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 1G
*************************** 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 1G
*************************** 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