Clustered index keys must not be used as include column

Mostly of tables have a clustered index. In addition, non-clustered indexes also created in need. To improve index performance, some columns may be included to non-clustered index.


At this point it is important that clustered index keys are not included. Because clustered index keys already exists in non-clustered indexes, including clustered index keys to non-clustered is useless.  Beside this, it has adverse effect, like index size gets bigger just for nothing.