当我发现ON比USING更有用的时候，我想在这里分享一下。它是在查询中引入OUTER连接时。

ON的好处在于，在维护OUTER连接的同时，允许限制查询OUTER连接到的表的结果集。试图通过指定WHERE子句来限制结果集，实际上会将OUTER连接更改为INNER连接。

尽管这可能是一个相对的极端情况。值得在.....

例如:

CREATE TABLE country (
   countryId int(10) unsigned NOT NULL PRIMARY KEY AUTO_INCREMENT,
   country varchar(50) not null,
  UNIQUE KEY countryUIdx1 (country)
) ENGINE=InnoDB;

insert into country(country) values ("France");
insert into country(country) values ("China");
insert into country(country) values ("USA");
insert into country(country) values ("Italy");
insert into country(country) values ("UK");
insert into country(country) values ("Monaco");


CREATE TABLE city (
  cityId int(10) unsigned NOT NULL PRIMARY KEY AUTO_INCREMENT,
  countryId int(10) unsigned not null,
  city varchar(50) not null,
  hasAirport boolean not null default true,
  UNIQUE KEY cityUIdx1 (countryId,city),
  CONSTRAINT city_country_fk1 FOREIGN KEY (countryId) REFERENCES country (countryId)
) ENGINE=InnoDB;


insert into city (countryId,city,hasAirport) values (1,"Paris",true);
insert into city (countryId,city,hasAirport) values (2,"Bejing",true);
insert into city (countryId,city,hasAirport) values (3,"New York",true);
insert into city (countryId,city,hasAirport) values (4,"Napoli",true);
insert into city (countryId,city,hasAirport) values (5,"Manchester",true);
insert into city (countryId,city,hasAirport) values (5,"Birmingham",false);
insert into city (countryId,city,hasAirport) values (3,"Cincinatti",false);
insert into city (countryId,city,hasAirport) values (6,"Monaco",false);

-- Gah. Left outer join is now effectively an inner join 
-- because of the where predicate
select *
from country left join city using (countryId)
where hasAirport
; 

-- Hooray! I can see Monaco again thanks to 
-- moving my predicate into the ON
select *
from country co left join city ci on (co.countryId=ci.countryId and ci.hasAirport)
; 

数据库表

为了演示USING和ON子句是如何工作的，让我们假设我们有下面的post和post_comment数据库表，它们通过post_comment表中的post_id外键列引用post表中的post_id主键列形成一对多的表关系:

父post表有3行:

| post_id | title     |
|---------|-----------|
| 1       | Java      |
| 2       | Hibernate |
| 3       | JPA       |

post_comment子表有3条记录:

| post_comment_id | review    | post_id |
|-----------------|-----------|---------|
| 1               | Good      | 1       |
| 2               | Excellent | 1       |
| 3               | Awesome   | 2       |

使用自定义投影的JOIN ON子句

传统上，在编写INNER JOIN或LEFT JOIN查询时，我们碰巧使用ON子句来定义连接条件。

例如，要获得评论及其相关的文章标题和标识符，我们可以使用下面的SQL投影查询:

SELECT
   post.post_id,
   title,
   review
FROM post
INNER JOIN post_comment ON post.post_id = post_comment.post_id
ORDER BY post.post_id, post_comment_id

然后，我们得到下面的结果集:

| post_id | title     | review    |
|---------|-----------|-----------|
| 1       | Java      | Good      |
| 1       | Java      | Excellent |
| 2       | Hibernate | Awesome   |

使用自定义投影的JOIN USING子句

当外键列与其引用的列具有相同的名称时，我们可以使用USING子句，如下例所示:

SELECT
  post_id,
  title,
  review
FROM post
INNER JOIN post_comment USING(post_id)
ORDER BY post_id, post_comment_id

并且，这个特定查询的结果集与前面使用ON子句的SQL查询相同:

| post_id | title     | review    |
|---------|-----------|-----------|
| 1       | Java      | Good      |
| 1       | Java      | Excellent |
| 2       | Hibernate | Awesome   |

USING子句适用于Oracle、PostgreSQL、MySQL和MariaDB。SQL Server不支持USING子句，因此需要使用ON子句。 USING子句可以与INNER、LEFT、RIGHT和FULL JOIN语句一起使用。

带有SELECT *的SQL JOIN ON子句

现在，如果我们改变之前的ON子句查询，使用select *选择所有列:

SELECT *
FROM post
INNER JOIN post_comment ON post.post_id = post_comment.post_id
ORDER BY post.post_id, post_comment_id

我们将得到以下结果集:

| post_id | title     | post_comment_id | review    | post_id |
|---------|-----------|-----------------|-----------|---------|
| 1       | Java      | 1               | Good      | 1       |
| 1       | Java      | 2               | Excellent | 1       |
| 2       | Hibernate | 3               | Awesome   | 2       |

可以看到，post_id是重复的，因为post和post_comment表都包含一个post_id列。

SQL JOIN USING子句与SELECT *

另一方面，如果我们运行一个SELECT *查询，其中包含JOIN条件的USING子句:

SELECT *
FROM post
INNER JOIN post_comment USING(post_id)
ORDER BY post_id, post_comment_id

我们将得到以下结果集:

| post_id | title     | post_comment_id | review    |
|---------|-----------|-----------------|-----------|
| 1       | Java      | 1               | Good      |
| 1       | Java      | 2               | Excellent |
| 2       | Hibernate | 3               | Awesome   |

您可以看到，这一次post_id列是重复数据删除的，因此结果集中只包含一个post_id列。

结论

如果数据库模式设计为外键列名与其引用的列匹配，并且JOIN条件只检查外键列值是否等于另一个表中它的镜像列的值，那么您可以使用USING子句。

否则，如果外键列名与引用列不同，或者希望包含更复杂的连接条件，则应该使用ON子句。

它主要是语法糖，但有几个差异是值得注意的:

ON是两者中更通用的一种。可以在一个列、一组列甚至一个条件上连接表。例如:

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

当两个表共享一个名称完全相同的列时，USING非常有用。在这种情况下，人们可以说:

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

一个额外的好处是不需要完全限定连接列:

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

为了说明，要使用ON完成上述操作，我们必须这样写:

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 ...

注意这部电影。SELECT子句中的film_id资格。只说film_id是无效的，因为这会导致歧义:

错误1052(23000):字段列表中的列“film_id”不明确

对于select *，使用ON时连接列在结果集中出现两次，而使用USING时只出现一次:

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>

简短的回答:

when子句有歧义 ON: when子句有不同的比较参数

当我发现ON比USING更有用的时候，我想在这里分享一下。它是在查询中引入OUTER连接时。

ON的好处在于，在维护OUTER连接的同时，允许限制查询OUTER连接到的表的结果集。试图通过指定WHERE子句来限制结果集，实际上会将OUTER连接更改为INNER连接。

尽管这可能是一个相对的极端情况。值得在.....

例如:

CREATE TABLE country (
   countryId int(10) unsigned NOT NULL PRIMARY KEY AUTO_INCREMENT,
   country varchar(50) not null,
  UNIQUE KEY countryUIdx1 (country)
) ENGINE=InnoDB;

insert into country(country) values ("France");
insert into country(country) values ("China");
insert into country(country) values ("USA");
insert into country(country) values ("Italy");
insert into country(country) values ("UK");
insert into country(country) values ("Monaco");


CREATE TABLE city (
  cityId int(10) unsigned NOT NULL PRIMARY KEY AUTO_INCREMENT,
  countryId int(10) unsigned not null,
  city varchar(50) not null,
  hasAirport boolean not null default true,
  UNIQUE KEY cityUIdx1 (countryId,city),
  CONSTRAINT city_country_fk1 FOREIGN KEY (countryId) REFERENCES country (countryId)
) ENGINE=InnoDB;


insert into city (countryId,city,hasAirport) values (1,"Paris",true);
insert into city (countryId,city,hasAirport) values (2,"Bejing",true);
insert into city (countryId,city,hasAirport) values (3,"New York",true);
insert into city (countryId,city,hasAirport) values (4,"Napoli",true);
insert into city (countryId,city,hasAirport) values (5,"Manchester",true);
insert into city (countryId,city,hasAirport) values (5,"Birmingham",false);
insert into city (countryId,city,hasAirport) values (3,"Cincinatti",false);
insert into city (countryId,city,hasAirport) values (6,"Monaco",false);

-- Gah. Left outer join is now effectively an inner join 
-- because of the where predicate
select *
from country left join city using (countryId)
where hasAirport
; 

-- Hooray! I can see Monaco again thanks to 
-- moving my predicate into the ON
select *
from country co left join city ci on (co.countryId=ci.countryId and ci.hasAirport)
; 

维基百科有以下关于使用的信息:

然而，USING结构不仅仅是语法糖，因为 属性的版本的结果集与之不同 明确的谓词。特别是USING中提到的任何列 List将只出现一次，带有一个非限定名称，而不是一次 对于连接中的每个表。在上面的例子中，会有一个单点 departmentd列，无员工。DepartmentID或 department.DepartmentID。

它所谈论的表格:

Postgres文档也很好地定义了它们:

The ON clause is the most general kind of join condition: it takes a Boolean value expression of the same kind as is used in a WHERE clause. A pair of rows from T1 and T2 match if the ON expression evaluates to true. The USING clause is a shorthand that allows you to take advantage of the specific situation where both sides of the join use the same name for the joining column(s). It takes a comma-separated list of the shared column names and forms a join condition that includes an equality comparison for each one. For example, joining T1 and T2 with USING (a, b) produces the join condition ON T1.a = T2.a AND T1.b = T2.b. Furthermore, the output of JOIN USING suppresses redundant columns: there is no need to print both of the matched columns, since they must have equal values. While JOIN ON produces all columns from T1 followed by all columns from T2, JOIN USING produces one output column for each of the listed column pairs (in the listed order), followed by any remaining columns from T1, followed by any remaining columns from T2.