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§ cs 303 · 10 guides

CS 303 — SQL I — Reading Data

SELECT, JOIN, GROUP BY, subqueries, CTEs, set operations — reading data out of relational stores. The 80% of SQL you'll use daily.

Level
10 guides
# Code Name Level Updated
1 —
CASE Expressions Inline if/else for SQL — pivot, bucket, gate aggregates, and the only way to write conditional logic inside a SELECT.
beginner
2 —
CTEs (WITH) Common Table Expressions name a subquery so the next query can read it — the cleanest way to break a 200-line monster into 20-line steps.
intermediate
3 —
GROUP BY and Aggregation Collapse many rows into one per group — SUM, COUNT, AVG, and the rules for what can sit beside them in SELECT.
beginner
4 —
HAVING vs WHERE WHERE filters rows before grouping; HAVING filters groups after. Mixing them up is the classic beginner bug.
beginner
5 —
JOINs — Inner, Left, Right, Full, Cross Combine rows from two tables — the four flavors, when each is right, and the cross-product trap.
beginner
6 —
NULLs and Three-Valued Logic NULL is not a value, it is the absence of one. Forgetting this is the source of half the wrong answers in production SQL.
beginner
7 —
SELECT, WHERE, ORDER BY The three clauses every SQL query starts with — pick columns, filter rows, sort the result.
beginner
8 —
Set Operations — UNION / INTERSECT / EXCEPT Stack two query results vertically — UNION glues, INTERSECT keeps the overlap, EXCEPT subtracts.
beginner
9 —
Subqueries — Scalar, IN, EXISTS, Correlated Nest queries inside queries — when to reach for a subquery vs a JOIN vs a CTE, and why correlated subqueries can ruin your day.
intermediate
10 —
What is SQL — and Why It Still Wins A map of SQL and the relational model — what they are, why they survived 50 years of "SQL killers", and how to actually learn them.
beginner