Chad Callihan shares some advice:
In my previous blog post, we saw how using INCLUDE to add a column to an index can make a difference compared to a key column. Let’s do a quick overview of INCLUDE and when it should be used.
Included columns are columns that can added to an index as non-key columns. They are only added to the leaf nodes of an index and have a bit more flexibility. Having trouble adding a particular data type to an index? Included columns can be data types unable to be added as key columns. Are you possibly maxed out on index key columns? Use INCLUDE to add any necessary columns.
Read on for an example and note the warning that you shouldn’t just add all of the columns to the INCLUDE clause.