SQL - select 5 columns where 3 are distinct using multiple JOINs - sql

Question

I searched Stack Overflow before asking and looked through the 5 most relevant questions, but they did not seem to answer this.
I need to select 5 columns from a table using multiple INNER JOINs, but I only want to get the records where 3 of the 5 columns are distinct. If I use:
"select DISTINCT pos_segments.pos, ",
" pos_segments.org, ",
" pos_segments.obj, ",
" pos_segments.proja, ",
" pos_segments.eff_dt ",
" from pos_segments ",
"INNER JOIN PersonnelPositions ",
"ON pos_segments.pos = PersonnelPositions.Position ",
"AND pos_segments.eff_dt = PersonnelPositions.EffectiveDate ",
"INNER JOIN Accounts ",
"ON PersonnelPositions.PayrollGLAccountId = Accounts.Id ",
" WHERE <where clause here>
I get back over 22k records. I need to get the pos_segments columns only where the combination of the following three columns are distinct:
pos_segments.pos, pos_segments.proja, pos_segments.eff_dt
These three columns taken together serve as a unique key for this table. How can I only get back the records which are distinct based on the combination of these three columns?
P.S. - we are using MS SQL Server
Thanks!

Answer 1

Edit: As Sean pointed out, the ORDER BY statement is not optional. I never tried without it so I wasn't sure. And it would appear that the code block below is more of a suggestion than a copy paste so take it with a grain of salt. However, this approach should still work with some tweaking to fit your application. Since you mentioned that it may be arbitrary which distinct row is grabbed, you can put either of the other two columns after the ORDER BY statement.
I very recently needed to solve a very similar issue. I was able to accomplish what I needed by using a partition in conjunction with a where statement. You code would be modified to look like this:
"select * from ("
"select ROW_NUMBER() OVER (PARTITION BY pos_segments.pos,
pos_segments.proja, pos_segments.eff_dt ORDER BY pos_segments.org)
AS row_num_throwaway"
" pos_segments.pos, ",
" pos_segments.org, ",
" pos_segments.obj, ",
" pos_segments.proja, ",
" pos_segments.eff_dt ",
" from pos_segments ",
"INNER JOIN PersonnelPositions ",
"ON pos_segments.pos = PersonnelPositions.Position ",
"AND pos_segments.eff_dt = PersonnelPositions.EffectiveDate ",
"INNER JOIN Accounts ",
"ON PersonnelPositions.PayrollGLAccountId = Accounts.Id ",
" WHERE <where clause here>)"
"WHERE row_num_throwaway = 1"
That first row_number partition line just finds distinct versions of the three columns specified and assigns a row number that counts up as more instances of that distinct group are found. By only looking where row_num_throwaway = 1, you are only getting the first time that combo is present. To ensure you retrieve the correct entry, you can always add an ORDER BY statement where I showed with the stars.
Hope this helps!