问题
In this case, a left join is the same as a right join?
mysql>
mysql>
mysql> use usenet;show tables;describe ARTICLE;describe NEWSGROUP;
Database changed
+------------------+
| Tables_in_usenet |
+------------------+
| ARTICLE |
| NEWSGROUP |
+------------------+
2 rows in set (0.00 sec)
+---------------+------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+---------------+------------+------+-----+---------+----------------+
| ID | bigint(20) | NO | PRI | NULL | auto_increment |
| MESSAGENUMBER | int(11) | YES | | NULL | |
| NEWSGROUP_ID | bigint(20) | YES | MUL | NULL | |
+---------------+------------+------+-----+---------+----------------+
3 rows in set (0.01 sec)
+-----------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-----------+--------------+------+-----+---------+----------------+
| ID | bigint(20) | NO | PRI | NULL | auto_increment |
| NEWSGROUP | varchar(255) | YES | | NULL | |
+-----------+--------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)
mysql>
mysql> select * from ARTICLE right join NEWSGROUP on ARTICLE.NEWSGROUP_ID=NEWSGROUP.ID;
+------+---------------+--------------+----+-------------------------------+
| ID | MESSAGENUMBER | NEWSGROUP_ID | ID | NEWSGROUP |
+------+---------------+--------------+----+-------------------------------+
| 1 | 4 | 1 | 1 | gwene.com.androidcentral |
| 2 | 5 | 1 | 1 | gwene.com.androidcentral |
| 3 | 6 | 1 | 1 | gwene.com.androidcentral |
| 4 | 7 | 1 | 1 | gwene.com.androidcentral |
| 5 | 8 | 1 | 1 | gwene.com.androidcentral |
| 6 | 9 | 1 | 1 | gwene.com.androidcentral |
| 7 | 10 | 1 | 1 | gwene.com.androidcentral |
| 8 | 11 | 1 | 1 | gwene.com.androidcentral |
| 9 | 4 | 2 | 2 | gwene.com.blogspot.emacsworld |
| 10 | 4 | 3 | 3 | gwene.com.blogspot.googlecode |
| 11 | 5 | 3 | 3 | gwene.com.blogspot.googlecode |
| 12 | 6 | 3 | 3 | gwene.com.blogspot.googlecode |
| 13 | 7 | 3 | 3 | gwene.com.blogspot.googlecode |
| 14 | 8 | 3 | 3 | gwene.com.blogspot.googlecode |
| 15 | 9 | 3 | 3 | gwene.com.blogspot.googlecode |
| 16 | 10 | 3 | 3 | gwene.com.blogspot.googlecode |
| 17 | 11 | 3 | 3 | gwene.com.blogspot.googlecode |
| 18 | 4 | 4 | 4 | gwene.com.economist |
| 19 | 5 | 4 | 4 | gwene.com.economist |
| 20 | 6 | 4 | 4 | gwene.com.economist |
| 21 | 7 | 4 | 4 | gwene.com.economist |
| 22 | 8 | 4 | 4 | gwene.com.economist |
| 23 | 9 | 4 | 4 | gwene.com.economist |
| 24 | 10 | 4 | 4 | gwene.com.economist |
| 25 | 11 | 4 | 4 | gwene.com.economist |
+------+---------------+--------------+----+-------------------------------+
25 rows in set (0.00 sec)
mysql>
mysql> select * from ARTICLE left join NEWSGROUP on ARTICLE.NEWSGROUP_ID=NEWSGROUP.ID;
+----+---------------+--------------+------+-------------------------------+
| ID | MESSAGENUMBER | NEWSGROUP_ID | ID | NEWSGROUP |
+----+---------------+--------------+------+-------------------------------+
| 1 | 4 | 1 | 1 | gwene.com.androidcentral |
| 2 | 5 | 1 | 1 | gwene.com.androidcentral |
| 3 | 6 | 1 | 1 | gwene.com.androidcentral |
| 4 | 7 | 1 | 1 | gwene.com.androidcentral |
| 5 | 8 | 1 | 1 | gwene.com.androidcentral |
| 6 | 9 | 1 | 1 | gwene.com.androidcentral |
| 7 | 10 | 1 | 1 | gwene.com.androidcentral |
| 8 | 11 | 1 | 1 | gwene.com.androidcentral |
| 9 | 4 | 2 | 2 | gwene.com.blogspot.emacsworld |
| 10 | 4 | 3 | 3 | gwene.com.blogspot.googlecode |
| 11 | 5 | 3 | 3 | gwene.com.blogspot.googlecode |
| 12 | 6 | 3 | 3 | gwene.com.blogspot.googlecode |
| 13 | 7 | 3 | 3 | gwene.com.blogspot.googlecode |
| 14 | 8 | 3 | 3 | gwene.com.blogspot.googlecode |
| 15 | 9 | 3 | 3 | gwene.com.blogspot.googlecode |
| 16 | 10 | 3 | 3 | gwene.com.blogspot.googlecode |
| 17 | 11 | 3 | 3 | gwene.com.blogspot.googlecode |
| 18 | 4 | 4 | 4 | gwene.com.economist |
| 19 | 5 | 4 | 4 | gwene.com.economist |
| 20 | 6 | 4 | 4 | gwene.com.economist |
| 21 | 7 | 4 | 4 | gwene.com.economist |
| 22 | 8 | 4 | 4 | gwene.com.economist |
| 23 | 9 | 4 | 4 | gwene.com.economist |
| 24 | 10 | 4 | 4 | gwene.com.economist |
| 25 | 11 | 4 | 4 | gwene.com.economist |
+----+---------------+--------------+------+-------------------------------+
25 rows in set (0.00 sec)
mysql>
mysql>
回答1:
Codeproject has this image which explains the simple basics of SQL joins, taken from: http://www.codeproject.com/KB/database/Visual_SQL_Joins.aspx SQL joins explained
回答2:
Not really, because RIGHT JOIN and LEFT JOIN are symmetric. That is:
A LEFT JOIN B = B RIGHT JOIN A
RIGHT JOIN is merely syntactic sugar. The optimizer can rewrite a right join to a left join:
mysql> explain extended select * from t right join t t2 using (c1)\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: t2
type: index
possible_keys: NULL
key: c2
key_len: 5
ref: NULL
rows: 4201
filtered: 100.00
Extra: Using index
*************************** 2. row ***************************
id: 1
select_type: SIMPLE
table: t
type: eq_ref
possible_keys: PRIMARY
key: PRIMARY
key_len: 4
ref: test.t2.c1
rows: 1
filtered: 100.00
Extra:
2 rows in set, 1 warning (0.00 sec)
Note the LEFT JOIN in the optimizer re-write (the tables are swapped):
mysql> show warnings\G
*************************** 1. row ***************************
Level: Note
Code: 1003
Message: select `test`.`t2`.`c1` AS `c1`,`test`.`t2`.`c2` AS `c2`,`test`.`t`.`c2` AS
`c2` from `test`.`t` `t2` left join `test`.`t` on((`test`.`t`.`c1` = `test`.`t2`.`c1`)) where 1
1 row in set (0.00 sec)
Notice that (A RIGHT JOIN B != A LEFT JOIN B) unless (A INNER JOIN B = A LEFT JOIN B). This is because A RIGHT JOIN B is not symmetrical with A LEFT JOIN B (it is symmetrical with B LEFT JOIN A).
In your case, A RIGHT JOIN B will be the same as A LEFT JOIN B unless there are NULL values in the columns you are joining. If there are NULL values, then A LEFT JOIN B will NOT be the same as A RIGHT JOIN B. If you add new articles without adding the associated newsgroup (or vice-versa) then the results would change too.
回答3:
With your current data, yes they are the same. However as NEWSGROUP_ID is nullable then they could be different as the data changes.
Personally I always use LEFT JOINs if possible (from primary table to child table), in fact I have only needed to use a RIGHT JOIN a couple of times in over 6 years of SQL development!
来源：https://stackoverflow.com/questions/11807226/right-join-versus-left-join