SQL Antipatterns, Volume 1

Solution: Use Null as a Unique Value

Most problems with null values are based on a common misunderstanding of the behavior of SQL’s three-valued logic. For programmers accustomed to the conventional true/false logic implemented in most other languages, this can be a challenge. You can handle null values in SQL queries with a little study of how they work.

Null in Scalar Expressions

Suppose Stan is thirty years old, while Oliver’s age is unknown. If you ask whether Stan is older than Oliver, the only possible answer is “I don’t know.” If you ask whether Stan is the same age as Oliver, the answer is also “I don’t know.” If you ask what is the sum of Stan’s age and Oliver’s age, the answer is the same.

Charlie’s age is ...