Eliminate partial duplicate rows from result set

Question

Hi,

I have a query that returns a result set similar to the one below (in reality it is far bigger, thousands of rows):

     A    | B  | C  |    D
     -----|----|----|-----
1    NULL | d0 | d0 | NULL
2    NULL | d0 | d1 | NULL
3    NULL | d0 | d2 |   a0
4      d0 | d1 | d1 | NULL
5      d0 | d2 | d2 |   a0

Two of the rows are considered duplicates, 1 and 2, because A, B and D are the same. To eliminate this, I could use SELECT DISTINCT A, B, D but then I do not get column C in my result set. Column C is necessary information for rows 3, 4 and 5.

So how do I come from the result set above to this one (the result appearing in C4 can also be NULL instead of d1):

     A    | B  | C    | D
     -----|----|------|-----
1    NULL | d0 | NULL | NULL
3    NULL | d0 | d2   |   a0
4      d0 | d1 | d1   | NULL
5      d0 | d2 | d2   |   a0

Kind regards,

Ronald

Answer 1

DECLARE @YourTable TABLE (
  A VARCHAR(2)
  , B VARCHAR(2)
  , C VARCHAR(2)
  , D VARCHAR(2))

INSERT INTO @YourTable VALUES (NULL, 'd0', 'd0', NULL)
INSERT INTO @YourTable VALUES (NULL, 'd0', 'd1', NULL)
INSERT INTO @YourTable VALUES (NULL, 'd0', 'd2', 'a0')
INSERT INTO @YourTable VALUES ('d0', 'd1', 'd1', NULL)
INSERT INTO @YourTable VALUES ('d0', 'd2', 'd2', 'a0')


SELECT A, B, C = MIN(C), D
FROM @YourTable
GROUP BY A, B, D

---

SELECT A, B, CASE WHEN MIN(C) = MAX(C) THEN MIN(C) ELSE NULL END, D
FROM @YourTable
GROUP BY A, B, D

---

SELECT A, B, CASE WHEN MIN(COALESCE(C, 'dx')) = MAX(COALESCE(C, 'dx')) THEN MIN(C) ELSE NULL END, D
FROM @YourTable
GROUP BY A, B, D

Answer 2

A subquery perhaps?

SELECT A,B,C,D FROM table1 WHERE EXISTS ( SELECT DISTINCT A,B,D FROM table1 );

Answer 3

if you have an unique id in the table, then i would go for something like this:

SELECT A,B,C,D FROM table WHERE id IN (SELECT DISTINCT A,B,D)

The problem is that you would always get the first value of C, not the frist one with an value.

Answer 4

The fact you have NULLs in A and D compicates matters for any EXISTS.

Any MIN/MAX solution on C may not give you NULL as I think you want. Otherwise, use MIN(C) and a simple group by.

You have to extract the unique keys first (A, B, D), then use that to determine extract the rows again and work out what to do with C

DECLARE @TheTable TABLE (
  A varchar(2) NULL,
  B varchar(2) NULL,
  C varchar(2) NULL,
  D varchar(2) NULL
)

INSERT INTO @TheTable VALUES (NULL, 'd0', 'd0', NULL)
INSERT INTO @TheTable VALUES (NULL, 'd0', 'd1', NULL)
INSERT INTO @TheTable VALUES (NULL, 'd0', 'd2', 'a0')
INSERT INTO @TheTable VALUES ('d0', 'd1', 'd1', NULL)
INSERT INTO @TheTable VALUES ('d0', 'd2', 'd2', 'a0')

SELECT DISTINCT
    T.A,
    T.B,
    CASE Number WHEN 1 THEN T.C ELSE NULL END,
    T.D
FROM
    (SELECT
        COUNT(*) AS Number,
        A, B, D
    FROM
        @TheTable
    GROUP BY
        A, B, D
    ) UQ
    JOIN
    @TheTable T ON ISNULL(T.A, '') = ISNULL(UQ.A, '') AND ISNULL(T.B, '') = ISNULL(UQ.B, '') AND ISNULL(T.D, '') = ISNULL(UQ.D, '')