Distinct type operands
A weakly typed distinct type operand can be used with arithmetic operators,
provided that the source type of the weakly typed distinct type is supported by the arithmetic
operator. 
A strongly typed distinct type cannot be used with arithmetic operators even
if its source data type is numeric. To perform an arithmetic operation, create a function with the
arithmetic operator as its source. For example, if there were distinct types INCOME and EXPENSES,
both of which had DECIMAL(8,2) data types, then the following user-defined function, REVENUE, could
be used to subtract one from the other.
CREATE FUNCTION REVENUE ( INCOME, EXPENSES )
RETURNS DECIMAL(8,2) SOURCE "-" ( DECIMAL, DECIMAL)Alternately, the - (minus) operator could be overridden using a user-defined function to subtract the new data types.
CREATE FUNCTION "-" ( INCOME, EXPENSES )
RETURNS DECIMAL(8,2) SOURCE "-" ( DECIMAL, DECIMAL)Alternatively, the distinct type can be cast to a built-in type, and the result can be used as an operand of an arithmetic operator.