create index

The INCLUDE clause, applicable only on unique indexes, specifies additional columns to be appended to the set of index key columns. Any columns included with this clause are not used to enforce uniqueness. These included columns can improve the performance of some queries through index only access.
This option might:
Eliminate the need to access data pages for more queries
Eliminate redundant indexes

https://www.ibm.com/support/knowledgecenter/SSEPGG_11.5.0/com.ibm.db2.luw.admin.dbobj.doc/doc/t0020190.html