Question

I have a table T with columns x,y,a,b such that

SELECT x,y,a,b FROM T
ORDER BY x,y,a,b

gives me the following table

 x    | y    | a    | b
 x1   | y1   | a1   | b1
 x1   | y1   | a1   | b2
 x1   | y1   | a2   | b1
 x1   | y2   | a1   | b1
 x1   | y2   | a1   | b2
 x1   | y2   | a2   | b1

How would I get the first row of each x,y group? That is, how would I get the following table

 x    | y    | a    | b
 x1   | y1   | a1   | b1
 x1   | y2   | a1   | b1

---

Here is a second example: For a table T such that

 x    | y    | a    | b
 x1   | y1   | a1   | b3
 x1   | y1   | a1   | b4
 x1   | y1   | a2   | b1
 x1   | y1   | a2   | b2
 x1   | y2   | a1   | b3
 x1   | y2   | a1   | b4
 x1   | y2   | a2   | b1
 x1   | y2   | a2   | b2

I am expecting to get

 x    | y    | a    | b
 x1   | y1   | a1   | b3
 x1   | y2   | a1   | b3

Answer 1

Considering SQL 2005 or Above:

SELECT T1.X,
       T1.Y,
       T1.A,
       T1.B
  FROM
      (SELECT X,
              Y,
              A,
              B,
              ROW_NUMBER() OVER (Partition BY X,Y Order By A,B) AS RowNum
         FROM T
     ) T1
WHERE T1.RowNum = 1