В FB 1.5 есть такое:
* New COALESCE internal function (SF #451917).
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.
Syntax:
COALESCE (value {, value} ... )
Notes:
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
3. The function has the same meaning as NVL one in Oracle.
Example:
SELECT
PROJ_NAME AS Projectname,
COALESCE(e.FULL_NAME, '[> not assigned <]') AS Employeename
FROM
PROJECT p LEFT JOIN EMPLOYEE e ON (e.EMP_NO = p.TEAM_LEADER)

* New NULLIF internal function (SF #451917).
Return a NULL value for a sub-expression if it has a specific value, otherwise return the value of the sub-expression.
Syntax:
NULLIF (value, value)
Note:
NULLIF (V1, V2) is equivalent to the following case specification:
CASE WHEN V1 = V2 THEN NULL ELSE V1 END
Example:
UPDATE PRODUCTS
SET STOCK = NULLIF(STOCK, 0)

* New CASE internal function (SF #451917).
Allow the result of a column to be determined by a the results of a case expression.
Syntax:
<case_specification> = <simple_case> | <searched_case>

<simple_case> = CASE value <simple_when_clause> ... [ELSE value] END
<simple_when_clause> = WHEN value THEN value

<searched_case> = CASE <searched_when_clause> ... [ELSE value] END
<searched_when_clause> = WHEN <search_condition> THEN value
Note:
The function has the same meaning as DECODE one in Oracle.
Examples:
1. Simple case:
SELECT
o.ID,
o.Description,
CASE o.Status
WHEN 1 THEN 'confirmed'
WHEN 2 THEN 'in production'
WHEN 3 THEN 'ready'
WHEN 4 THEN 'shipped'
ELSE 'unknown status ''' || o.Status || ''''
END
FROM
Orders o
2. Searched case:
SELECT
o.ID,
o.Description,
CASE
WHEN (o.Status IS NULL) THEN 'new'
WHEN (o.Status = 1) THEN 'confirmed'
WHEN (o.Status = 3) THEN 'in production'
WHEN (o.Status = 4) THEN 'ready'
WHEN (o.Status = 5) THEN 'shipped'
ELSE 'unknown status ''' || o.Status || ''''
END
FROM
Orders o