NULL Semantics and IS NULL in SQL
NULL represents the absence of a known value. Every NULL-related behavior in PostgreSQL follows from that single fact.
NULL is SQL's way of marking a value as missing.
In real databases, missing data is everywhere. A customer signed up but never filled in their city. An employee was just hired and doesn't have a manager yet. A session started but hasn't ended. None of those are blank strings or zeros. The data simply isn't there. SQL stores that absence as NULL.
Knowing how to handle NULL is essential for any analyst working with real data. The gaps show up constantly: incomplete customer profiles, optional fields, records that are still in-progress. When you need to find those rows, or exclude them, you use IS NULL and IS NOT NULL.
To find rows where a column has no value:
SELECT name, email
FROM customers
WHERE city IS NULLSQL checks each row in customers. If city has no value recorded, the row passes. You get every customer with a missing city.
The inverse works the same way:
Swap IS NULL for IS NOT NULL when you want only rows that have a value — filtering out anything that's incomplete or uncategorized.
The one thing that trips people up: WHERE city = NULL never returns anything.
When SQL tries to compare a column to NULL using =, it can't produce a true or false answer. An unknown value compared to anything gives an unknown result. WHERE drops rows where the condition isn't clearly true. So = NULL fails silently, every time, for every row. IS NULL works because it doesn't try to compare. It asks a direct question: is this value absent? That always has a clear answer.
A 'customers' table has a 'city' column that is sometimes NULL. Which condition returns only rows where city is missing?
9 NULL Semantics and IS NULL practice problems
Write a query to return the name and email of every customer whose city has not been recorded.
Write a query to return the name and title of every such employee.
Write a query to return the ID and start time of every open session.
Write a query to return each uncategorized product's name, its original price, and its clearance price.
Write a query to return the name and title of every qualifying employee.
Write a query to return the ID and name of every root category.
Write a query to return the ID, user ID, and start time of every session that has ended.
Write a query to return the ID of every such order.
Write a query to return the ID and name of every customer with a recorded city value.
These problems are part of the NULL Semantics and IS NULL lesson in SQLMaxx, with instant grading and a worked solution on each.
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