Can Firebirds SELECT FIRST accept a variable?

Question

http://www.firebirdsql.org/refdocs/langrefupd20-select.html#langrefupd20-first-skip

The manual says that FIRST accepts "Any expression evaluating to an integer." Shouldn't this mean a variable too?

In the following stored procedure, I get an error trying to supply :DAYS to FIRST.

Token unknown - line 10, column 18
:

Line 10 column 18 is the : in front of DAYS...

SET TERM ^ ;

CREATE PROCEDURE P_STOCK_MDA 
 ( STOCK BIGINT, TRADE_DATE DATE, DAYS SMALLINT ) 
RETURNS 
 ( AVG_CLOSE NUMERIC(6,2) )
AS 
BEGIN
  SELECT AVG(STOCK_ADJ_CLOSE) FROM (
    SELECT FIRST :DAYS STOCK_ADJ_CLOSE
    FROM STOCK_DAILY yd 
    WHERE yd.STOCK_STOCK=:STOCK AND yd.TRADE_DATE<=:TRADE_DATE
    ORDER BY yd.TRADE_DATE DESC
  ) INTO AVG_CLOSE;
END^

Answer 1

You need to enclose the parameter in parentheses to get it working:

SELECT FIRST (:DAYS) STOCK_ADJ_CLOSE

Full code:

SET TERM ^ ;

CREATE PROCEDURE P_STOCK_MDA 
 ( STOCK BIGINT, TRADE_DATE DATE, DAYS SMALLINT ) 
RETURNS 
 ( AVG_CLOSE NUMERIC(6,2) )
AS 
BEGIN
  SELECT AVG(STOCK_ADJ_CLOSE) FROM (
    SELECT FIRST (:DAYS) STOCK_ADJ_CLOSE
    FROM STOCK_DAILY yd 
    WHERE yd.STOCK_STOCK=:STOCK AND yd.TRADE_DATE<=:TRADE_DATE
    ORDER BY yd.TRADE_DATE DESC
  ) INTO AVG_CLOSE;
END^

SET TERM ; ^

The documentation does say :

If <int-expr> is an integer literal or a query parameter, the “()” may be omitted

However I assume this only applies to the ? query parameter in DSQL, not to the named parameters in PSQL.