Queries

Every SQL query starts with SELECT and FROM.

Pattern:

SELECT field1, field2
FROM tableName;

Examples (from a Books table with fields BookID, Title, Author, Year, Price):

-- Show every book's title and author
SELECT Title, Author
FROM Books;

-- Show ALL fields for every book
SELECT *
FROM Books;

The asterisk * means 'all fields'. Use it when you want everything; otherwise list the specific fields needed (uses less bandwidth, makes intent clearer).

Cambridge tip. Keywords (SELECT, FROM, WHERE, ORDER BY) are conventionally CAPITALISED; field/table names follow the database's casing. SQL is case-insensitive in keywords.

WHERE filters which rows are returned. The query only shows rows where the condition is TRUE.

Pattern:

SELECT field1, field2
FROM tableName
WHERE condition;

Examples:

-- Books published after 2020
SELECT Title, Year
FROM Books
WHERE Year > 2020;

-- Books by a specific author
SELECT *
FROM Books
WHERE Author = "Pratchett";

-- Cheap books
SELECT Title, Price
FROM Books
WHERE Price < 10;

Comparison operators.

Operator	Meaning
=	Equals
<> (or !=)	Not equal to
<	Less than
>	Greater than
<=	Less than or equal
>=	Greater than or equal
BETWEEN x AND y	Within range (inclusive)
LIKE pattern	Matches a wildcard pattern (% = any sequence)

Strings vs numbers. STRINGS need quotes ("London"); NUMBERS don't (Year > 2020). Some DBMSes are forgiving; Cambridge mark scheme expects the convention.

Combining conditions:

-- Books AFTER 2020 AND under £10
WHERE Year > 2020 AND Price < 10

-- Books by ONE of two authors
WHERE Author = "Pratchett" OR Author = "Adams"

-- Books NOT in 2024
WHERE NOT Year = 2024

AND requires BOTH conditions true. OR requires AT LEAST ONE true. NOT inverts.

Cambridge tip. Translate the natural-language question carefully. 'Books from 2020 and 2021' could be Year >= 2020 AND Year <= 2021 — using AND for an inclusive range, not OR.

ORDER BY sorts the returned rows.

Pattern:

SELECT fields
FROM tableName
WHERE condition
ORDER BY field [ASC|DESC];

Examples:

-- Books sorted by year, oldest first (default ASC)
SELECT Title, Year
FROM Books
ORDER BY Year;

-- Books sorted by year, newest first
SELECT Title, Year
FROM Books
ORDER BY Year DESC;

-- Books sorted by author A-Z, then by year newest first
SELECT *
FROM Books
ORDER BY Author ASC, Year DESC;

Defaults.

• ASC (ascending) = A-Z, 0-9, oldest-first. Default if not specified.
• DESC (descending) = Z-A, 9-0, newest-first.

Multiple sort keys. Comma-separated. Sorts by the first key; for ties, falls back to the second; and so on.

Cambridge tip. When the question says 'most recent first' or 'highest-priced first', you need DESC. Without it, the default ASC reverses the requested order.