SQL
The language every backend, data, and analytics interview still opens with. Joins, windows, and query plans — not just SELECT *.
Mastery unlocks after a 90% Quiz score
Real-world analogy
A database is a warehouse with labelled aisles. A good query is a picking list. A bad query is walking every aisle with a flashlight.
◎ Why it matters in production
Missing indexes, SELECT *, and unbounded joins are how production dashboards time out at 9am. Interviewers ask SQL because it shows whether you think in sets — or in nested for-loops.
Fundamental concepts you will master
Set thinking
SQL is not a loop. Filters, projections and joins describe what you want, not how to walk it.
Join types
Inner, left, right, full, cross — and the silent row-loss that happens when you pick the wrong one.
Indexes
B-trees under the table. Why WHERE on an unindexed column is a full scan, every time.
Window functions
RANK, LAG, running totals — analytics without collapsing the grain of the table.
Step-by-step curriculum
The relational model
Tables, keys, and why normalisation still matters when the interview whiteboard comes out.
Filtering & aggregation
WHERE vs HAVING, GROUP BY gotchas, and NULL three-valued logic.
Joins in anger
Multi-table joins, fan-out, and how to prove a result with a tiny dataset.
Windows & CTEs
Readable queries that replace correlated subqueries and temporary tables.
Plans & indexes
Read an EXPLAIN. Add the index that actually gets used.
Ready to see it in action?
Build your own path, step through operations, and watch mastery unlock after 90%.
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