What I need is to group by "fld4" if and only if its value is "TL". If its value is different than "TL" then no grouping should be performed on this field.
I will very much appreciate ypur feedback.
I'm not totally clear on your requirement but would the HAVING keyword solve the problem? That's kind of like a WHERE but in the group by phase of the query..
SELECT fld1, fld2, fld3, fld4 FROM tblTable1
INNER JOIN ............
GROUP BY fld1, fld2, fld3, fld4
HAVING fld4='TL'
SELECT fld1, fld2, fld3, CASE WHEN fld4 = 'TL' THEN fld4 ELSE pkcol END AS fld4_or_pk
FROM/JOIN
GROUP BY 1, 2, 3, 4
You need to make sure that the value in the fourth column is unique if the value of fld4 is anything other than 'TL'. The easiest way is to simply use a relevant primary key as the substitute value.
You perform this case against the actual data and then reference the columns by position in the group by.
Sample data and expected output will likely be needed should you require further assistance; your problem specification is lacking - specifically the expected values for fld4 and what you expect to output in the fourth column of the final query. Also, your example does not have any actual aggregate functions? Are you using this simply to remove duplicate rows from the output? If so you should consider if DISTINCT ON () or a different query altogether will perform better.