Introduction and advanced level of MySQL (11)

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Query result set as ⼀ Virtual tables directly participate in the query

Eg： Query the learning of each class ⽣ Number , And sort in reverse order according to the number
Eg2: List the men in each class ⽣⼥⽣⽐ example
/* Subquery ：
Query nesting

1. Query results are used directly as virtual tables
2. The query result is used as the value of the judgment condition
3. The query result is used as the value of the assignment update set a= value ？ You can also use subquery
   Subquery ：
   The location of the queried table
   Query condition location
   delete Search condition location of
   update The location of the search condition
   update set The position of assignment can also
   insert The position of writing value can also be used */

--  The query result is used as a table , Alias is required 
17 --  Query the number of students in each class , Sort by flashback 
18 select a.classname,count(b.studentid)
19 from class a
20 left join studentinfo b on a.classid=b.classid
21 group by a.classid,a.classname
22 order by count(b.studentid) desc
234 select * from(
25 select a.classname,count(b.studentid) as stucount
26 from class a
27 left join studentinfo b on a.classid=b.classid
28 group by a.classid,a.classname
29 )a
30 order by a.stucount desc
312 --  List the proportion of boys and girls in each class 
33 select nvcount/nancount,a.classid from(
34 select count(*) as nvcount,gender,classid
35 from studentinfo
36 where gender=' Woman '
37 group by classid
38 )a
39 left join(
40 select count(*) as nancount,gender,classid
41 from studentinfo
42 where gender=' male '
43 group by classid
44 )b on a.classid=b.classid