MySQL JOIN ON vs USING?

It is mostly syntactic sugar, but a couple differences are noteworthy:

ON is the more general of the two. One can join tables ON a column, a set of columns and even a condition. For example:

SELECT * FROM world.City JOIN world.Country ON (City.CountryCode = Country.Code) WHERE ...

USING is useful when both tables share a column of the exact same name on which they join. In this case, one may say:

SELECT ... FROM film JOIN film_actor USING (film_id) WHERE ...

An additional nice treat is that one does not need to fully qualify the joining columns:

SELECT film.title, film_id -- film_id is not prefixed
FROM film
JOIN film_actor USING (film_id)
WHERE ...

To illustrate, to do the above with ON, we would have to write:

SELECT film.title, film.film_id -- film.film_id is required here
FROM film
JOIN film_actor ON (film.film_id = film_actor.film_id)
WHERE ...

Notice the film.film_id qualification in the SELECT clause. It would be invalid to just say film_id since that would make for an ambiguity:

ERROR 1052 (23000): Column ‘film_id’ in field list is ambiguous

As for select *, the joining column appears in the result set twice with ON while it appears only once with USING:

mysql> create table t(i int);insert t select 1;create table t2 select*from t;
Query OK, 0 rows affected (0.11 sec)

Query OK, 1 row affected (0.00 sec)
Records: 1  Duplicates: 0  Warnings: 0

Query OK, 1 row affected (0.19 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> select*from t join t2 on t.i=t2.i;
+------+------+
| i    | i    |
+------+------+
|    1 |    1 |
+------+------+
1 row in set (0.00 sec)

mysql> select*from t join t2 using(i);
+------+
| i    |
+------+
|    1 |
+------+
1 row in set (0.00 sec)

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