<...>Firebird is very strict with datatypes, and when you have different datatype in queries that make up the UNION it throws an error. It can happen even if difference is something trivial like CHAR(5) vs CHAR(6). It also happens for decimal and double datatypes, and (one of the most annoying) when you put NULL or zero.

Firebird 2 has much better type coercion as it detects the largest datatype, and allows smaller ones to fit automatically, i.e. if you have CHAR(5) in one query and CHAR(6) in other, it will automatically set view's column to be CHAR(6). Please note that datatypes have to be compatible for this to work (for example, you cannot union a Blob and Decimal column).