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 SQL Language StructureCommon Language Elements 

Implicit Conversion During String Concatenation

When multiple data elements are being concatenated, all non-string data will undergo implicit conversion to string, if possible.

Example
SELECT 30||' days hath September, April, June and November' CONCAT$
  FROM RDB$DATABASE;

CONCAT$
------------------------------------------------
30 days hath September, April, June and November

Custom Data Types — Domains

In Firebird, the concept of a “user-defined data type” is implemented in the form of the domain.Creating a domain does not truly create a new data type, of course.A domain provides the means to encapsulate an existing data type with a set of attributes and make this “capsule” available for reuse across the whole database.If several tables need columns defined with identical or nearly identical attributes, a domain makes sense.

Domain usage is not limited to column definitions for tables and views.Domains can be used to declare input and output parameters and variables in PSQL code.