SQL: Using NULL values vs. default values

Question

What are the pros and cons of using NULL values in SQL as opposed to default values?

Answer 1

It depends on the situation, but it's really ultimately simple. Which one is closer to the truth?

A lot of people deal with data as though it's just data, and truth doesn't matter. However, whenever you talk to the stakeholders in the data, you find that truth always matters. sometimes more, sometimes less, but it always matters.

A default value is useful when you may presume that if the user (or other data source) had provided a value, the value would have been the default. If this presumption does more harm then good, then NULL is better, even though dealing with NULL is a pain in SQL.

Note that there are three different ways default values can be implemented. First, in the application, before inserting new data. The database never sees the difference between a default value provided by the user or one provided by the app!

Second, by declaring a default value for the column, and leaving the data missing in an insert.

Third, by substituting the default value at retrieval time, whenever a NULL is detected. Only a few DBMS products permit this third mode to be declared in the database.

In an ideal world, data is never missing. If you are developing for the real world, required data will eventually be missing. Your applications can either do something that makes sense or something that doesn't make sense when that happens.