firebird-mirror/doc/sql.extensions/README.order_by_expressions_nulls

SQL Language Extension: ORDER BY clause can specify expressions and nulls placement

Syntax
========

SELECT ... FROM .... ORDER BY order_list ....;

order_list : order_item [, order_list];

order_item : <expression> [order_direction] [nulls_placement]

order_direction : ASC | DESC;

nulls_placement : NULLS FIRST | NULLS LAST;

   The ORDER BY clause lets you specify any valid expressions to 
   sort query results. If expression is consisted of a single number
   it is interpreted as column number. The nulls_placement clause
   controls ordering of nulls in result set. They can be sorted
   either above (NULLS FIRST) or below (NULLS LAST) of all other values.
   Behaviour when nulls_placement is unspecified is NULLS LAST.   

Author:
   Nickolay Samofatov <skidder@bssys.com>


N O T E S
=========

 - Index is never used for ordering if you specify NULLS FIRST
   as a nulls_placement
 - Results are undefined if you ask engine to sort results using 
   non-deterministic UDF or stored procedure.
 - Amount of procedure invocations is undefined if you ask engine to 
   sort results using UDF or stored procedure in any case even
   if you reference column calling procedure by number
 - You can use only numbers to reference columns to sort unions

Examples
========


A)
  SELECT * FROM MSG
  ORDER BY PROCESS_TIME DESC NULLS FIRST

B)
  SELECT FIRST 10 * FROM DOCUMENT
  ORDER BY STRLEN(DESCRIPTION) DESC

C)
  SELECT DOC_NUMBER, DOC_DATE FROM PAYORDER
  UNION ALL
  SELECT DOC_NUMBER, DOC_DATA FROM BUDGORDER
  ORDER BY 2 DESC NULLS LAST, 1 ASC NULLS FIRST