Question

I have a table with fields TransactionID, Amount and ParentTransactionID
The transactions can be cancelled so a new entry posted with amount and ParentTransactionID as cancelled TransactionID.

Lets say a transaction

1 100 NULL

I cancelled the above entry, it will like

2 -100 1

Again cancelled the above transaction, so it should like

3 100 2

When I fetch I should get the record 3 as ID 1 and 2 got cancelled.
result should be

3 100 2

If I cancelled the 3rd entry no records should return.

SELECT * FROM Transaction t
WHERE NOT EXISTS (SELECT TOP 1 NULL FROM Transaction pt
WHERE (pt.ParentTransactionID = t.TransactionID OR t.ParentTransactionID = pt.TransactionID)
AND ABS(t.Amount) = ABS(pt.Amount))

This works if only one level of cancel is made.

Answer 1

If all transactions are cancelled by a new transaction setting ParentTransactionId to the transaction it cancels, it can be done using a simple LEFT JOIN;

SELECT t1.* FROM Transactions t1
LEFT JOIN Transactions t2
  ON t1.TransactionId = t2.ParentTransactionId
WHERE t2.TransactionId IS NULL;

t1 being the transaction we’re currently looking at and t2 being the possibly cancelling transaction. If there is no cancelling transaction (ie the TransactionId for t2 does not exist), return the row.

I’m not sure about your last statement though, If I cancelled the 3rd entry no records should return.. How would you cancel #3 without adding a new transaction to the table? You may have some other condition for a cancel you’re not telling us about…?

Simple SQLfiddle demo.

EDIT: Since you don’t want cancelled transactions (or rather transactions with an odd number of cancellations), you need a quite a bit more complicated recursive query to figure out whether to show the last transaction or not;

WITH ChangeLog(TransactionID, Amount, ParentTransactionID, 
               IsCancel, OriginalTransactionID) AS
(
  SELECT TransactionID, Amount, ParentTransactionID, 0, TransactionID
  FROM Transactions WHERE ParentTransactionID IS NULL
  UNION ALL
  SELECT t.TransactionID, t.Amount, t.ParentTransactionID, 
         1-c.IsCancel, c.OriginalTransactionID
  FROM Transactions t
  JOIN ChangeLog c ON c.TransactionID = t.ParentTransactionID
) 
SELECT c1.TransactionID, c1.Amount, c1.ParentTransactionID
FROM ChangeLog c1
LEFT JOIN ChangeLog c2
  ON c1.TransactionID < c2.TransactionID
     AND c1.OriginalTransactionID = c2.OriginalTransactionID
WHERE c2.TransactionID IS NULL AND c1.IsCancel=0

This will, in your example with 3 transactions, show the last row, but if the last row is cancelled, it won’t return anything.

Since SQLfiddle is up again, here is a fiddle to test with.

A short explanation of the query may be in order even if a bit hard to do in a simple way; it defines a recursive “view”, ChangeLog that tracks cancels and the original transaction id from the original to the last transaction in a series (a series is all transactions with the same OriginalTransactionId). After that, it joins ChangeLog with itself to find the last entry (ie all transactions that don’t have a cancelling transaction). If the last entry found in a series is not a cancel (IsCancel=0) it will show up.