Вообще теперь надо указывать версию FireBird у полуторки есть приятные расширения :) coalesce например

Function:
Allow a column value to be calculated by a number of expressions,
the first expression returning a non NULL value is returned as the
column value

<case abbreviation> ::=
| COALESCE <left paren> <value expression> { <comma> <value expression> }... <right paren>

Syntax Rules:
1) COALESCE (V1, V2) is equivalent to the following <case specification>:
CASE WHEN V1 IS NOT NULL THEN V1 ELSE V2 END
2) COALESCE (V1, V2,..., Vn), for n >= 3, is equivalent to the following
<case specification>:
CASE WHEN V1 IS NOT NULL THEN V1 ELSE COALESCE (V2,...,Vn) END