Question

Let me try to explain the scenario. I have two tables A (Columns – A1, A2, A3) & B (Columns – B1, B2, B3). I need to join table A with table B on A1.B2. For every join, table B has one or two records with different values for B3(X or Y). I wanna write one query where the JOIN query needs to pick the row with B3=X(if there’s no other row with B3=Y); If two rows exists (B3=X & B3=Y), then the query needs to pick only the row with B3=Y (ignoring the row with B3=X).

Let me try to give some values to the tables & explain a little bit more.

Table A
********
A1   A2   A3
1    11   111
2    22   222
3    33   333
4    44   444

Table B
********
B1   B2   B3
6    1    X
7    1    Y
8    2    X
9    3    X
10   3    Y
11   4    X

Again.. JOIN is on A1.B2. The result should be as following,

JOIN Results
*************
A1   A2   A3   B1   B2   B3
1    11   111  7    1    Y
2    22   222  8    2    X
3    33   333  10   3    Y
4    44   444  11   4    X

Let me know if you guys have any clarification about my question.

Thanks in advance.
Yogi

Answer 1

You can pick the rows from table B with the ROW_NUMBER function if you partition by the join column and order by your “picking order” column:

SELECT b1, b2, b3,
       ROW_NUMBER() OVER (PARTITION BY b2 ORDER BY b3 DESC) as rn
  FROM b;

1 Y 1
1 X 2
2 X 1
3 Y 1
3 X 2
4 X 1

Then you can filter the first row, the one with rn=1:

SELECT b1, b2, b3
  FROM (SELECT b1, b2, b3,
               ROW_NUMBER() OVER (PARTITION BY b2 ORDER BY b3 DESC) as rn
          FROM b)
 WHERE rn=1;

7  1 Y
8  2 X
10 3 Y
11 4 X

The filtered rows can then be joined to table a:

SELECT *
  FROM a 
  JOIN (
        SELECT b1, b2, b3
          FROM (SELECT b1, b2, b3,
                       ROW_NUMBER() OVER (PARTITION BY b2 ORDER BY b3 DESC) as rn
                  FROM b
                )
          WHERE rn=1
       ) bfilter  ON a.a1 = bfilter.b2;

1 11 111  7 1 Y
2 22 222  8 2 X
3 33 333 10 3 Y
4 44 444 11 4 X   

If ‘X’ and ‘Y’ are not actual values, you can extend the ORDER clause with a CASE statement to allow for general values:

 ROW_NUMBER() OVER (PARTITION BY b2 ORDER BY 
   CASE b3 WHEN 'Y' THEN 1
           WHEN 'X' THEN 2
           ...
    END ASC)

Edit:

 SELECT a1, a2, a3, b1, b2, b3
   FROM (
         SELECT a1, a2, a3, b1, b2, b3,
                ROWNUMBER() OVER (PARTITION BY a1 ORDER BY 
                   CASE WHEN a2=... AND b3=... THEN 1
                        WHEN a2=... AND b3=... THEN 2
                        ...
                   END ASC)
          FROM a JOIN b ON a.a1 = b.b2
        ) 
  WHERE rn = 1;