options for returning distinct values across an inner join

Question

I have two tables.

Table A contains UserID, UserName

Table B contains ID, FK_UserID, ClientName

I need to return a list of distinct A.UserName where A.Username exists in table B and has at least one ClientName attached to them, but in my query, only look at distinct B.ClientName.

My thoughts were:

Select Distinct A.UserName from A as A
Inner Join B as B
on A.UserID = B.FK_UserID

But that only distincts on table A

My next thought was:

Select Distinct Username from A
where UserID In 
(
Select FK_UserID, distinct ClientName from B)

I was told that there is a way to do a distinct on both tables in the join, because if table A is 300 rows, and table B is 3 BILLION rows, my original query is going to take awhile.

The person specifically wants me to use an inner join...

Answer 1

Your original query is:

Select Distinct A.UserName
from A as A Inner Join
     B as B
     on A.UserID = B.FK_UserID;

This can be a problem, if there are many matches in b. Using in isn't quite the right solution. Instead, use exists:

select a.UserName
from a
where exists (select 1
              from b
              where b.fk_UserID = a.UserId
             )

Then, be sure that you have an index on b(fk_UserId).

This should do about 300 lookups in the index. That should be quite fast.

My advice for the person who told you to use the inner join: Write a special version for that person. For the many minutes or hours that it takes to run, let other people use the faster version using exists.