Once you have declared the possible record types as field groups and defined their structure, you can fill the extract dataset using the following statements:
When the first EXTRACT statement occurs in a program, the system creates the extract dataset and adds the first extract record to it. In each subsequent EXTRACT statement, the new extract record is added to the dataset.
Each extract record contains exactly those fields that are contained in the fg field group, plus perhaps the fields of the header field group. The fields occur as a sort key at the beginning. If you do not explicitly specify an fg field group, the
statement is a shortened form of the statement
When you extract the data, the record is filled with the current values of the corresponding fields.
As soon as the system has processed the first EXTRACTstatement for an fg field group, the structure of the corresponding extract record in the extract dataset is fixed. You can no longer insert new fields into the fg and header field groups. If you try to modify one of the field groups afterwards and use it in another EXTRACTstatement, a runtime error occurs.
By processing EXTRACT statements several times using different field groups, you fill the extract dataset with records of different length and structure. Since you can modify field groups dynamically up to their first usage in an EXTRACT statement, extract datasets provide the advantage that you need not determine the structure at the beginning of the program.
Assume the following program is linked to the logical database F1S.
NODES: spfli, sflight.
FIELD-GROUPS: header, flight_info, flight_date.
INSERT: spfli-carrid spfli-connid sflight-fldate
There are three field groups. The INSERT statement assigns fields to two of the field groups. During the GET events, the
system fills the extract dataset with two different record types. The records
of the field group flight_info consist of five fields: spfli-carrid, spfli-connid, sflight-fldate, spfli-cityfrom, and spfli-cityto.
The first three fields belong to the prefixed field group header. The records of the field group flight_date consist only of the three fields of the header field group. The following figure shows the structure of the extract dataset: