Learn fundamentals

Questions that build the mental models behind the language. No hand-holding - answer, see why, move on.

SQL

SELECT, filtering, joins, aggregation, window functions, and the NULL edge cases that catch everyone off guard.

Each topic builds on the one before. Start at step 1 and work down.

Foundation

where everything starts

WHERE clause

the absent value

COUNT, SUM, AVG

GROUP BY

HAVING

Core

most tested concept

cleaner than nested subqueries

hierarchies and sequences

UNION, INTERSECT, EXCEPT

Advanced

hardest topic, do last