To define a sparse key:
file control. select fd_name assign to "user.dat" organization is indexed record key is fd-name-key alternate record key is fd-name-alt-key with duplicates suppress when all "A". ... fd fd-name. 01 fd-name-record. 03 fd-name-key pic X(3).
In this example, if a record is written for which the value of the alternate key is all A's, the actual key value is not stored in the index file.