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 Data Types and SubtypesData Definition (DDL) Statements 

AT Time Zone Expression

Syntax
<at expr> ::= <expr> AT { TIME ZONE <time zone string> | LOCAL }

The AT expression expresses a datetime value in a different time zone, while keeping the same UTC instant.

AT translates a time/timestamp value to its corresponding value in another time zone.If LOCAL is used, the value is converted to the session time zone.

When expr is a WITHOUT TIME ZONE type, expr is first converted to a WITH TIME ZONE in the session time zone and then transformed to the specified time zone.

Examples
select time '12:00 GMT' at time zone '-03:00' from rdb$database;
select current_timestamp at time zone 'America/Sao_Paulo' from rdb$database;
select timestamp '2018-01-01 12:00 GMT' at local from rdb$database;

Conditional Expressions

A conditional expression is one that returns different values according to how a certain condition is met.It is composed by applying a conditional function construct, of which Firebird supports several.This section describes only one conditional expression construct: CASE.All other conditional expressions apply internal functions derived from CASE and are described in Conditional Functions.