MySQL计数查询与组不返回任何内容

Question

I am trying to understand why this query in MySQL does not return any result at all whereas I think it should be returning one row with 0 as the column value

select count(ps.id) as totalcount 
from tblpresi ps
inner join users_staging.tbluser u on u.id = ps.userid
where ps.id = 3678
group by ps.id

The same query without the "Group By" works as expected.

Any ideas?

Answer 1

No, it won't return that, since your query does not returns results at all (probably there is no ps.id = 3678, or your inner join rules out every row in the result set).

If you change your query to:

select count(1) as totalcount 
  from tblpresi ps
 inner join users_staging.tbluser u on u.id = ps.userid
 where ps.id = 3678
 group by ps.id

You will get one row with 0 .

Do keep in mind that if your ps.id is unique in the result set (I don't know if that inner join you have will return multiple rows with the same ps.id ) your group by clause is not necessary.

Answer 2

Since you say you expect 1 row with zero count, I assume there is no ps.id of 3678.

If you group-by on an empty result-set, there can be no groups. The group by doesn't "know" about the where-clause, it's only looking through the result-set.