Lesson 3 of 7
Lesson 3 — SQL Basics: SELECT, WHERE, ORDER BY
Ask a database questions in its own language and get exactly the rows you want.
Learn it
SQL (Structured Query Language) is how you talk to a relational database.
SELECT chooses columns, FROM chooses the table, WHERE filters rows, ORDER BY sorts and LIMIT caps the result.
SQL is declarative: you say what you want, not how to fetch it. The query planner works out the how.
Use single quotes for text, % as a wildcard with LIKE, and IS NULL (never = NULL) to test for missing values.
Key terms
- SQL
- Structured Query Language, the standard language for relational databases.
- DDL / DML
- Data Definition Language (structure) vs Data Manipulation Language (data).
- Predicate
- The condition in a WHERE clause that each row must satisfy.
- NULL
- Unknown/missing — compared with IS NULL, not = NULL.
- Parameterised query
- A query with placeholders for user input; blocks SQL injection.
Build a query one clause at a time
- 1FROM: SELECT * FROM student; — every row, every column.
- 2SELECT: SELECT name, year_group FROM student; — fewer columns, less data moved.
- 3WHERE: … WHERE year_group = 10 AND house = 'Ash';
- 4ORDER BY: … ORDER BY name ASC;
- 5LIMIT: … LIMIT 10; — top ten only, great while testing.
A realistic query
sqlSELECT name,
year_group,
house
FROM student
WHERE year_group >= 10
AND house IN ('Ash', 'Birch')
AND left_school_on IS NULL
ORDER BY year_group DESC, name ASC
LIMIT 20;Filter first, sort second, limit last. IS NULL finds students who have not left — using = NULL would silently return nothing.
Try it
Complete the query that finds active Year 11 students, newest first.
SELECT name FROM student
WHERE year_group = 11
AND left_school_on ______
ORDER BY joined_on DESC;Challenge
Write SQL for a shop's `orders` table (id, customer_id, total, status, placed_at) that returns the 5 largest completed orders placed this year, showing only id, customer_id and total.
Pick whichever way suits you — every mode earns the same bonus XP.
Write at least 40 more characters to submit.
Mark your own work
Guided walkthrough — 0/3 clues revealed
- Clue 1 locked — reveal it only if you get stuck.
- Clue 2 locked — reveal it only if you get stuck.
- Clue 3 locked — reveal it only if you get stuck.
Each clue costs 4 XP (never below 20 XP). You'd earn 40 XP right now.