Question

How do I add the sums from each results, so that the end results would give me a table like below?

Date | Sum1       | Sum2
1108  | Total sum1 | Total sum2
etc. 

DECLARE @StartDate      int
DECLARE @EndDate  int

SET         @StartDate  = XXXXXX
SET         @EndDate    = XXXXXX

SELECT                  ODATE,
                        SUM(CASE WHEN [COLUMN1] IN (X,X) THEN 1 ELSE 0 END)+ SUM(CASE WHEN [COLUMN3] IN (X) THEN 1 ELSE 0 END) AS SUM1,
                      SUM(CASE WHEN [COLUMN2] IN (X,X) THEN 1 ELSE 0 END) AS SUM2


FROM              [Db1]..
WHERE             ODATE Between @StartDate And @EndDate

UNION ALL
SELECT                  ODATE,
                        SUM(CASE WHEN [COLUMN1] IN (X,X) THEN 1 ELSE 0 END)+ SUM(CASE WHEN [COLUMN3] IN (X) THEN 1 ELSE 0 END) AS SUM1,
                      SUM(CASE WHEN [COLUMN2] IN (X,X) THEN 1 ELSE 0 END) AS SUM2        

FROM              [Db2]..
WHERE             ODATE Between @StartDate And @EndDate

UNION ALL
SELECT                  ODATE,
                        SUM(CASE WHEN [COLUMN1] IN (X,X) THEN 1 ELSE 0 END)+ SUM(CASE WHEN [COLUMN3] IN (X) THEN 1 ELSE 0 END) AS SUM1,
                      SUM(CASE WHEN [COLUMN2] IN (X,X) THEN 1 ELSE 0 END) AS SUM2        

FROM              [Db3]..
WHERE             ODATE Between @StartDate And @EndDate

Answer 1

Wrap your select, and run a GROUP BY over it

DECLARE @StartDate      int
DECLARE @EndDate  int

SET         @StartDate  = XXXXXX
SET         @EndDate    = XXXXXX

SELECT ODATE, SUM(SUM1) Sum1, SUM(SUM2) Sum2
(
  ... your big select ...
) X
GROUP BY ODATE;