Question

In my table I need to know if each ID has one and only one ID_name. How can I write such query?

I tried:

select ID, count(distinct ID_name) as count_name 
from table 
group by ID 
having count_name > 1

But it takes forever to run.

Any thoughts?

Answer 1

select  ID
from    YourTable
group by
        ID
having  count(distinct ID_name) > 1

or

select  *
from    YourTable yt1
where   exists
        (
        select  *
        from    YourTable yt2
        where   yt1.ID = yt2.ID
                and yt1.ID_Name <> yt2.ID_Name
        )

Now, most ID columns are defined as primary key and are unique. So in a regular database you’d expect both queries to return an empty set.