“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.