A odd case about mysql sub query

Hi guys, I have a table like this: +----+-------+-------+ | id | col_a | col_b | +----+-------+-------+ | 1 | a | b | | 2 | a | b | | 3 | a | c | | 4 | a | c | +----+-------+-------+ execute sql a: > select * from test where id not in (select id from (select id from test > group by col_a,col_b) as a); can get result +----+-------+-------+ | id | col_a | col_b | +----+-------+-------+ | 2 | a | b | | 4 | a | c | +----+-------+-------+ while execute sql b: > select * from test where id not in (select id from (select * from test) as > a group by col_a,col_a); get empty result. I'm very confused with this, somebody has idea about why? I tried in 10.1.13-MariaDB and mysql5.6. -- Best Regards, Haiwen