Commented by: @dyemanov

And what's wrong with the CASE function? Unlike DECODE, it's standard, this is why anything else is unlikely to be considered.

Commented by: @hvlad

There is standard CASE expression already implemented.
I personally see no reason to introduce ugly syntax sugar.
But, it is already implemented in FB 2.1

Commented by: @pmakowski

see release notes :
DECODE is a shortcut for a CASE ... WHEN ... ELSE expression.
Format:

DECODE( <expression>, <search>, <result>
[ , <search>, <result> ... ] [, <default> ]

Example

select decode(state, 0, 'deleted', 1, 'active', 'unknown') from things;