SQL — SELECT statements

SQL (Structured Query Language) is the standard language for querying relational databases. The most common operation is the SELECT statement, which retrieves data without changing it.

Basic structure

SELECT column1, column2
FROM tableName
WHERE condition
ORDER BY column;

• SELECT — which columns you want.
• FROM — which table to read from.
• WHERE — filter rows by condition (optional).
• ORDER BY — sort the results (optional).

Use * for "all columns": SELECT * FROM Students;.

Filtering with WHERE

Comparison operators: =, <> (or !=), <, <=, >, >=. Combine conditions with AND and OR; negate with NOT.

SELECT Name, DOB
FROM Students
WHERE TutorGroup = '10A';

SELECT * FROM Books
WHERE Author = 'Tolkien' AND Year > 1950;

SELECT *
FROM Students
WHERE TutorGroup = '10A' OR TutorGroup = '10B';

LIKE for pattern matching

SELECT * FROM Books WHERE Title LIKE 'The%';

• % matches any sequence of characters.
• _ matches a single character.

So 'The%' finds anything starting with "The".

IN — list membership

SELECT * FROM Students WHERE TutorGroup IN ('10A', '10B', '10C');

Equivalent to using OR repeatedly.

BETWEEN — range

SELECT * FROM Books WHERE Year BETWEEN 1990 AND 2000;

Inclusive of both endpoints.

Sorting with ORDER BY

SELECT Name, Mark FROM Students ORDER BY Mark DESC;

• ASC — ascending (default).
• DESC — descending.

You can sort by multiple columns:

SELECT * FROM Students ORDER BY TutorGroup ASC, Name ASC;

Selecting specific columns

SELECT Name, DOB FROM Students;

Returns just those two columns, not all data — better for performance and clarity.

Joining tables (Higher / extension)

SELECT Students.Name, Subjects.Name AS Subject, Enrolments.Grade
FROM Students
JOIN Enrolments ON Students.StudentID = Enrolments.StudentID
JOIN Subjects   ON Enrolments.SubjectID = Subjects.SubjectID
WHERE Students.TutorGroup = '10A';

A JOIN combines rows across tables using their key relationships. AQA's GCSE focuses on simple SELECTs but joins appear in some questions.

Aggregate functions (extension)

SELECT COUNT(*) FROM Students;
SELECT AVG(Mark) FROM Enrolments WHERE SubjectID = 'S01';
SELECT MIN(Year), MAX(Year) FROM Books;

COUNT, AVG, MIN, MAX, SUM — useful summaries.

BookID | Title              | Author        | Year
1      | The Hobbit         | Tolkien       | 1937
2      | LOTR               | Tolkien       | 1954
3      | Foundation         | Asimov        | 1951
4      | I, Robot           | Asimov        | 1950
5      | Dune               | Herbert       | 1965

Title       | Year
The Hobbit  | 1937
LOTR        | 1954

Style tips

• Use uppercase for SQL keywords by convention.
• Use single quotes for string literals ('10A'), not double.
• End each statement with a semicolon.
• Indent for readability when statements get long.