MySql Conditional Group By - mysql

Question

Please consider my table (this is just a simplified version, in my project, I got 600,000+ records):
Id TransactionId TransactionTypeId Description
1 1 1 Description1
2 1 1 Description2
3 1 2 Description3
4 1 2 Description4
5 1 1 Description5
6 1 2 Description6
7 2 1 Description7
8 2 1 Description8
9 2 2 Description9
10 2 2 Description10
What I need to do is to when TransactionTypeId = 1, I need to get the latest data from that table. Otherwise, when TransactionTypeId <> 1, I need to get them all.
For this instance, I have this query:
SELECT MAX(T.Id)
, SUBSTRING_INDEX(GROUP_CONCAT(T.TransactionId ORDER BY T.Id DESC), ',', 1) AS TransactionId
, SUBSTRING_INDEX(GROUP_CONCAT(T.TransactionTypeId ORDER BY T.Id DESC), ',', 1) AS TransactionTypeId
, SUBSTRING_INDEX(GROUP_CONCAT(T.Description ORDER BY T.Id DESC), ',', 1) AS Description
FROM Transactions T
GROUP BY T.TransactionId
, CASE WHEN T.TransactionTypeId = 1 THEN T.TransactionTypeId END
ORDER BY T.TransactionId, T.TransactionTypeId
And my desired result would be:
Id TransactionId TransactionTypeId Description
5 1 1 Description5
3 1 2 Description3
4 1 2 Description4
6 1 2 Description6
8 2 1 Description8
9 2 2 Description9
10 2 2 Description10
But my problem is, even when TransactionTypeId <> 1, the query still groups them.
My query returns:
Id TransactionId TransactionTypeId Description
5 1 1 Description5
6 1 2 Description6
8 2 1 Description8
10 2 2 Description10
I know I can use UNION here, to seperate queries for the TransactionTypeId column, but I can't, it took me more than 5 minutes to get the results. Is there any possible (if there's none, I've got no choice but to use UNION) way to solve this?
Thanks :)
UPDATE #1
Here's my query when I'm using UNION.
SELECT Transaction.Id
, Transaction.TransactionId
, Transaction.TransactionTypeId
, Transaction.Description
FROM (
SELECT MAX(T.Id)
, SUBSTRING_INDEX(GROUP_CONCAT(T.TransactionId ORDER BY T.Id DESC), ',', 1) AS TransactionId
, SUBSTRING_INDEX(GROUP_CONCAT(T.TransactionTypeId ORDER BY T.Id DESC), ',', 1) AS TransactionTypeId
, SUBSTRING_INDEX(GROUP_CONCAT(T.Description ORDER BY T.Id DESC), ',', 1) AS Description
FROM Transactions T
WHERE T.TransactionTypeId = 1
GROUP BY T.TransactionId
UNION
SELECT T.Id
, T.TransactionId
, T.TransactionTypeId
, T.Description
FROM Transactions T
WHERE T.TransactionTypeId <> 1
) Transaction
ORDER BY Transaction.TransactionId, Transaction.TransactionTypeId

Answer 1

TRY WITH THIS
SELECT MAX(T.Id)
, SUBSTRING_INDEX(GROUP_CONCAT(T.TransactionId ORDER BY T.Id DESC), ',', 1) AS TransactionId
, SUBSTRING_INDEX(GROUP_CONCAT(T.TransactionTypeId ORDER BY T.Id DESC), ',', 1) AS TransactionTypeId
, SUBSTRING_INDEX(GROUP_CONCAT(T.Description ORDER BY T.Id DESC), ',', 1) AS Description
FROM Transactions T
GROUP BY CASE WHEN T.TransactionTypeId = 1 THEN CONCAT(T.TransactionId,'-',T.TransactionTypeId) ELSE T.Id END
ORDER BY T.TransactionId, T.TransactionTypeId

Answer 2

Try this one. The only slowing factor I see is the final sorting - so you may try it first without the ORDER BY:
SELECT T.Id
, T.TransactionId
, 1 AS TransactionTypeId
, T.Description
FROM Transactions T
JOIN
( SELECT MAX(Id) AS Id
FROM Transactions
WHERE TransactionTypeId = 1
GROUP BY TransactionId
) AS grp
ON grp.Id = T.Id
UNION ALL
SELECT T.Id
, T.TransactionId
, T.TransactionTypeId
, T.Description
FROM Transactions T
WHERE T.TransactionTypeId <> 1
ORDER BY TransactionId, TransactionTypeId
You can also use this (which is non UNION but has an OR so I can't be sure about performance) query:
SELECT T.Id
, T.TransactionId
, T.TransactionTypeId
, T.Description
FROM Transactions T
WHERE T.TransactionTypeId <> 1
OR NOT EXISTS
( SELECT *
FROM Transactions tm
WHERE tm.TransactionTypeId = 1
AND tm.TransactionId = T.TransactionId
AND tm.Id > T.Id
)
ORDER BY T.TransactionId, T.TransactionTypeId