The ON clause
- SQL course
- Beginner
- Lesson 103/115
- 4 min
Choose which column to match on.
Table aliases
In JOINs you alias table names for brevity. With FROM orders o you give the alias o.
SELECT o.id, c.name
FROM orders o
JOIN customers c ON o.customer_id = c.id;This query orders aliased as o, customers as c.
o.id, c.name is selected from orders o. customers c is joined on o.customer_id = c.id, keeping only matching rows.
Table aliases — details
Aliases shorten long table names and make queries readable. ON and SELECT use the alias; the AS keyword is optional.
Practice Table aliases
INNER JOIN orders o and customers c; select o.id and c.name.
Starter code
-- join with alias
SELECT o.id, c.name
FROM orders o
JOINSample solution
SELECT o.id, c.name
FROM orders o
JOIN customers c ON o.customer_id = c.idExpected output
id name 1 Ayşe 2 Alex
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 The ON clause: match the terms
| Term | Meaning |
|---|---|
alias |
short name |
JOIN |
join |
ON |
condition |
AS |
optional |
2 Alias
Question
SELECT o.id FROM orders ____ JOIN customers c ON o.customer_id = c.id;
Answer
SELECT o.id FROM orders o JOIN customers c ON o.customer_id = c.id;Explanation: The o written after the table name is an alias; it lets you write o.id instead of the full name.
3 What does a table alias do?
- It adds a column
- Gives the table a short name Correct answer
- Deletes rows
- Creates an index
Explanation: An alias shortens a table name.