Comma vs JOIN
A query to grab the list of phone numbers for clients who ordered in the last two weeks might be written in a couple of ways. Here are two:
SELECT * FROM clients, orders, phoneNumbers WHERE clients.id = orders.clientId AND clients.id = phoneNumbers.clientId AND orderPlaced >= NOW() - INTERVAL 2 WEEK;
SELECT * FROM clients INNER JOIN orders ON clients.id = orders.clientId INNER JOIN phoneNumbers ON clients.id = phoneNumbers.clientId WHERE orderPlaced >= NOW() - INTERVAL 2 WEEK;
Does it make a difference? Not much as written. But unless you're using a version older than 3.23.17 or 4.0.11, use the second form. Why?
- Readability. Once the WHERE clause contains more than two conditions, it becomes tedious to pick out the difference between business logic (only dates in the last two weeks) and relational logic (which fields relate clients to orders). Using the JOIN syntax with an ON clause makes the WHERE list shorter, and makes it very easy to see how tables relate to each other.
- Flexibility. Let's say we need to see all clients even if they don't have a phone number in the system. With the second version, it's easy; just change
INNER JOIN phoneNumbersto
LEFT JOIN phoneNumbers. Try that with the first version, and MySQL version 5.0.12+ will issue a syntax error because of the change in precedence between the comma operator and the JOIN keyword. The solution is to rearrange the FROM clause or add parentheses to override the precedence, and that quickly becomes frustrating.
- Portability. The changes in 5.0.12 were made to align with SQL:2003. If your queries use standard syntax, you will have an easier time switching to a different database should the need ever arise.
There is also an interesting blog entry "MySQL joins: ON vs. USING vs. Theta-style" regarding this topic.