TSQL - If..Else statement inside Table-Valued Functions - cant go through

Question

Before posting I have read few articles about developing USD functions, but have not encountered solutions for my problem... which is as follows:

I have a very simple database, which stores basketball players and consists of ID, Age, Height and Name column. What I would like to do is to implement a function 'height' with one parameter @set varchar(10), that depending one @set value will trigger off different select statements

what I was trying to implement was in psuedo-code:

CREATE FUNCTION [dbo].[age](@set varchar(10))
RETURNS TABLE
AS
BEGIN

    IF  (@set = 'tall')
         SELECT * from player where height > 180

    ELSE IF (@set = 'average')
         SELECT * from player where height >= 155 and height <=175

    ELSE IF (@set = 'low')
         SELECT * from player where height < 155
END

Could anyone give me a hint how to implement it?

Answer 1

You were close. Using a multi-statement table-valued function requires the return table to be specified and populated in the function:

CREATE FUNCTION [dbo].[age](@set varchar(10))
RETURNS @Players TABLE
(
    -- Put the players table definition here
) 
AS
BEGIN

    IF  (@set = 'tall')
         INSERT INTO @Players SELECT * from player where height > 180

    ELSE IF (@set = 'average')
         INSERT INTO @Players SELECT * from player where height >= 155 and height <=175

    ELSE IF (@set = 'low')
         INSERT INTO @Players SELECT * from player where height < 155

    RETURN -- @Players (variable only required for Scalar functions)

END

I would recommend using an inline TVF as Richard's answer demonstrates. It can infer the table return from your statement.

Note also that a multi-statement and inline TVFs are really quite different. An inline TVF is less of a black-box to the optimizer and more like a parametrized view in terms of the optimizer being able to rearrange things with other tables and views in the same execution plan.

Answer 2

The simplest form is always the best

CREATE FUNCTION [dbo].[age](@set varchar(10))
RETURNS TABLE
AS RETURN
SELECT * from player
where ((@set = 'tall' and height > 180)
   or (@set = 'average' AND height >= 155 and height <=175)
   or (@set = 'low' AND height < 155))
GO

This form is called INLINE table function, which means SQL Server is free to expand it to join player directly to other tables in-line of a greater query, making it perform infinitely¹ better than a multi-statement table valued function.

You may prefer this though, so that your ranges are complete (you have a gap between 175 and 180)

where ((@set = 'tall' and height > 180)
   or (@set = 'average' AND height >= 155 and height <= 180)
   or (@set = 'low' AND height < 155))

SQL Server takes care of short circuiting the branches when the variable @set is parsed.

^{1 exaggeration, but only slightly}

Answer 3

Why are you hardcoding this, create a heights table and then grab all the heights that are valid for the range

SELECT * from player p
join Heights h on p.height between h.heightStart and h.heightEnd 
WHERE h.height  = @set

Answer 4

This should work.

SELECT * FROM player 
WHERE
  height > CASE 
            WHEN @set = 'tall' THEN 180
            WHEN @set = 'average' THEN 154
            WHEN @set = 'low' THEN 0
          END

I'll leave the < case for your enjoyment.