A join combines rows from two or more tables by matching values in a shared key column. INNER JOIN keeps only the rows that match on both sides, LEFT JOIN keeps every row from the left table and fills the missing columns with NULL, and an anti-join finds rows with no match at all by adding WHERE right.id IS NULL. These questions cover the join shapes that actually show up in analytics work: fact-to-dimension lookups, self-joins over a hierarchy, and three- and four-table chains.
What you will practice
•Pick the right join type by asking which side you are allowed to lose rows from
•Spot and fix row multiplication caused by a non-unique join key
•Write anti-joins with LEFT JOIN ... IS NULL or NOT EXISTS to find missing records
•Self-join a table to compare a row against another row in the same table, such as employee to manager
•Chain three or more tables and keep the ON conditions and aliases readable
