加入两个以上表时USING和ON之间的区别

Question

假设我有三个表,其中包含以下数据:

CREATE TABLE movies (
  movie_id INT,
  movie_name VARCHAR(255),
  PRIMARY KEY (movie_id)
);
CREATE TABLE movie_ratings (
  movie_rating_id INT,
  movie_id INT,
  rating_value TINYINT,
  PRIMARY KEY (movie_rating_id),
  KEY movie_id (movie_id)
);
CREATE TABLE movie_actors (
  movie_actor_id INT,
  movie_id INT,
  actor_id INT,
  PRIMARY KEY (movie_actor_id),
  KEY movie_id (movie_id)
);
INSERT INTO movies VALUES (1, 'Titanic'),(2,'Star Trek');
INSERT INTO movie_ratings VALUES (1,1,5),(2,1,4),(3,1,5);
INSERT INTO movie_actors VALUES (1,1,2),(2,2,2);

如果我想获得每部电影的平均评分和演员数量,我可以使用以下方法JOINs:

SELECT m.movie_name, AVG(rating_value) AS avgRating, COUNT(actor_id) AS numActors
FROM movies m
LEFT JOIN movie_ratings r ON m.movie_id = r.movie_id
LEFT JOIN movie_actors a ON m.movie_id = a.movie_id
GROUP BY m.movie_id;

我们称之为查询A.查询A可以用USING这样重写:

SELECT m.movie_name, AVG(rating_value) AS avgRating, COUNT(actor_id) AS numActors
FROM movies m
LEFT JOIN movie_ratings r USING (movie_id)
LEFT JOIN movie_actors a USING (movie_id)
GROUP BY m.movie_id;

我们称之为查询B.

这两个查询都返回1作为电影'星际迷航'的numActors.所以让我们稍微修改一下这个查询:

SELECT m.movie_name, AVG(rating_value) AS avgRating, COUNT(actor_id) AS numActors
FROM movies m
LEFT JOIN movie_ratings r ON m.movie_id = r.movie_id
LEFT JOIN movie_actors a ON r.movie_id = a.movie_id
GROUP BY m.movie_id;

让我们调用这个查询C.而不是m.movie_id = a.movie_id我现在正在做r.movie_id = a.movie_id.对于查询C,numActors为0.

我的问题是:

1. 如何使用USING？编写查询？我可以吗？
2. 是USING本质上做一个ON与当前表中提到的表FROM？
3. 如果#2的答案是肯定的,那么USING当使用隐式JOIN和多个表时,该FROM怎么办？

Answer 1

1. Can C be rewritten using USING?

Yes, you can, using a nested join:

SELECT m.movie_name, AVG(rating_value) AS avgRating, COUNT(actor_id) AS numActors
FROM movies m
LEFT JOIN (
  movie_ratings r 
  LEFT JOIN movie_actors a USING (movie_id)
) USING (movie_id)
GROUP BY m.movie_id

2. Is USING essentially doing an ON with the current table and the table mentioned in FROM?

No. MySQL Documentation says:

The evaluation of multi-way natural joins differs in a very important way that affects the result of NATURAL or USING joins and that can require query rewriting. Suppose that you have three tables t1(a,b), t2(c,b), and t3(a,c) that each have one row: t1(1,2), t2(10,2), and t3(7,10). Suppose also that you have this NATURAL JOIN on the three tables:

SELECT ... FROM t1 NATURAL JOIN t2 NATURAL JOIN t3;

Previously, the left operand of the second join was considered to be t2, whereas it should be the nested join (t1 NATURAL JOIN t2). As a result, the columns of t3 are checked for common columns only in t2, and, if t3 has common columns with t1, these columns are not used as equi-join columns. Thus, previously, the preceding query was transformed to the following equi-join:

SELECT ... FROM t1, t2, t3 WHERE t1.b = t2.b AND t2.c = t3.c;

So basically, in older versions of MySQL your query B was not the same as query A, but as query C!

3. What does USING do when an implicit JOIN is used and multiple tables are in the FROM?

Again, citing the MySQL Documentation:

Previously, the comma operator (,) and JOIN both had the same precedence, so the join expression t1, t2 JOIN t3 was interpreted as ((t1, t2) JOIN t3). Now JOIN has higher precedence, so the expression is interpreted as (t1, (t2 JOIN t3)). This change affects statements that use an ON clause, because that clause can refer only to columns in the operands of the join, and the change in precedence changes interpretation of what those operands are.

It's all about join-order and precedence. So basically t1, t2 JOIN t3 USING (x) would do t2 JOIN t3 USING(x) first and join that with t1.