Microsoft® SQL Server 2012 Unleashed

Column Sets

Column sets provide an alternative way to view and work with all the sparse columns in a table. The sparse columns are aggregated into a single untyped XML column, which simplifies working with many sparse columns in a table. The XML column used for a column set is similar to a calculated column in that it is not physically stored, but unlike calculated columns, it is updateable. Applications may see some performance improvement when they select and insert data by using column sets on tables that have lots of columns.

There are some restrictions on column sets:

You cannot add a column set to a table that already has sparse columns. ...