The concept of “Null” is frequently misunderstood by people who are new to working with databases. Simply put, Null means “unassigned” or “no value”. In most types of databases (dBase and Paradox are exceptions that I know of, there are probably others…), fields that have not had a value put in them return Null when included in a select query.
Null is not the same as an empty string. It is not the same as 0 in a numeric field. It is NOT the same as False.
Most types of databases provide a means for preventing Nulls – when defining a table the fields can be specified as “Not Null” which means that the field must always have a value. The application that adds or updates data in the database is then responsible for making sure that the data is provided for those fields. However, not all databases or applications are set up this way. Also, when you use “outer” joins to join the tables in your report, there will be Nulls when the data in one table doesn’t have a match in the other table.
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