If you have a wide existing table containing lots of frequently null columns, it is likely you can make it more efficient if you re-arrange the physical table so that most of the nulls are contiguous to the right hand side of the table. If the table is over 254 columns in certain RDBMSes, you may profit from breaking the table into two pieces such that the overflow is less often referenced and when you need the whole it is via a view slicing the two physical tables together one-for-one.
--Database tables and NULLs, stackexchange.com
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