Conditional SQL ORDER BY ASC/DESC for alpha columns

16
votes

Writing a stored procedure in MS SQL Server 2008 R2, I want to avoid using DSQL...

I would like the sort method (ASC or DESC) to be conditional.

Now, with a numeric column I would simply use a case statement and negate the value to emulate ASC or DESC... That is:

... ORDER BY CASE @OrderAscOrDesc WHEN 0 THEN [NumericColumn] ELSE -[NumericColumn] END ASC

What is an appropriate method for doing this with an alpha column?

EDIT: I thought of a clever way but it seems terribly inefficient... I could insert my ordered alpha column into a temp table with an autonumber then sort by the autonumber using the method described above.

EDIT2:

What do you guys think of this approach?

ORDER BY CASE @OrderAscOrDesc WHEN 0 THEN [AlphaColumn] ELSE '' END ASC,
CASE @OrderAscOrDesc WHEN 0 THEN '' ELSE [AlphaColumn] END DESC

I don't know if forcing a sort on a uniform column is more efficient than deriving numbers from sorted strings though

2
sql-serverstored-proceduressql-order-byconditional

2 Answers

36
votes

One option

;WITH cQuery AS
(
   SELECT
       *,
       ROW_NUMBER() OVER (ORDER BY SortColumn) AS RowNum
   FROM
       MyTable
)
SELECT
   *
FROM
   cQuery
ORDER BY
   RowNum * @Direction --1 = ASC or -1 = DESC

Or CASE which IMHO is a bit uglier

ORDER BY
  CASE WHEN 'ASC' THEN SortColumn ELSE '' END ASC,
  CASE WHEN 'DESC' THEN SortColumn ELSE '' END DESC
1
votes

This is one of those cases when specific solutions may be preferable preferable to generic ones, especially when we deal with large amounts of data. I would:

IF @OrderAscOrDesc = 0 THEN BEGIN
  SELECT ...
  FROM  ... 
  ORDER BY [AlphaColumn] ASC
END ELSE BEGIN
  SELECT ...
  FROM  ... 
  ORDER BY [AlphaColumn] DESC
END

If you have an index on [AlphaColumn], you might sometimes get a better plan with a more specific query, than with a generic one-size-fits-all one.

Edit: to facilitate code reuse, you can wrap your select in an inline UDF - it will perform just as well:

IF @OrderAscOrDesc = 0 THEN BEGIN
  SELECT ...
  FROM  YourInlineUdf(...)
  ORDER BY [AlphaColumn] ASC
END ELSE BEGIN
  SELECT ...
  FROM  YourInlineUdf(...) 
  ORDER BY [AlphaColumn] DESC
END