Question

I want to insert in sql query something like that:

Select * from Users where id=[if @userId>3 then @userId else "donnt use this condition"] and Name=[switch @userId  
case 1:"Alex"
case 2:"John"
default:"donnt use this condition"];

How can i do it?

yet another similar question

When showAll is false it works ok but when showAll is true it returns nothing. Why and how to make it working right? IsClosed column has a bit type.

Select * from orders where IsClosed=CASE WHEN @showAll='false' THEN 'false' ELSE NULL END;

Answer 1

This will perform horribly:

Select * 
  from Users 
 where (@userid > 3 AND id = @userId)
    OR (@userId BETWEEN 1 AND 2 AND name = CASE 
                                             WHEN @userId = 1 THEN 'Alex' 
                                             ELSE 'John' 
                                           END)

The best performing option is dynamic SQL:

SQL Server 2005+

DECLARE @SQL NVARCHAR(4000)
    SET @SQL = 'SELECT u.*
                  FROM USERS u
                 WHERE 1 = 1 '

    SET@SQL = @SQL + CASE 
                       WHEN @userId > 3 THEN ' AND u.id = @userId '
                       ELSE ''
                     END

    SET@SQL = @SQL + CASE @userId
                       WHEN 1 THEN ' AND u.name = ''Alex'' '
                       WHEN 2 THEN ' AND u.name = ''John'' '
                       ELSE ''
                     END

BEGIN

 EXEC sp_executesql @SQL, N'@userId INT', @userId

END

For more info on SQL Server’s dynamic SQL support, read “The Curse and Blessings of Dynamic SQL“