MySQL Learning (a) use of-group by

Business Scenario One: Query the main table and bring out the number of child tables associated with the main table

Implementation method: Grouping group by child table foreign key

SQL is as follows:

1 Select 2 main.id ID,3 main.name Name,4 ifnull (Sub.num, ' 0 ') Num5  from 6 Xx_main Main7  Left Join(8   　　 Select Count(1) num,main_id fromXx_subGroup  bymain_id9 ) sub on main.id = sub.main_idTen whereMain.id=#{id}

The results of the operation are as follows:

Id	Name	Num
89	King	3
90	Meteor	1

Analysis: GROUP BY statements group result sets based on one or more columns, and we can use functions such as count,sum,avg on grouped columns

Concluding remarks: Personal sentiment, lack of place also hope you feel free, thank you!

MySQL Learning (a) use of-group by