How to select only one row if a value of a column occurse multiple times in the result?

I have a table like this on:

AttributeA	AttributeB	AttributeC	AttributeD
A	B	EQUALITY	D
123	B	EQUALITY	D
456	B	C	D
…	…	…	…

My goal is, to create a Select-Query where the result only contains one row with an equal AttributeC – no matter of the count of rows with the same value in this column.

The other Attributes do not matter. So I want this:

AttributeA	AttributeB	AttributeC	AttributeD
A or 123	B	EQUALITY	D
ABC	DEF	GHI	JKL
…	…	…	…

I have already tried some WHERE-Clauses, but I found no way to specify that I want to have the row atleast once in my result.

Something like this was my plan with an unique ID.

SELECT * FROM Table1
WHERE Table1.ID = (SELECT TOP(1) FROM Table1 as tempTable where Table1.ID = tempTable.ID)

I think I know, why this doesn’t work, but I could not found a way to make it work. (and I’m not sure, that it is possible with a WHERE-Clause).

Thank you for your help.

Answer

You can use row_number():

select t1.*
from (select t1.*,
             row_number() over (partition by attributeC order by attributeC) as seqnum
      from table1 t1
     ) t1
where seqnum = 1;

A somewhat more concise, but less performant method is:

select top (1) with ties t1.*
from table1 t1
order by row_number() over (partition by attributeC order by attributeC) ;