SQL Antipatterns, Volume 1

Objective: Distinguish Missing Values

It’s inevitable that some data in your database has no value. Either you need to insert a row before you have discovered the values for all the columns, or else some columns have no meaningful value in some legitimate circumstances. SQL supports a special null value, corresponding to the NULL keyword.

There are many ways you can use a null value productively in SQL tables and queries:

• You can use null in place of a value that is not available at the time the row is created, such as the date of termination for an employee who is still working.

• A given column can use a null value when it has no applicable value on a given row, such as the fuel efficiency rating for a car that is fully electric.

• A function ...