SQL IF else在where语句中

Question

ALTER PROCEDURE [dbo].[sp_CourseEvalRpt_2]
    @coursetitle varchar(100) = null,
    @datestart varchar(8),
    @dateend varchar(8)
AS
    select distinct 
        t_empname, t_leader, T_Dept, t_submit 
    from 
        tbltrain
    where 
        t_category like 'course%' 
        and 
           if (@coursetitle <> '') t_course = @coursetitle end

How can I eliminate the SQL where criteria when parameter name @coursetitle is empty? I don't want to add the t_course condition when @coursetitle is empty, so system will display all data rather than set the t_course=''.

Answer 1

Try changing your sql to this:

select distinct t_empname,
       t_leader,
       T_Dept,
       t_submit 
  from tbltrain
 where t_category like 'course%' 
   and ( (@coursetitle<>''and t_course=@coursetitle) 
         or @coursetitle='')

Answer 2

You can simplify the where clause to:

where t_category like 'course%' and
      (@coursetitle = '' or t_course = @coursetitle) 

If you used NULL instead of an empty string for @coursetitle , you could phrase it as:

where t_category like 'course%' and
      t_course = coalesce(@coursetitle, t_course)

Answer 3

How about using dynamic SQL ? so that you can perform a clear IF ELSE condition.

Just add SQLQuery variable for the SQL query and use dynamic SQL

ALTER PROCEDURE [dbo].[sp_CourseEvalRpt_2]

@coursetitle varchar(100)=null,
@datestart varchar(8),
@dateend varchar(8),
@SQLQuery varchar(max) -->> Additional variable for dynamic SQL
AS

SET @SQLQuery = 'select distinct t_empname,t_leader,T_Dept,t_submit 
                 from tbltrain where t_category like course%'

IF ISNULL(@coursetitle,'') <> ''
    BEGIN
       SET @SQLQuery = @SQLQuery + 'AND t_course = ' + @coursetitle 
    END

EXEC (@SQLQuery)