AI engineering glossary · SQL databases
What is NULL?
NULL: A special marker meaning a value is missing or unknown, not zero or an empty string. Comparisons with it yield unknown rather than true or false, so you must test it with IS NULL. Aggregates such as AVG ignore it.
NULL: an example
A customer who has not given a phone number has phone = NULL; WHERE phone = NULL matches nothing, while WHERE phone IS NULL finds them.
Related terms
More SQL databases terms
- Object-relational mapper
- One-to-many relationship
- ORDER BY
- Parameterised query
- PARTITION BY
- Phantom read
- PostgreSQL
- Primary key
Where you learn it
NULL is part of the AI Engineering Program, which teaches it with notes, a hands-on lab and an exam. The first lesson is free.