# Comma vs JOIN

A query to retrieve 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:

```sql
SELECT *
FROM
  clients,
  orders,
  phoneNumbers
WHERE
  clients.id = orders.clientId
  AND clients.id = phoneNumbers.clientId
  AND orderPlaced >= NOW() - INTERVAL 2 WEEK;
```

```sql
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 you should use the second form, for these reasons:

* 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 phoneNumbers` to `LEFT JOIN phoneNumbers`. Try that with the first version, and recent MySQL versions 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. If your queries use standard SQL syntax, you will have an easier time switching to a different database should the need ever arise.

## See Also

* ["MySQL joins: ON vs. USING vs. Theta-style"](https://code.openark.org/blog/mysql/mysql-joins-on-vs-using-vs-theta-style) — A blog entry about this topic.
* [Joining Tables with JOIN Clauses Guide](https://mariadb.com/docs/server/mariadb-quickstart-guides/mariadb-join-guide)
* [More Advanced Joins](https://mariadb.com/docs/server/mariadb-quickstart-guides/more-advanced-joins)
* [JOIN Syntax](https://mariadb.com/docs/server/reference/sql-statements/data-manipulation/selecting-data/joins/join-syntax)
* [Joins, Subqueries and SET](https://mariadb.com/docs/server/reference/sql-structure/joins-subqueries-set)

*The initial version of this article was copied, with permission, from* [*Comma\_vs\_JOIN*](https://hashmysql.org/wiki/Comma_vs_JOIN) *on 2012-10-05.*

<sub>*This page is licensed: CC BY-SA / Gnu FDL*</sub>

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