Question

I’m a sql newbie, I use mssql2005

I like to do join Action depnding on input parameter.

CREATE PROCEDURE SelectPeriodicLargeCategoryData 
    @CATEGORY_LEVEL CHAR(1),
    @CATEGORY_CODE VARCHAR(9)
AS

...


JOIN CATEGORY_AD_SYS CAS WITH(NOLOCK)
ON CA.CATEGORY_ID = [[[[[   HERE      ]]]]

above the sql.
if @CATEGORY_LEVEL = 'L' then I like to join on CAS.LCATEGORY

else if @CATEGORY_LEVEL = 'M' then I like to join on CAS.MCATEGORY

else if @CATEGORY_LEVEL = 'S' then I like to join on CAS.SCATEGORY

…

how can I do this?

Answer 1

You could use a CASE expression such as:

CASE @CATEGORY_LEVEL  
     WHEN 'L' THEN CAS.LCATEGORY
     WHEN 'M' THEN CAS.MCATEGORY
     WHEN 'S' THEN CAS.SCATEGORY
END

I’m not sure how fast that would be in a JOIN‘s ON condition (depends on how smart the query optimizer is about it, of course, so you’d better check by measuring with real data) — if it turns out to have unacceptable performance, I guess you could use completely different SELECT statements depending on @CATEGORY_LEVEL as a last resort.