ORDER BY AND CASE IN SQL SERVER

Question

I need to have an order by functionality inside a stored procedure. A value is posted to a webservice and based on that value I have to order the results in a certain way i.

Answer 1

Let see if this is what you need:

DECLARE @t table(col1 int, col2 int, col3 int,col4 int);
DECLARE @OrderBy varchar(4) = 'Col3';

insert into @t 
values
 (1,2,3,4),
 (6,3,8,5),
 (7,4,1,7),
 (3,7,0,1),
 (9,8,3,6);

 SELECT 
     CASE @OrderBy
        WHEN 'Col2' THEN Col2
        WHEN 'Col3' THEN Col3
        WHEN 'Col4' THEN Col4
        ELSE Col1 END as OrderCol
    ,* 
 FROM @t
 ORDER BY OrderCol desc;

 SELECT * 
 from @t
 ORDER BY CASE @OrderBy
    WHEN 'Col2' THEN Col2
    WHEN 'Col3' THEN Col3
    WHEN 'Col4' THEN Col4
    ELSE Col1 END desc;

Answer 2

CASE is an expression and has to produce a result of a single well defined type. So as long as the types of all columns are compatible, they can all be placed into a single CASE expression.

If that's not the case then you need to split it up and use multiple expressions. Say that Col1 and Col3 have compatible types (whether the same or you're happy for one to convert to the other) and that Col2 and Col4 have incompatible types (both between themselves and with Col1 and Col3), then we need three expressions:

ORDER BY 
    CASE @OrderBy
        WHEN 'Col1' THEN Col1
        WHEN 'Col3' THEN Col3
    END
    , CASE @OrderBy WHEN 'Col2' THEN Col2 END
    , CASE @OrderBy WHEN 'Col4' THEN Col4 END
    , Col1

(I've also include a final expression of Col1 so that your "fallback" sort still occurs)

For each of the CASE expressions above, if no match occurs then the expression returns NULL - and all NULLs sort together, so that that entire CASE expression then has no overall effect on the sorting.

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From CASE:

Return Types

Returns the highest precedence type from the set of types in result_expressions and the optional else_result_expression.