"Difference between extract and collect statements
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:
EXTRACT <fg>.
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
field group <fg>, plus the fields of the field group HEADER (if one exists). The
fields from HEADER occur as a sort key at the beginning of the record. If you do
not explicitly specify a field group <fg>, the EXTRACT statement is a shortened
form of the statement extracts used in field groups (version 2.x - obsolete
these days as noone uses field groups anymore), and collect is used to
accumulate the contents of a field if X no. of keys are the same.
EXTRACT HEADER.
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 EXTRACT
statement for a field group <fg>, the structure of the corresponding extract
record in the extract dataset is fixed. You can no longer insert new fields into
the field groups <fg> and HEADER. If you try to modify one of the field groups
afterwards and use it in another EXTRACT statement, 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.
Collect:
When the line is inserted, the system checks whether there is already a table
entry that matches the key. If there is no corresponding entry already in the
table, the COLLECT statement has the same effect as inserting the new line. If
an entry with the same key already exists, the COLLECT statement does not append
a new line, but adds the contents of the numeric fields in the work area to the
contents of the numeric fields in the existing entry.
You should only use the COLLECT statement if you want to create summarized
tables. If you use other statements to insert table entries, you may end up with
duplicate entries.
