If you have invoked a database, you have the necessary privileges to use the FOR Statement with Segmented Strings. Oracle Rdb defines a special name to refer to the segments of a segmented string. This value expression is equivalent to a field name; it names the "fields" or segments of the string. Furthermore, because segments can vary in length, Oracle Rdb also defines a name for the length of a segment. The statement inside the segmented string FOR loop must use these names to refer to the segments of the string. These names are: RDB$VALUE The value stored in a segment of a segmented string RDB$LENGTH The length in bytes of a segment Within a single context, such as the context of a single request, if an arithmetic expression contains the MISSING operator, the resulting expression will evaluate to MISSING. In the following example, A.FIELD_1 contains missing (unknown) values, and the query correctly interprets the values in A.FIELD_1 as missing (unknown), causing the expression A.FIELD_3 = VARIABLE + A.FIELD_1 to evaluate to MISSING: RDO> FOR A IN RELATION_A cont> MODIFY A USING cont> A.FIELD_3 = VARIABLE + A.FIELD_1 cont> END_MODIFY cont> END_FOR However, in nested queries that use multiple database requests, such as the following example, if B.FIELD_2 contains missing (unknown) values, the expression A.FIELD_3 = VARIABLE + B.FIELD_2 returns different results. The second query (which begins with FOR A) retrieves a value, in this case the value defined as the field's MISSING_VALUE, from B.FIELD_2 for its record selection expression (RSE). However, because of RDO language limitations, the second query cannot use the fact that the field B.FIELD_2 has an unknown value and instead uses the missing value defined for the field with the DEFINE FIELD or CHANGE FIELD statement. Using this value for B.FIELD_2 instead of treating the value as unknown means that the A.FIELD_3 = VARIABLE + B.FIELD_2 expression does not evaluate to MISSING. RDO> FOR B IN RELATION_B cont> FOR A IN RELATION_A WITH A.FIELD_1 = B.FIELD_1 cont> MODIFY A USING cont> A.FIELD_3 = VARIABLE + B.FIELD_2 cont> END_MODIFY cont> END_FOR cont> END_FOR The workaround is to use the SQL interface to Oracle Rdb. You can use the SQL indicator variables to detect the NULL attribute of the column (field) and therefore set the appropriate value for the column.