Comma JOIN
- SQL course
- Intermediate
- Lesson 16/115
- 4 min
Learn the comma form of JOIN.
Comma JOIN
Writing tables separated by a comma is an implicit CROSS JOIN. Modern SQL prefers explicit CROSS JOIN.
SELECT c.name, p.name
FROM customers c, products p;This query gives a cartesian product with old syntax.
c.name, p.name is selected from customers c, products p.
Comma JOIN — details
With a WHERE filter the comma form becomes an INNER JOIN. But readability is poor.
For example, FROM orders o, customers c WHERE o.customer_id = c.id is an older style; if you forget the WHERE it silently becomes a CROSS JOIN.
Practice Comma JOIN
Replace comma FROM with explicit CROSS JOIN.
Starter code
-- explicit cross join
SELECT c.name, p.name
FROM customers cSample solution
SELECT c.name, p.name
FROM customers c
CROSS JOIN products p;Expected output
name name Ayşe Kalem Ayşe Defter
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What is comma-separated FROM equivalent to?
- INNER JOIN
- LEFT JOIN
- CROSS JOIN Correct answer
- UNION
Explanation: Comma FROM is an implicit CROSS JOIN.
2 Replace comma with
Question
SELECT c.name, p.name
FROM customers c, products p
____;
Answer
SELECT c.name, p.name
FROM customers c, products p
CROSS JOIN;Explanation: CROSS JOIN is the right answer.
3 Comma JOIN: match the terms
| Term | Meaning |
|---|---|
comma |
implicit cross |
CROSS JOIN |
explicit |
WHERE |
filter |
ON |
condition |