You can define keys which allow duplicate values. However, do not allow duplicates on primary keys as the value of a record's primary key must be unique.
When you use duplicate keys, be aware that there is a limit on the number of times you can specify the same value for an individual key. Each time you specify the same value for a duplicate key, an increment of one is added to the key's occurrence number. The maximum number of duplicate values permitted for an individual key varies according to the type of indexed file. For a full list of indexed file types and their characteristics, see the topic Types of Indexed Files.
The duplicate occurrence value is a unique identifier, which is added to the key, which has duplicates in order to make it unique. When a new key is added, where keys of the same value exist, then the occurrence value of the last added key is incremented by one and used for the newly added key. If keys are deleted, no change is made to the keys, which are not deleted and so if the highest occurrence of the series of duplicates is not deleted, then the highest value does not change. If the highest occurrence is deleted, then the highest occurrence value reduces to the next lower still existing occurrence value.
Your COBOL system uses the occurrence number to ensure that duplicate key records are read in the order in which they were created. Because of this, you cannot reuse an occurrence number whose record you have deleted. Therefore, you can reach the maximum number of duplicate values, even if some of those keys have already been deleted.
Some types of indexed file contain a duplicate occurrence record in the data file. Where an indexed file contains a duplicate occurrence record, each record in the data file is followed by a system record. This system record holds, for each duplicate key in that record, the occurrence number of the key. This number is just a counter of the number of times that key value has been used during the history of the file. The presence of the duplicate occurrence record makes REWRITE and DELETE operations on a record with many duplicates much faster but causes the data records of such files to be larger than those of a standard file.
To enable duplicate values to be specified for alternate keys, use WITH DUPLICATES in the ALTERNATE RECORD KEY clause in the SELECT clause:
file-control. select idxfile assign to "idx.dat" organization is indexed record key is idxfile-record-key alternate record key is idxfile-alt-key with duplicates.