使用参数的WHERE子句中的SQL CASE

Question

I have 4 date columns in my Date table, I want to write a query that will hit one of these columns depending on that user passes as a parameter: here is a part of my code:

Select ...
FROM...
WHERE
CASE 
WHEN @timePeriod =  'Cal' THEN d.CalendarYear  = @year 
WHEN @timePeriod =  'Fin' THEN d.FinancialYear = @year 
WHEN @timePeriod =  'Mar' THEN d.YearEndedMarch = @year 
WHEN @timePeriod =  'Sep' THEN d.YearEndedSep = @year 
END

So the user will provide a TimePeriod, eg 'Fin' and the actual year 'FY2013 - 2014'. Thus, if the user proivdes @timePeriod = 'Fin' I want the query to filter the Date table by @year.

Here is a sample of what Im trying to get: I have some data in the Data column, and then I want the query to filter the results by one of these 4 calendar columns.

Answer 1

You do do this without a CASE statement and using simple logic:

Select ...
FROM...
WHERE
    (@timePeriod =  'Cal' AND d.CalendarYear  = @year)
OR  (@timePeriod =  'Fin' AND d.FinancialYear = @year) 
OR  (@timePeriod =  'Mar' AND d.YearEndedMarch = @year) 
OR  (@timePeriod =  'Sep' AND d.YearEndedSep = @year) 

[Given the cardinality for all these columns is very similar (or identical) you should not have to worry about parameter sniffing caching an inappropriate query plan.]

Answer 2

Yet another variation:

Select ...
FROM...
WHERE @year = 
  CASE 
     WHEN @timePeriod =  'Cal' THEN d.CalendarYear
     WHEN @timePeriod =  'Fin' THEN d.FinancialYear
     WHEN @timePeriod =  'Mar' THEN d.YearEndedMarch
     WHEN @timePeriod =  'Sep' THEN d.YearEndedSep
  END

And this one, which may be faster than one using CASE or OR :

 Select ...
 FROM...
 WHERE  @timePeriod = 'Cal' and d.CalendarYear = @year
 UNION ALL
 Select ...
 FROM ....
 WHERE  @timePeriod = 'Fin' and d.FinancialYear = @year
 UNION ALL
 ... etc...

The idea behind this second one is that each SELECT is very simple and can take advantage of any indexes you might have, while the UNION ALL combines them without attempting to filter out duplicates. Since @timePeriod can only have one value at a time, no records will be returned for 3 of the queries, and there'll be no duplicates to require removing.

Answer 3

Use sub-query:

  Select ...
  FROM
  (
  SELECT
  CASE 
  WHEN @timePeriod =  'Cal' THEN d.CalendarYear
  WHEN @timePeriod =  'Fin' THEN d.FinancialYear
  WHEN @timePeriod =  'Mar' THEN d.YearEndedMarch
  WHEN @timePeriod =  'Sep' THEN d.YearEndedSep
  END as date
  FROM...) t
  WHERE t.date = @year

Answer 4

Hello we can make query more faster by using Union all with in the Subquery i am just giving syntax like example because there is no availability of data

 Select t.TimePeriod,t.CalendarYear
  FROM
  (
  SELECT TIMEID,
   @timePeriod =  'Cal' FROM @x where  t.CalendarYear = @year
   UNION ALL
  SELECT TIMEID,
   @timePeriod =  'FIN' FROM @x where  t.CalendarYear = @year
    UNION ALL
  SELECT TIMEID,
   @timePeriod =  'MAR' FROM @x where  t.CalendarYear = @year
    UNION ALL
  SELECT TIMEID,
   @timePeriod =  'Sep' FROM @x where  t.CalendarYear = @year

) t
  WHERE t.date = @year