Real data is rarely tidy. A column may contain NULL, an empty string, or whitespace; other values may belong to a set, fall in a range, or match a text pattern. This chapter builds on WHERE conditions and introduces operators for these common filtering tasks.
The lesson uses a small, deliberately messy users dataset. Examples are broadly supported SQL unless a dialect is identified; data types, date/time handling, pattern escaping, and other details can vary between DBMSs.