using与on的区别

SELECT * FROM film JOIN film_actor ON (film.film_id = film_actor.film_id)
SELECT * FROM film JOIN film_actor USING (film_id)
SELECT * FROM film, film_actor WHERE film.film_id = film_actor.film_id

语法糖

ON

使用 JOIN ... ON 可以将表关联的条件和记录过滤条件分开，将上面的语句重写后的结果如下：

SELECT * FROM film JOIN film_actor ON (film.film_id = film_actor.film_id) WHERE actor_id = 17 AND film.length > 120

USING

有一种特殊情况，当两个要关联表的字段名是一样的，我们可以使用 USING ，可减少 SQL 语句的长度:

SELECT * FROM film JOIN film_actor USING (film_id) WHERE actor_id = 17 AND film.length > 120

这个时候括号就是必须的了

USING 和 ON

下面语句是可行的：

SELECT film.title, film_id FROM film JOIN film_actor USING (film_id) WHERE actor_id = 17 AND film.length > 120;

但下面这个就不行：

SELECT film.title, film_id FROM film JOIN film_actor ON (film.film_id = film_actor.film_id) WHERE actor_id = 17 AND film.length > 120;

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

因为 USING "知道" film_id 字段在两个表中都有，所以没有指定确切的表都没关系，两个值必须一致就是

ON 比较灵活，必须指明要关联的表和字段名

当使用 USING 时，字段在结果中出现一次：

SELECT * FROM film JOIN film_actor USING (film_id) WHERE actor_id = 17 AND film.length > 120 LIMIT 1\G

*************************** 1. row ***************************
             film_id: 96
               title: BREAKING HOME
         description: A Beautiful Display of a Secret Agent And a Monkey who must Battle a Sumo Wrestler in An Abandoned Mine Shaft
        release_year: 2006
         language_id: 1
original_language_id: NULL
     rental_duration: 4
         rental_rate: 2.99
              length: 169
    replacement_cost: 21.99
              rating: PG-13
    special_features: Trailers,Commentaries
         last_update: 2006-02-15 05:03:42
            actor_id: 17
         last_update: 2006-02-15 05:05:03

使用 ON 时，字段会出现两次：

SELECT * FROM film JOIN film_actor ON film.film_id = film_actor.film_id WHERE actor_id = 17 AND film.length > 120 LIMIT 1\G

*************************** 1. row ***************************
             film_id: 96
               title: BREAKING HOME
         description: A Beautiful Display of a Secret Agent And a Monkey who must Battle a Sumo Wrestler in An Abandoned Mine Shaft
        release_year: 2006
         language_id: 1
original_language_id: NULL
     rental_duration: 4
         rental_rate: 2.99
              length: 169
    replacement_cost: 21.99
              rating: PG-13
    special_features: Trailers,Commentaries
         last_update: 2006-02-15 05:03:42
            actor_id: 17
             film_id: 96
         last_update: 2006-02-15 05:05:03