the following error will occur ：

The execution of the syntax sequence is and  MySQL  Of  select  Statement execution order related ,select  The order of execution is as follows ：

• from  Stage

• where  Stage

• group  Stage

• having  Stage

• select  Stage

• order by  Stage

• limit  Stage

Be careful ： among  select  A special , When writing query statements , Write it first , Other grammatical order shall be consistent with the execution order .

2.group by  or  having  Can be used alone

having  and  group by  Can be used alone , The following query is shown ：

3.group by  and  having  You can use aliases

** When  having  When used alone , Its role and  where  similar , But there are subtle differences .** For example  where  Alias cannot be used in , but  having  and  group by  But it can be another name . Let's create a test table to demonstrate , Build table  SQL  as follows ：

drop table if exists student_score;
create table student_score(
 id int primary key auto_increment comment ' Primary key ',
 name varchar(250) comment ' full name ',
 math decimal(4,1) comment ' Math scores ',
 chinese decimal(4,1) comment ' Chinese achievement '
);
insert into student_score(name,math,chinese) values(' Zhang San ',50,50),(' Li Si ',80,80),(' Wang Wu ',90,90);


The data in the table is shown in the figure below ：

When we use the total score alias  total  Respectively in  where  and  having  When used in , The query results are as follows ：

As can be seen from the above results ,having  Queries can use  select  Alias in , and  where  Cannot use alias . except  having  You can use something other than an alias ,group by  You can also use aliases , As shown in the figure below ：

Why aliases are available ？

where  Alias cannot be used in , This sum  MySQL  Statement execution order ,MySQL  The execution order of the statements is as follows ：

• from  Stage

• where  Stage

• group  Stage

• having  Stage

• select  Stage

• order by  Stage

• limit  Stage

in other words ,** In execution  where  When inquiring ,select  Not yet implemented , So in  where  You want to use a that has not yet been executed  select  Aliases in are not acceptable .** From the above execution sequence, you can see ,having  Execution is also  select  Before , Why can it be used  select  The alias in ？

This is because  MySQL  stay  5.7.5  After that, it was extended , Allow in  having  Use aliases , There are corresponding instructions in the official documents , As shown in the figure below ：

MySQL  Official document address ：

https://dev.mysql.com/doc/refman/5.7/en/group-by-handling.html

PS：group by  Be able to use the principle and  having  similar .

summary

SQL  Statements must be written in this order ：select、from、where、group by、having、order by、limit. among  having  or  group by  Can be used alone , And in  MySQL 5.7.5  after ,group by  and  having  You can use alias queries , but  where  Cannot use alias .

It's up to you to judge right and wrong , Disdain is to listen to people , Gain or loss is more important than number .

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