• Included columns are available only to cover to query (prevent bookmark lookups), but not for seeks. Included columns can assist in exploiting the fact that clustered indexes append its key as seeks (non-unique indexes; it appends as includes for unique indexes) at the end by limiting columns only needed to prevent bookmark lookups to the leaf level. I have several indexes with one or two seek columns and 3+ include columns (for covering the query) that rely on the clustered index key for 3rd or 4th seek columns. If these 'cover the query' only columns were seeks, it would ruin clustered index exploitation for additional seek columns.