SQL (OCR GCSE Computer Science): Revision Note
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
|
| Example |
|---|---|---|
SELECT | Retrieves data from a database table | SELECT * SELECT name, age |
FROM | Specifies the tables to retrieve data from | SELECT * SELECT name, age |
WHERE | Filters the data based on a specified condition | SELECT * |
The '*' symbol is called a 'wildcard', it selects all fields in the table
Examples
Select all the fields from the Customers table
Command:
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:
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]
Answer
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
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