Stuck on this union / except - sql

Question

Trying to find the best way to proceed with this, for some reason it is really tripping me up.
I have data like this:
transaction_id(pk) decision_id(pk) accepted_ind
A 1 NULL
A 2 <blank>
A 4 Y
B 1 <blank>
B 2 Y
C 1 Y
D 1 N
D 2 O
D 3 Y
Each transaction is guaranteed to have decision 1
There can be multiple decision possibilities (what-if's) type of scenarios
Accepted can have multiple values or be blank or NULL but only one can be accepted_ind = Y
I am trying to write a query to:
Return one row for each transaction_id
Return the decision_id where the accepted_ind = Y or if the transaction has no rows accepted_ind = Y, then return the row with decision_id = 1 (regardless of value in the accepted_ind)
I have tried:
1. Using logical "or" to pull the records, kept getting duplicates.
2. Using a union and except but can not quite get the logic down correctly.
Any assistance is appreciated. I am not sure why this is tripping me up so much!
Adam

Answer 1

Try this. Basically the WHERE clause says:
Where Accepted = 'Y'
OR
There is no accepted row for this transaction and the decision_id = 1
SELECT Transaction_id, Decision_ID, Accepted_id
FROM MyTable t
WHERE Accepted_ind = 'Y'
OR (NOT EXISTS (SELECT 1 FROM MyTable t2
WHERE Accepted_ind = 'Y'
and t2.Transaction_id = t.transaction_id)
AND Decision_id = 1)

Answer 2

This approach uses ROW_NUMBER() and therefore will only work on SQL Server 2005 or later
I have modified your sample data as as it stands, all transaction_id have a Y indicator!
DECLARE #t TABLE (
transaction_id NCHAR(1),
decision_id INT,
accepted_ind NCHAR(1) NULL
)
INSERT #t VALUES
( 'A' , 1 , NULL ),
( 'A' , 2 , '' ),
( 'A' , 4 , 'Y' ),
( 'B' , 1 , '' ),
( 'B' , 2 , 'N' ), -- change from your sample data
( 'C' , 1 , 'Y' ),
( 'D' , 1 , 'N' ),
( 'D' , 2 , 'O' ),
( 'D' , 3 , 'Y' )
And here is the query itself:
SELECT transaction_id, decision_id, accepted_ind FROM (
SELECT transaction_id, decision_id, accepted_ind,
ROW_NUMBER() OVER (
PARTITION BY transaction_id
ORDER BY
CASE
WHEN accepted_ind = 'Y' THEN 1
WHEN decision_id = 1 THEN 2
ELSE 3
END
) rn
FROM #t
) Raw
WHERE rn = 1
Results:
transaction_id decision_id accepted_ind
-------------- ----------- ------------
A 4 Y
B 1
C 1 Y
D 3 Y
The ROW_NUMBER() clause gives a 'priority' to each criterion you mention; we then ORDER BY to pick the best, and take the first row.

Answer 3

There's probably a neater/more efficient query, but I think this will get the job done. It assumes the table name is Decision:
SELECT CASE
WHEN accepteddecision.transaction_id IS NOT NULL THEN
accepteddecision.transaction_id
ELSE firstdecision.transaction_id
END AS transaction_id,
CASE
WHEN accepteddecision.decision_id IS NOT NULL THEN
accepteddecision.decision_id
ELSE firstdecision.decision_id
END AS decision_id,
CASE
WHEN accepteddecision.accepted_ind IS NOT NULL THEN
accepteddecision.accepted_ind
ELSE firstdecision.accepted_ind
END AS accepted_ind
FROM decision
LEFT OUTER JOIN (SELECT *
FROM decision AS accepteddecision
WHERE accepteddecision.accepted_ind = 'Y') AS
accepteddecision
ON accepteddecision.transaction_id = decision.transaction_id
LEFT OUTER JOIN (SELECT *
FROM decision AS firstdecision
WHERE firstdecision.decision_id = 1) AS firstdecision
ON firstdecision.transaction_id = decision.transaction_id
GROUP BY accepteddecision.transaction_id,
firstdecision.transaction_id,
accepteddecision.decision_id,
firstdecision.decision_id,
accepteddecision.accepted_ind,
firstdecision.accepted_ind

Answer 4

Out of interest, the following uses UNION and EXCEPT (plus a JOIN) as specified in the question title:
WITH T AS (SELECT * FROM (
VALUES ('A', 1, NULL),
('A', 2, ''),
('A', 4, 'Y'),
('B', 1, ''),
('B', 2, 'Y'),
('C', 1, 'Y'),
('D', 1, 'N'),
('D', 2, 'O'),
('D', 3, 'Y'),
('E', 2, 'O'), -- smaple data extended
('E', 1, 'N') -- smaple data extended
) AS T (transaction_id, decision_id, accepted_ind)
)
SELECT *
FROM T
WHERE accepted_ind = 'Y'
UNION
SELECT T.*
FROM (
SELECT transaction_id
FROM T
WHERE decision_id = 1
EXCEPT
SELECT transaction_id
FROM T
WHERE accepted_ind = 'Y'
) D
JOIN T
ON T.transaction_id = D.transaction_id
AND T.decision_id = 1;