✅ Quiz 12 — Databases
Think first, then open the fold. Getting one wrong and re-reading the section is the whole point.
1. In plain words: what is a database, and what are its two ingredients?
Show answer
An organized collection of data, plus the guardian software that stores it, protects it, enforces the rules, and answers questions about it — the filing cabinet *and* the meticulous librarian, sold as a package.2. Give two reasons a shared spreadsheet fails as the data home for an app with 10,000 users.
Show answer
Any two of: (1) no referee for simultaneous edits — users trample each other's changes; (2) no rule enforcement — nothing stops "banana" in a date column or duplicate customer numbers; (3) no protection against half-saves — a crash mid-edit can corrupt or lose data with no all-or-nothing guarantee.3. A money transfer subtracts $100 from one account and adds $100 to another. What does wrapping those two steps in a transaction guarantee?
Show answer
That both steps happen or neither does — all-or-nothing. Even if the power dies between them, the database rolls the half-done work back, so money can never vanish from one account without arriving in the other. (The famous "ACID" letters are shorthand for this family of promises.)4. Why does the orders table store customer_id = 1 instead of copying Amara’s
name and address onto every order?
Show answer
So each fact lives in exactly one place. If Amara moves, one row in `customers` gets updated and every order automatically "knows", because orders only point to her by ID. Copies drift out of sync — the day you miss one, the cake ships to the old address.5. What is a primary key? What everyday document did the lesson compare it to?
Show answer
A column whose value uniquely identifies each row — no two rows may share it. The lesson compared it to a passport number: two people can share a name, never a passport number.6. Translate this query into plain English:
SELECT name FROM customers WHERE city = 'Lagos';
Show answer
"Librarian, from the customers table, give me the name of every customer whose city is Lagos." SELECT = which column I want; FROM = which table; WHERE = which rows qualify.7. Why do engineers get nervous around DELETE — and what one missing word turns it from routine into catastrophe?
Show answer
DELETE removes rows permanently, with no confirmation and no undo. Without a WHERE clause, `DELETE FROM customers;` removes *every* row in the table. The missing word is WHERE. That's why teams rely on backups, permissions, and the habit of running the WHERE as a SELECT first.8. What is a database index, what does it speed up, and what two costs does it add?
Show answer
A separate, sorted lookup structure — like the index at the back of a book — that lets the database jump to matching rows instead of scanning the whole table. It speeds up reads on the indexed column. Costs: extra storage space, and extra upkeep on every insert or update, which slows writes slightly.9. Filing cabinet vs. flexible closet: what’s the core difference between relational and NoSQL databases, and name one example of a NoSQL product.
Show answer
Relational databases require every record to fit strict pre-defined tables (the pre-printed forms in the cabinet) — rigid but consistent and great for cross-referencing. NoSQL databases store flexible labeled documents or key-value pairs (boxes in a closet) — each record can have its own shape, at the price of fewer guarantees. Examples: MongoDB (documents) or Redis (key-value).10. Your friend keeps all family photos on one laptop. Apply the 3-2-1 idea to their situation.
Show answer
Keep 3 copies (laptop + external drive + cloud), on 2 different kinds of storage (the drive and the cloud count), with 1 copy far away (the cloud, or a drive at a relative's house). One flood or one theft should never be able to reach all copies.11. Name two different jobs whose daily work involves querying databases.
Show answer
Backend engineers (most API calls end in a database query) and data analysts (who spend all day asking business questions in SQL). Also acceptable: operations/ database administrators who keep the librarian healthy and backed up.12. 🗣️ Explain it to a friend. Your cousin asks: “Why do banks need ‘databases’? Couldn’t they keep everyone’s balance in a big Excel file?” Answer out loud in under a minute. Bonus points for using the librarian, the two-hands money move, or the passport number.