- LeetCode 40 – Combination Sum II
Combination Sum with two changes: each element used once, and the input may contain duplicates. The first is one character; the second is one line — and `i > start` rather than `i > 0` is the most misunderstood condition in the backtracking family. Getting it wrong does not duplicate answers, it loses them, which is far harder to notice.
- AWS – The CLI: Profiles, Queries and the Flags That Bite
Named profiles so you never run a command against the wrong account, SSO login, and `--query` — JMESPath — which turns a screenful of JSON into the one field you wanted. Then the flags that have cost real time on this site: `s3 sync` skips unchanged files so metadata never updates, `--exact-timestamps` is not a tidy-up, and `--metadata-directive REPLACE` without `--content-type` rewrites every object.
- MySQL – CROSS JOIN
Every row on the left paired with every row on the right. The Cartesian product, how CROSS JOIN differs from a comma join with no WHERE (it does not), the row count arithmetic that makes an accidental one so expensive, and the case where you actually want it: generating a complete grid — every product against every size — to find the combinations that are missing.
- MySQL – RIGHT JOIN
RIGHT JOIN is LEFT JOIN with the tables the other way round, and that is very nearly the whole lesson. What it does, the identical query written both ways, why you will almost never see one in a real codebase, and the one honest argument for it — a long chain of joins where flipping the order would mean rewriting every line.
- MySQL – LEFT JOIN
Keeping every row on the left whether or not the right side matches. Where the NULLs come from, the guest-order case the pizza schema is built around, finding rows with NO match using `WHERE right.id IS NULL`, and the single most common LEFT JOIN mistake: putting a condition on the right-hand table in WHERE instead of ON, which turns the whole thing back into an INNER JOIN.