Combine two tables for one output

Question

Say I have two tables:

KnownHours:

ChargeNum    CategoryID    Month    Hours
111111       1             2/1/09   10
111111       1             3/1/09   30
111111       1             4/1/09   50
222222       1             3/1/09   40
111111       2             4/1/09   50

UnknownHours:

ChargeNum   Month   Hours
111111      2/1/09  70
111111      3/1/09  40.5
222222      7/1/09  25.5

I need to group these hours, ignoring Month, into a single data table so that my expected result is the following:

ChargeNum    CategoryID     Hours
111111       1              90
111111       2              50
111111       Unknown        110.5
222222       1              40
222222       Unknown        25.5

I cannot seem to figure this out. Any help would be greatly appreciated!

EDIT: I need to sum the hours for each ChargeNum/Category combination. I updated the sample data to reflect this.

Answer 1

You'll need to use UNION to combine the results of two queries. In your case:

SELECT ChargeNum, CategoryID, SUM(Hours)
FROM KnownHours
GROUP BY ChargeNum, CategoryID
UNION ALL
SELECT ChargeNum, 'Unknown' AS CategoryID, SUM(Hours)
FROM UnknownHours
GROUP BY ChargeNum

Note - If you use UNION ALL as in above, it's no slower than running the two queries separately as it does no duplicate-checking.