Why this chapter matters
Key metrics usually require data spread across entities such as users, orders, plans, and events.
What you will learn
- Choose INNER and LEFT joins based on whether unmatched rows must remain visible.
- Write explicit ON clauses that protect row cardinality.
- Diagnose duplicate rows caused by one-to-many joins.
Lessons in this chapter
- Join types and result meaningInterpret how PostgreSQL join types include or exclude unmatched rows. Read the full guide →
- Join keys and grainMatch tables on stable keys and check the grain of each dataset before joining.
- Preventing accidental fan-outUse pre-aggregation or unique keys to avoid inflated counts after joins.
- Readable multi-table SQLAlias tables clearly and keep join predicates separate from business filters.
Study task
Join users, subscriptions, and invoices in PostgreSQL to list active subscribers and their latest paid invoice date.
Chapter checkpoint
Which join keeps all rows from the left table even when there is no match on the right in PostgreSQL?
LEFT JOIN keeps all rows from the left table and fills unmatched right-side columns with NULL.