SQL (OCR GCSE Computer Science): Revision Notes

SQL

SQL (Structured Query Language) is a standard language used to manage and query databases. It allows users to retrieve, manipulate, and update data stored in relational databases. One of the most common uses of SQL is to search for data within tables.

Using SQL to Search for Data

• SQL allows you to extract specific information from a database using queries.
• The most basic SQL query uses the SELECT statement to choose which columns of data you want to retrieve.
• You can refine your search by using the WHERE clause, which sets conditions for the data returned.

Key SQL Commands

SELECT

• This command is used to specify the columns of data you want to retrieve.
• You can select specific columns or all columns using * (which means all columns). Example:

SELECT * FROM Customers;

This will retrieve all columns from the Customers table.

FROM

• This command specifies the table you want to retrieve the data from.
• It follows the SELECT statement to indicate where the data is stored. Example:

SELECT ContactName, Address FROM Customers;

This retrieves only the ContactName and Address columns from the Customers table.

WHERE

• This command is used to philtre the results of a query based on specific conditions.
• It limits the data returned by the SELECT statement, ensuring that only rows meeting the condition are displayed. Example:

SELECT ContactName, Address FROM Customers WHERE ContactName = 'Mr Smith';

This retrieves the ContactName and Address of customers where the ContactName is "Mr Smith."

SQL Operators

Operator	Description	Example
=	Equal to	WHERE City = 'London'
<> or !=	Not equal to	WHERE Age != 30
>	Greater than	WHERE Age > 18
<	Less than	WHERE Age < 65
>=	Greater than or equal to	WHERE Price >= 10
<=	Less than or equal to	WHERE Price <= 50
BETWEEN	Used to search for values within a range	WHERE Age BETWEEN 18 AND 30
LIKE	Searches for patterns in text fields	WHERE ContactName LIKE 'Smi%'
IN	Specify multiple values for a column	WHERE City IN ('London', 'Paris')

Boolean Operators

Boolean Operator	Description	Example
AND	Both conditions must be true	WHERE Country = 'UK' AND Age >= 18
OR	At least one condition must be true	WHERE Age < 18 OR Age > 65
Parentheses ()	Used to group conditions for complex queries	WHERE Country = 'England' AND (City = 'London' OR City = 'Oxford')