无法在MySQL选择查询中使用Case语句？

Question

Here is my query and Grouping by Transaction type C for Credit and D for Debit and

SELECT 
case 
WHEN ACT.TRANSACTIONTYPE='C' THEN SUM(ACT.AMOUNT) END AS CreditBalance,
case 
WHEN ACT.TRANSACTIONTYPE='D' THEN SUM(ACT.AMOUNT) END AS DebitBalance
FROM accounttransaction ACT 
WHERE ACT.TRANSACTIONTYPE IS NOT NULL
GROUP BY ACT.TRANSACTIONTYPE;

Below is my result

CreditBalance DebitBalance
16536          NULL
NULL           9673

Now I want to Subtract the balance of the A/C then I need to show as Current Balance Account transaction table includes Accountnumber,transactiontype and amount

Answer 1

You are grouping by TransactionType , so the results show up on different rows. You need to remove the group by and move the condition expressions inside the sum() :

select sum(case when  ACT.TRANSACTIONTYPE = 'C' then act.amount end) as CreditBalance,
       sum(case when  ACT.TRANSACTIONTYPE = 'D' then act.amount end) as DebitBalance
from AccountTransaction act
WHERE act.TRANSACTIONTYPE IS NOT NULL;

Answer 2

Do you just want the grand total of all the credit / debit amounts?

If so maybe the following:-

SELECT Accountnumber, SUM(CASE WHEN transactiontype = 'C' THEN amount ELSE -1 * amount END) 
FROM accounttransaction
WHERE transactiontype IS NOT NULL
GROUP BY Accountnumber

Or to give the credit and debit amounts separately as well as the overall total

SELECT Accountnumber, 
    SUM(CASE WHEN transactiontype = 'C' THEN amount ELSE 0 END) AS CreditTotal,
    SUM(CASE WHEN transactiontype = 'D' THEN amount ELSE 0 END) AS DebitTotal,
    SUM(CASE WHEN transactiontype = 'C' THEN amount WHEN transactiontype = 'D' THEN -1 * amount ELSE 0 END) AS OverallTotal
FROM accounttransaction
WHERE transactiontype IS NOT NULL
GROUP BY Accountnumber