SQL (OCR GCSE Computer Science)

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)

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

Examples

• Select all the fields from the Customers table

Command:

SELECT * FROM Customers;

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:

SELECT ID, name, age  FROM Customers WHERE Age > 25;

Output:

ID	Name	Age
1	John Doe	30
3	Peter Lee	40