Problem with Full Outer Join not working as expected - sql

Question

I have a query to subtract current balance from one table with previous balance from another history table. When a particular product has no current balance but has previous balance, I am able to subtract it correctly...in this case it will be like 0 - 100.
However, when the product has current balance but no previous balance, I am unable to get the result. My query does not even select the current balance even though I have done a full outer join on both tables.
Following is my query:
SELECT DATEPART(yyyy, #ExecuteDate) * 10000 + DATEPART(mm, #ExecuteDate) * 100 + DATEPART(dd, #ExecuteDate) AS Period_Key,
CASE WHEN GL.GL_Acct_Key IS NULL THEN 0 ELSE GL.GL_Acct_Key END AS GL_Acct_Key,
CASE WHEN BANK.Bank_Type_Key IS NULL THEN 0 ELSE BANK.Bank_Type_Key END AS Bank_Type_Key,
CASE WHEN TSC.TSC_Key IS NULL THEN 0 ELSE TSC.TSC_Key END AS TSC_Key,
ISNULL(FT.CurrentBalance,0) - ISNULL(HIST.CurrentBalance,0) AS Actual_Income_Daily,
CASE WHEN BR.Branch_Key IS NULL THEN 0 ELSE BR.Branch_Key END AS Branch_Key
FROM WSB_Stage.dbo.Stage_TS_Daily_Income_Hist HIST
FULL OUTER JOIN WSB_Stage.dbo.Stage_TS_Daily_Income FT
ON FT.GLAcctID = HIST.GLAcctID AND
FT.BankType = HIST.BankType AND
FT.BranchNumber = HIST.BranchNumber
LEFT OUTER JOIN WSB_Mart.dbo.Dim_Branch BR
ON HIST.BranchNumber = BR.Branch_Code
LEFT OUTER JOIN WSB_Mart.dbo.Dim_GL_Acct GL
ON HIST.GLAcctID = GL.Acct_Code
LEFT OUTER JOIN WSB_Mart.dbo.Dim_Bank_Type BANK
ON HIST.BankType = BANK.Bank_Type_Code
LEFT OUTER JOIN WSB_Stage.dbo.Param_Branch_TSC_Map BRTSC
ON HIST.BranchNumber = BRTSC.BranchNumber
LEFT OUTER JOIN WSB_Mart.dbo.Dim_TSC TSC
ON BRTSC.RegionCode = TSC.TSC_Code
WHERE HIST.TransactionDate = #PreviousDate
AND GL.Acct_Type_Code = 'Interest'
AND BANK.Bank_Type_Key = 1

Answer 1

You are checking a attribute of the HIST table in the WHERE clause. If there is no entry in the HIST table, the clause doesn't match and thus discards the row.
Replace
WHERE HIST.TransactionDate = #PreviousDate
with
WHERE (HIST.TransactionDate IS NULL OR HIST.TransactionDate = #PreviousDate)

Answer 2

It's because of:
WHERE HIST.TransactionDate = #PreviousDate
This forces Hist.TransactionDate not to be null.
You could use
WHERE (HIST.TransactionDate = #PreviousDate OR HIST.TransactionDate IS NULL)
or change the join to:
FULL OUTER JOIN WSB_Stage.dbo.Stage_TS_Daily_Income FT
ON FT.GLAcctID = HIST.GLAcctID AND
FT.BankType = HIST.BankType AND
FT.BranchNumber = HIST.BranchNumber AND
HIST.TransactionDate = #PreviousDate

Answer 3

Thanks for the help but I couldn't get it to work the way I wanted using the below answers. Finally, I decided to go the long way and declare two temporary tables to hold current and previous balances. I think I want to stay as far away from outer joins as possible ;p
Code is below:
INSERT INTO #PreviousGL
SELECT GLAcctID,
BankType,
BranchNumber,
ISNULL(CurrentBalance,0) AS Current_Balance
FROM WSB_Stage.dbo.Stage_TS_Daily_Income_Hist
WHERE TransactionDate = #PreviousDate
INSERT INTO #CurrentGL
SELECT GLAcctID,
BankType,
BranchNumber,
ISNULL(CurrentBalance,0) AS Current_Balance
FROM WSB_Stage.dbo.Stage_TS_Daily_Income
INSERT INTO #DailyIncomeGL
SELECT CASE WHEN CURR.GLAcctID IS NULL THEN PREV.GLAcctID
WHEN PREV.GLAcctID IS NULL THEN CURR.GLAcctID
WHEN CURR.GLAcctID IS NULL AND PREV.GLAcctID IS NULL THEN 0
ELSE CURR.GLAcctID
END AS GLAcctID,
CASE WHEN CURR.BankType IS NULL THEN PREV.BankType
WHEN PREV.BankType IS NULL THEN CURR.BankType
WHEN CURR.BankType IS NULL AND PREV.BankType IS NULL THEN ''
ELSE CURR.BankType
END AS BankType,
CASE WHEN CURR.BranchNumber IS NULL THEN PREV.BranchNumber
WHEN PREV.BranchNumber IS NULL THEN CURR.BranchNumber
WHEN CURR.BranchNumber IS NULL AND PREV.BranchNumber IS NULL THEN 0
ELSE CURR.BranchNumber
END AS BranchNumber,
ISNULL(CURR.CurrentBal,0) - ISNULL(PREV.CurrentBal,0) AS Actual_Income_Daily
FROM #CurrentGL CURR
FULL OUTER JOIN #PreviousGL PREV
ON CURR.GLAcctID = PREV.GLAcctID AND
CURR.BankType = PREV.BankType AND
CURR.BranchNumber = PREV.BranchNumber