UNION_COLNAME_7 in macro DSN6SPRM

Specifies the behavior for result column names in UNION queries.

NO: If the column name is the same across all sub-queries in the UNION query, the result column name is that column name. Otherwise, the result column is unnamed. NO is the default value. DB2® Version 8 introduced this behavior.
YES: The column name that is returned in the SQLNAME field of an SQLDA following a DESCRIBE statement, where the result table that is described is the result of a union, is the column name from the first subquery of the union operation. DB2 used this behavior in DB2 Version 7 and earlier.