Database Concepts

Databases · 5 question types

Practise

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Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 4% of your exam marks.

stable

Rare

Stable4%

SQL SELECT queries and database structure (tables, fields, records) appear as 4 to 6 mark questions.

A typical SQL query has up to three clauses:

SELECT  field1, field2, ...
FROM    TableName
WHERE   condition;

Each line has a specific job:

Clause	What it does
SELECT	Lists which fields (columns) to display in the result
FROM	Names the table to read from
WHERE	Filters which records (rows) to include

The semicolon at the end is the SQL "end of statement" marker.

SELECT every field

To return every column for every record, use the wildcard * after SELECT:

SELECT *
FROM Customers;

This returns the whole table.

SELECT specific fields

To return only certain columns, list them by name:

SELECT FirstName, LastName, Age
FROM Customers;

Only those three columns appear in the result, even if the table has many more.

SELECT with a WHERE filter

To return only certain rows, add a WHERE clause with a condition:

SELECT *
FROM Customers
WHERE Age > 25;

Only customers older than 25 are returned.

Exam tip

Writing a SELECT query

What comes up: write a SQL query to retrieve specific fields from a table under a given condition (typically 3 marks, one per clause).

Write (three marks): (1) SELECT followed by the field names to display — or * to display every field; (2) FROM followed by the table name; (3) WHERE followed by the filter condition, with any text value in single quotes (e.g. WHERE City = 'Lagos').

Watch out: two common slip-ups cost marks independently. First, omitting quotes around text values — writing WHERE City = Lagos instead of WHERE City = 'Lagos' makes the database look for a column called Lagos, breaking the query. Second, using * when specific fields were asked for (or listing specific fields when the question says "all details") — read the question carefully to match what SELECT should list.

A customers table to practise on

Consider this Customers table:

ID	Name	Age	City	Country
101	Maya Hassan	32	Toronto	Canada
102	Liam O'Brien	25	Dublin	Ireland
103	Hiroshi Tanaka	47	Osaka	Japan
104	Priya Gupta	19	Mumbai	India

Query 1: SELECT * FROM Customers; returns all five fields for all four records (the whole table).

Query 2: SELECT ID, Name, Age FROM Customers WHERE Age > 25;

ID	Name	Age
101	Maya Hassan	32
103	Hiroshi Tanaka	47

(Liam O'Brien is excluded because his age is exactly 25, while the query asked for strictly greater than 25. Priya Gupta is excluded because 19 < 25.)