6. Stage 4--Optimize the Database Schema : Performance Considerations : Nulls
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The use of nulls should be avoided wherever possible. Every column that is defined as WITH NULL will entail an additional column being created to support that null. This not only affects storage requirements, but has a detrimental impact on performance.
Nulls are often seen as a result of poor database design. If they must be used, however, try to consider using a value that would otherwise be out-of-range for the column instead of defining it as NULL. For example in a “number_of_dependents” column, where valid values are >=0, using a negative number to represent NULL is a better option.