The concept of included columns in indexes was introduced in SQL Server 2005 and is also available in SQL Server 2008 and 2012. We can include non-key columns in a non-clustered index, as they are not counted in its index size.
There is a limitation wherein the maximum number of columns allowed is 16 and the maximum size of the index key column allowed is 900 bytes, so it is not a good idea to have an index with many unnecessary or non-key columns.
Only include the key column in the Index part, and to avoid the lookup of a non-key column, keep another non-key column in the
INCLUDE part of a non-clustered Index. This is because any column given in the
INCLUDE part of a non-clustered index doesn't ...