Mysql leftist multi-table query where condition example _mysql

Copy Code code as follows:

SELECT * FROM _test a LEFT join _test B on a.id=b.id where a.level= ' and a.month= ' and b.level= ' 03 ' ;

Select a.*,b.* from (SELECT * to _test where level= ' and month= ') as a LEFT join (SELECT * from _test where level= ' ' and month= ') as B on a.id=b.id;

The results of these two statements are different and should be left join when the condition is that multiple tables MySQL automatically turns inline, the second statement achieves the correct purpose (in order to find the difference of two data).