SQL Server Conditional Order By

Question

I have a SQL query in SQL Server 2005 that is breaking when I include a conditional order by. When I remove the order by, the query works. When I explicitly write the order by condition (e.g. order by p.Description) it works. When I include the conditional order by, I get the error,

'Conversion failed when converting character string to smalldatetime data type'

SQL Server isn't showing me which line of code caused this error. I'm wondering how I can fix this so I can use the conditional order by or troubleshoot which column is failing in the conversion.

declare @SearchTerm nvarchar(255)
declare @SortBy nvarchar(255)
declare @Months int
declare @VendorID int
declare @ProductID int

set @SearchTerm = 'focus'
set @SortBy = 'product'
set @Months = 3
set @VendorID = null
set @ProductID = null

-- This makes it so the @Month will filter by n number of months ago.
declare @PreviousMonths datetime
if @Months is null
    begin
        set @PreviousMonths = 24
    end
else
    begin
        set @PreviousMonths = DateAdd(month, -@Months, GetDate())
    end

select
    a.dsAlertID as AlertID,
    a.ProductID,
    v.VendorID,
    p.Description as ProductName,
    v.LongName as VendorName,
    a.Introduction,
    a.Writeup,
    a.DateAdded 
from
    ev_ds_Alerts a
left outer join
    tblProducts p on a.ProductID = p.ProductID
left outer join
    tblVendors v on v.VendorID = p.VendorID
where
    ( @SearchTerm is null or ( a.Writeup like '% ' + @SearchTerm + '%' or a.Introduction like '% ' + @SearchTerm + '%') )
    and (( @Months is null ) or ( @Months is not null and a.DateAdded >= @PreviousMonths))
    and (( @VendorID is null ) or ( @VendorID is not null and v.VendorID = @VendorID ))
    and (( @ProductID is null ) or ( @ProductID is not null and p.ProductID = @ProductID ))
order by
    case @SortBy
        when 'product' then p.Description
        when 'vendor' then v.LongName
        else a.DateAdded
    end

-- order by p.Description or v.LongName works when explicitly writing them out!

Answer 1

Per the previous answer, try:

order by
    case @SortBy
        when 'product' then p.Description
        when 'vendor' then v.LongName
        else convert(VARCHAR(25),a.DateAdded,20)

This should give you the sort you want, as it will format the date string yyyy-mm-dd hh:mm:ss.

Answer 2

If you care about performance, you might want a different approach: 1. Wrap your select in an inline TVF 2. Use two different SELECTs, so that they can get two different plans, potentially more efficient that the generic one-size-fits-all one plan you are getting now:

IF @SortBy='product' BEGIN
  SELECT AlertID,
(snip)
  FROM MyTvf
  ORDER BY Description ;
  RETURN @@ERROR ;
END 

IF @SortBy='Vendor' BEGIN
  SELECT AlertID,
(snip)
  FROM MyTvf
  ORDER BY LongName ;
  RETURN @@ERROR ;
END