SELECT a.Date, a.DistID, a.BranchId, SUM(a.CreditAmount) AS CreditAmount, NULL AS DebitAmount
FROM a
GROUP BY a.Date, a.DistID, a.BranchId
UNION
SELECT b.Date, b.DistID, b.BranchId, NULL, SUM(b.DebitAmount)
FROM b
GROUP BY b.Date, b.DistID, b.BranchId