SQL Basics

SQL (Structured Query Language) is a language used to search and retrieve data from a database. Rather than writing a loop in Python to scan every record, you write a single SQL statement that describes the data you want — the database engine finds it.

OCR J277 requires knowledge of three SQL keywords: SELECT, FROM, and WHERE.

OCR J277 scope: you need SELECT, FROM, and WHERE only. Other SQL commands (INSERT, UPDATE, DELETE, ORDER BY, JOIN) are not required.

All examples in this lesson use this Students table:

StudentID	Name	Age	Subject	Grade
1	Alice	16	Maths	A
2	Bob	17	English	B
3	Carol	16	Maths	B
4	Dan	17	Science	A
5	Eve	16	English	A

SELECT specifies which columns to retrieve. FROM specifies which table to search.

Retrieve all columns from a table:

SELECT * FROM Students;

* means "all columns." This returns every field for every row in the Students table.

Retrieve specific columns only:

SELECT Name, Grade FROM Students;

Result:

Name	Grade
Alice	A
Bob	B
Carol	B
Dan	A
Eve	A

Retrieve one column:

SELECT Name FROM Students;

Returns only the Name column for all five students.

SELECT * is useful for exploring data. For real applications, listing specific columns (SELECT Name, Grade) is preferred — it makes the query intent clear and avoids returning unnecessary data.

WHERE adds a condition that each row must satisfy to be included in the results. Only rows where the condition is true are returned.

Basic WHERE syntax:

SELECT columns FROM table WHERE condition;

Example 1 — retrieve all students aged 16:

SELECT * FROM Students WHERE Age = 16;

Result:

StudentID	Name	Age	Subject	Grade
1	Alice	16	Maths	A
3	Carol	16	Maths	B
5	Eve	16	English	A

Example 2 — retrieve names of students who achieved grade A:

SELECT Name FROM Students WHERE Grade = "A";

Result: Alice, Dan, Eve.

String values in WHERE conditions are enclosed in double or single quotes: WHERE Name = "Alice". Numbers are written without quotes: WHERE Age = 16.

The three keywords work together as a single statement. The most useful queries specify both the columns to return and the rows to include.

Example — retrieve the name and subject of all 17-year-old students:

SELECT Name, Subject FROM Students WHERE Age = 17;

Result:

Name	Subject
Bob	English
Dan	Science

Example — retrieve all details of students studying Maths:

SELECT * FROM Students WHERE Subject = "Maths";

Result:

StudentID	Name	Age	Subject	Grade
1	Alice	16	Maths	A
3	Carol	16	Maths	B

Worked example — a teacher wants names and grades of students in English:

SELECT Name, Grade FROM Students WHERE Subject = "English";

Result: Bob B, Eve A.

WHERE conditions can be combined using AND and OR to filter on more than one criterion.

AND — both conditions must be true:

SELECT Name FROM Students WHERE Age = 16 AND Grade = "A";

Result: Alice, Eve. (Both are aged 16 and have grade A. Carol is 16 but grade B — excluded.)

OR — at least one condition must be true:

SELECT Name FROM Students WHERE Subject = "Maths" OR Subject = "Science";

Result: Alice, Carol, Dan. (Students studying Maths or Science.)

Combining AND and OR:

SELECT Name, Grade FROM Students WHERE Age = 17 AND Grade = "A";

Result: Dan only. (Bob is 17 but grade B.)

Use AND to narrow results (fewer rows returned), OR to broaden them (more rows returned). Both can be used in the same query.

Worked example 1 — find all Grade A students aged 16, return their names and subjects:

SELECT Name, Subject FROM Students WHERE Grade = "A" AND Age = 16;

Step-by-step:

• SELECT Name, Subject → return only these two columns
• FROM Students → search the Students table
• WHERE Grade = "A" AND Age = 16 → include rows where grade is A and age is 16

Check each row:

• Alice: Grade A ✓, Age 16 ✓ → included
• Bob: Grade B ✗ → excluded
• Carol: Grade B ✗ → excluded
• Dan: Age 17 ✗ → excluded
• Eve: Grade A ✓, Age 16 ✓ → included

Result: Alice (Maths), Eve (English).

Worked example 2 — retrieve all information about students who are either in Maths or aged 17:

SELECT * FROM Students WHERE Subject = "Maths" OR Age = 17;

Included: Alice (Maths ✓), Bob (Age 17 ✓), Carol (Maths ✓), Dan (Age 17 ✓). Eve: English, Age 16 — excluded.

1. Forgetting quotes around string values in WHERE

WHERE Name = Alice is incorrect — the database looks for a column named Alice. Write WHERE Name = "Alice". Numbers do not use quotes: WHERE Age = 16.

2. Confusing AND and OR

WHERE Subject = "Maths" AND Subject = "Science" returns zero rows — no student can simultaneously be in both subjects. For "Maths or Science", use OR.

3. Misreading SELECT *

SELECT * returns all columns, not all rows. Filtering rows is done with WHERE. Without a WHERE clause, all rows are returned regardless.

4. Wrong capitalisation of keywords

SQL keywords (SELECT, FROM, WHERE, AND) are conventionally written in uppercase, but most databases accept lowercase. In exam answers, write them in uppercase to match the expected style.

Mistake	Correction
WHERE Name = Alice	WHERE Name = "Alice"
WHERE Subject = "Maths" AND Subject = "Science"	WHERE Subject = "Maths" OR Subject = "Science"
SELECT * FROM Students to get only names	SELECT Name FROM Students