Drag the edges to resize the window.

In Projects, you can keep track of your progress as you go throught the tasks. Check each item as you complete it!

Query Results

Run a query to see results.

Database Schema

Schema undefined.

Code Editor

Learn

Conditional Aggregates

NULL

While working with databases, it's common to have empty or unknown "cells" in data tables. For example, in our first flights table, we keep track of when flights arrive in a column called arr_time:

alt text

In this table, the third flight was canceled and therefore did not have an arrive time. We'll leave this cell empty, which is denoted as NULL.

What do we do when we need to test whether a value is or is not null? We use the special keywords IS NULL or IS NOT NULL in the WHERE clause (= NULL does not work).