SQL Intro

SQL is the language used to ask questions of a database and to change the data it holds. A database stores its data in tables, and each table holds rows and columns.

A first query

This query asks for the name of the customer with id 1. The answer is one row with one column.

SELECT name FROM customers WHERE customer_id = 1;

One column of a table

This query asks for the name of every supplier. The suppliers table holds three rows, so the answer holds three rows.

SELECT name FROM suppliers;

A question with a filter

The WHERE part narrows a question to the rows that match. This query asks for the name of the customer who lives in Japan.

SELECT name FROM customers WHERE country = 'Japan';

Example

SELECT city FROM customers WHERE name = 'Ana Silva';

Key points

Database: a stored collection of data organised in tables
Table: a set of rows and columns, like a price list
Row: one record in a table, like one customer
Column: one kind of value in a table, like a name
Query: a question written in SQL that returns rows
The sample shop database holds four tables named customers, products, orders and suppliers.
SQLite: the small database engine that runs the queries in this course

In this example, the query asks the customers table for one column, the city. The WHERE part picks the row whose name is Ana Silva. The answer is the single value Lisbon.

Try it Yourself

Edit the code, then press Run.

Running is simulated · DB Fiddle

SELECT city FROM customers WHERE name = 'Ana Silva';

The customers table

customer_id	name	city	country
1	Ana Silva	Lisbon	Portugal
2	Ben Carter	Austin	USA
3	Chen Wei	Toronto	Canada
4	Dana Novak	Prague	Czechia
5	Eli Ross	Austin	USA
6	Fatima Noor	Dubai	UAE
7	Gus Meyer	Berlin	Germany
8	Hana Sato	NULL	Japan

Exercise

What does a query return?

Rows from a table A web page A printed report A new database