Question

this is my query

Create FUNCTION [dbo].[CountUses](@couponid INT)
RETURNS INT
AS
   BEGIN
   RETURN
    (
    SELECT  c.Id,
     c.Name,
     c.CreateDate,
     Count(cu.id) NofUses
  FROM   Coupon as  c
     JOIN CouponUse as cu
       ON c.id = cu.couponid
 GROUP  BY c.Id,
     c.Name,
     c.CreateDate
     )
 END 

its giving the error Only one expression can be specified in the select list when the subquery is not introduced with EXISTS. where is the problem ?
Aside from the main answer, I’d also appreciate any comments you may have about optimizing my query…

Answer 1

If you want your function to return more than one value, then you need to look at Table-Valued Functions.

These types of functions return a table and not just one value. Your current function is set up as a scalar function so it can only return one value.

If you want a scalar value – let’s say just the count then your function will be similar to this:

Create FUNCTION [dbo].[CountUses](@couponid INT)
RETURNS INT
AS
   BEGIN
   RETURN
    (
      SELECT Count(cu.id) NofUses  --- this can only return one column
      FROM   Coupon as  c
      JOIN CouponUse as cu
        ON c.id = cu.couponid
      WHERE cu.couponid = @couponid
     )
   END 

If you intention is to return a table of data, then your function will be similar to this:

Create FUNCTION [dbo].[CountUses](@couponid INT)
RETURNS @new_table table
(
  id int,
  name varchar(50),
  CreateDate datetime,
  NofUsers int
)
AS
BEGIN
  INSERT INTO @new_table
  SELECT c.Id,
       c.Name,
       c.CreateDate,
       Count(cu.id) NofUses
      FROM   Coupon as  c
      JOIN CouponUse as cu
        ON c.id = cu.couponid
      WHERE cu.couponid = @couponid
      GROUP  BY c.Id,  c.Name,  c.CreateDate

   RETURN
END