SQL: IF clause within WHERE clause

You should be able to do this without any IF or CASE

 WHERE 
   (IsNumeric(@OrderNumber) AND
      (CAST OrderNumber AS VARCHAR) = (CAST @OrderNumber AS VARCHAR)
 OR
   (NOT IsNumeric(@OrderNumber) AND
       OrderNumber LIKE ('%' + @OrderNumber))

Depending on the flavour of SQL you may need to tweak the casts on the order number to an INT or VARCHAR depending on whether implicit casts are supported.

This is a very common technique in a WHERE clause. If you want to apply some "IF" logic in the WHERE clause all you need to do is add the extra condition with an boolean AND to the section where it needs to be applied.

You don't need a IF statement at all.

WHERE
    (IsNumeric(@OrderNumber) = 1 AND OrderNumber = @OrderNumber)
OR (IsNumeric(@OrderNumber) = 0 AND OrderNumber LIKE '%' + @OrderNumber + '%')

Use a CASE statement
UPDATE: The previous syntax (as pointed out by a few people) doesn't work. You can use CASE as follows:

WHERE OrderNumber LIKE
  CASE WHEN IsNumeric(@OrderNumber) = 1 THEN 
    @OrderNumber 
  ELSE
    '%' + @OrderNumber
  END

Or you can use an IF statement like @N. J. Reed points out.