SQL (OCR GCSE Computer Science)

Revision Note

Test yourself Flashcards

Written by: Robert Hampton

Reviewed by: James Woodhouse

SQL

What is SQL?

SQL (Structured Query Language) is a programming language used to interact with a DBMS.
SQL allows users to locate specific information in a database table using these basic SQL commands:
- Select - what field(s) do you want to retrieve?
- From - which table(s) do you want to search?
- Where - what condition is there?

Selecting Data Commands

Command	Description	Example
SELECT	Retrieves data from a database table	SELECT * (retrieves all data from the table) SELECT name, age (retrieves names and ages from the table)
FROM	Specifies the tables to retrieve data from	SELECT * FROM users; (retrieves all data from the 'users' table) SELECT name, age FROM users; (retrieves names and ages from the 'users' table)
WHERE	Filters the data based on a specified condition	SELECT * FROM users WHERE age > 30; (Retrieves all users older than 30)

Command

Description

Example

SELECT

Retrieves data from a database table

SELECT *
(retrieves all data from the table)

SELECT name, age
(retrieves names and ages from the table)

FROM

Specifies the tables to retrieve data from

SELECT *
FROM users;
(retrieves all data from the 'users' table)

SELECT name, age
FROM users;
(retrieves names and ages from the 'users' table)

WHERE

Filters the data based on a specified condition

SELECT *
FROM users
WHERE age > 30;
(Retrieves all users older than 30)

The '*' symbol is called a 'wildcard', it selects all fields in the table

Examples

Select all the fields from the Customers table

Command:

SQL query snippet: "SELECT * FROM Customers;" in a code editor highlighting "SELECT" in purple and "FROM" in pink, both on a black background.

Output:

ID	Name	Age	City	Country
1	John Doe	30	New York	USA
2	Jane Doe	25	London	UK
3	Peter Lee	40	Paris	France

Select the ID, name & age of customers who are older than 25

Command:

SQL query written on a black background with colored keywords. The query selects the ID, name, and age from the Customers table where age is greater than 25.

Output:

ID	Name	Age
1	John Doe	30
3	Peter Lee	40

Worked Example

The database table Stock stores the current stock levels of products currently on sale.

ProdName	Quantity	Price
Biscuits	143	0.99
Bread	87	1.49
Milk	34	1.10
Pasta	421	0.89
Ketchup	287	2.99

Complete the SQL query to return the product name and quantity of all products that's price is less than £1 [3]

Table with three rows labeled SELECT, FROM, and WHERE, commonly used in SQL query structure. Each label is in a separate cell in the first column.

Answer

SQL query for selecting "ProdName" and "Quantity" from "Stock" table where "Price" is less than 1, shown in a table format.

Guidance

Spelling of field names and table name must be exact
Capitalisation must match field and table names to be awarded marks
Table name is always in the question in different format

Last updated: 29 July 2024

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