MySQL grouping query

Group query

Group query mainly involves two clauses , Namely ：group by and having

group by

Get the total salary of each job , Position name and total salary are required to be displayed

Group the raw data by position department and salary

Raw data

  After grouping

 having

If you want to filter the grouped data again, you need to use having Clause

The average salary for each position is greater than 2000

Execution order of grouping functions ：

1. Query data according to criteria

2. grouping

3. use having Filter , Get the right data

 select summary

A complete select The statement format is as follows ：

select Field

from Table name

where …….

group by ……..

having …….( It exists to filter the grouped data — It cannot appear alone )

order by ……..

The execution order of the above statements

1. First, execute where Statement to filter raw data
2. perform group by Grouping
3. perform having Operate on grouped data
4. perform select Select data
5. perform order by Sort

principle ： Can be in where Data filtered in , As far as possible in where Medium filtration , More efficient .having The filtering of is dedicated to filtering the data after grouping .