Slava Murygin discusses sparse columns:
“Sparsing” is the way SQL Server optimizes spacing for NULL values at the cost of overhead for non-NULL values.
In other words, if you expect having in your column more nulls than not nulls you can SPARSE that column to optimize the space.I’ve seen the situations when a lot of columns in Data Mart tables were almost completely filled with NULLS and I started wondering if “SPARSE” can be a good tool to gain some space.
Read the whole thing. I am not a fan of sparse columns because they prohibit things like page-level compression. Be sure to read the restrictions on using sparse columns before you give them a try; on net, I think they’re more trouble than they’re worth except in edge cases like extremely denormalized tables collecting thousands of data points from sensors.