MySQL left join multi-Table query where condition writing example, mysql left join

MySQL left join multi-Table query where condition writing example, mysql left join

Copy codeThe Code is as follows:
Select * from _ test a left join _ test B on. id = B. id where. level = '20' and. month = '04 'and B. level = '20' and B. month = '03 ';

Select. *, B. * from (select * from _ test where level = '20' and month = '04 ') as a left join (select * from _ test where level = '20' and month = '03') as B on. id = B. id;

The results of these two statements are different. It should be caused by the condition that mysql is automatically converted to inline During left join, the second statement can achieve the correct purpose (in order to find the difference between the two data ).