Question
In a MySQL JOIN, what is the difference between ON and USING()? As far
as I can tell, USING() is just more convenient syntax, whereas ON allows a
little more flexibility when the column names are not identical. However, that
difference is so minor, you'd think they'd just do away with USING().
Is there more to this than meets the eye? If yes, which should I use in a given situation?
Answer
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)
mysql>
