mysql - MySQL条件分组依据

请考虑我的表（这只是一个简化版本，在我的项目中，我得到了600000多条记录）：

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

我需要做的是，当TransactionTypeId = 1时，我需要从该表中获取最新数据。否则，当TransactionTypeId <> 1时，我需要把它们都弄到手。
在本例中，我有一个查询：

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

我想要的结果是：

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

但我的问题是，即使当TransactionTypeId <> 1时，查询仍然将它们分组。
我的查询返回：

Id  TransactionId   TransactionTypeId   Description
5   1               1                   Description5
6   1               2                   Description6
8   2               1                   Description8
10  2               2                   Description10

我知道我可以在这里使用UNION来分隔TransactionTypeId列的查询，但是我不能，我花了5分钟才得到结果。有没有可能（如果没有，我别无选择，只能使用UNION）的方法来解决这个问题？
谢谢：）
更新#1
下面是我使用UNION时的查询。

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

最佳答案

试试这个

    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