ORACLE Special JOIN - sql

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;

Answer 2

You can use left outer joins as follows
select A.A1, A.A2, A.A3,
nvl(BT1.B1, BT2.B1),
nvl(BT1.B2, BT2.B2),
nvl(BT1.B3, BT2.B3) from A
left outer join B BT1 on A.A1 = BT1.B2 and BT1.B3 = 'Y'
left outer join B BT2 on A.A1 = BT2.B2 and BT2.B3 = 'X'
A good explanation of the various joins is at http://www.codinghorror.com/blog/2007/10/a-visual-explanation-of-sql-joins.html

Answer 3

Here is, how I would do it:
Make the join
group by B2
take the max(B3)
That way you ensure that X is only picked, when there is no alphabetically higher value (Y) available

Answer 4

With UNION
select a.*,b.* from a,b
where a.a1=b.b2
and b.b3='Y'
union
select a.*,b.* from a,b
where a.a1=b.b2
and not exists (select bb.br from b bb where bb.b2=a.a1 and bb.b3='Y')
Without UNION
select a.*,b.* from a,b
where a.a1=b.b2
and (b.b3='Y'
or not exists (select bb.b3 from b bb where bb.b2=a.a1 and bb.b3='Y'))
The constraint here is that B has exactly 1 or 2 rows for each A's row