Choosing the Right Join Type
A decision framework for picking the correct join from a word problem.
Picking the Join From a Word Problem
The highest-value join skill in an interview is not syntax, it is translating a plain-English requirement into the correct join type. Most candidates know how each join works but freeze when they must choose one under pressure.
This lesson gives you a repeatable decision framework so you can name the right join in seconds.
The Core Question: Which Rows Must Survive?
Every join choice reduces to one question: which side's unmatched rows do you need to keep?
- Keep only matched rows on both sides:
INNER JOIN. - Keep all left rows, matched or not:
LEFT JOIN. - Keep all rows from both sides:
FULL OUTER JOIN. - Keep every combination:
CROSS JOIN.
Decide what must survive first; the join type follows automatically.
All lessons in this course
- CROSS JOIN and Cartesian Products
- SELF JOIN for Hierarchies
- Comparing Rows Within One Table
- Choosing the Right Join Type