A · QueryBeginner30 min read
5. Checkpoint A — SELECT & JOINs
SQL · 48 pages source format
Before aggregations and mutations, prove you can read relational data with filters and joins. Fix gaps before Phase B.
What you'll learn
- Read-path self-check
Pass criteria
All six must be true. Fix failures before GROUP BY chapter.
- 10+ saved queries in sql-notes/queries/ with descriptive filenames
- SCHEMA.md documents all sandbox tables and relationships
- WHERE query with AND/OR, NULL check, and LIKE pattern
- INNER JOIN and LEFT JOIN each used correctly
- Anti-join query: rows in A with no match in B (LEFT JOIN + IS NULL)
- Can draw a 3-table ER diagram and explain join keys aloud
Do this now
Audit sql-notes repo. Mark pass/fail in README.
Clear?
Query review
Re-read saved queries. Add comments explaining business question each answers. Remove SELECT * where present.
Do this now
Add -- comment header to each .sql file: purpose, date, author.
Clear?
Explain joins
Interview staple: "Explain INNER vs LEFT JOIN." Use customers/orders example.
Do this now
Record 90-second explanation without notes.
Clear?