Home >Database >Mysql Tutorial >Mysql's detailed explanation of JOIN
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1 Overview
- JOINExecution order of statements
- INNER/LEFT/RIGHT/FULL JOIN## The difference between
#ON- and WHERE
2 JOIN execution sequence
SELECT <row_list> FROM <left_table> <inner|left|right> JOIN <right_table> ON <join condition> WHERE <where_condition>
Its execution sequence is as follows
(The first one executed in the SQL statement is always the FROM clause):
Create a user information table:
CREATE TABLE `user_info` ( `userid` int(11) NOT NULL, `name` varchar(255) NOT NULL, UNIQUE `userid` (`userid`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
CREATE TABLE `user_account` ( `userid` int(11) NOT NULL, `money` bigint(20) NOT NULL, UNIQUE `userid` (`userid`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
Whatever Import some data:
select * from user_info; +--------+------+ | userid | name | +--------+------+ | 1001 | x | | 1002 | y | | 1003 | z | | 1004 | a | | 1005 | b | | 1006 | c | | 1007 | d | | 1008 | e | +--------+------+ 8 rows in set (0.00 sec) select * from user_account; +--------+-------+ | userid | money | +--------+-------+ | 1001 | 22 | | 1002 | 30 | | 1003 | 8 | | 1009 | 11 | +--------+-------+ 4 rows in set (0.00 sec)
A total of 8 users have user names, and 4 users have account balances.
Get the user name and balance with userid 1003, the SQL is as follows
:SELECT i.name, a.money FROM user_info as i LEFT JOIN user_account as a ON i.userid = a.userid WHERE a.userid = 1003;
SELECT * FROM user_info as i LEFT JOIN user_account as a ON 1; +--------+------+--------+-------+ | userid | name | userid | money | +--------+------+--------+-------+ | 1001 | x | 1001 | 22 | | 1002 | y | 1001 | 22 | | 1003 | z | 1001 | 22 | | 1004 | a | 1001 | 22 | | 1005 | b | 1001 | 22 | | 1006 | c | 1001 | 22 | | 1007 | d | 1001 | 22 | | 1008 | e | 1001 | 22 | | 1001 | x | 1002 | 30 | | 1002 | y | 1002 | 30 | | 1003 | z | 1002 | 30 | | 1004 | a | 1002 | 30 | | 1005 | b | 1002 | 30 | | 1006 | c | 1002 | 30 | | 1007 | d | 1002 | 30 | | 1008 | e | 1002 | 30 | | 1001 | x | 1003 | 8 | | 1002 | y | 1003 | 8 | | 1003 | z | 1003 | 8 | | 1004 | a | 1003 | 8 | | 1005 | b | 1003 | 8 | | 1006 | c | 1003 | 8 | | 1007 | d | 1003 | 8 | | 1008 | e | 1003 | 8 | | 1001 | x | 1009 | 11 | | 1002 | y | 1009 | 11 | | 1003 | z | 1009 | 11 | | 1004 | a | 1009 | 11 | | 1005 | b | 1009 | 11 | | 1006 | c | 1009 | 11 | | 1007 | d | 1009 | 11 | | 1008 | e | 1009 | 11 | +--------+------+--------+-------+ 32 rows in set (0.00 sec)
ON i .userid = a.userid After filtering, vt2 is as follows:
+--------+------+--------+-------+ | userid | name | userid | money | +--------+------+--------+-------+ | 1001 | x | 1001 | 22 | | 1002 | y | 1002 | 30 | | 1003 | z | 1003 | 8 | +--------+------+--------+-------+
SimilarlyIn this example, LEFT JOIN is used, so the left tableuser_info
Add the remaining rows to generate table vt3: +--------+------+--------+-------+
| userid | name | userid | money |
+--------+------+--------+-------+
| 1001 | x | 1001 | 22 |
| 1002 | y | 1002 | 30 |
| 1003 | z | 1003 | 8 |
| 1004 | a | NULL | NULL |
| 1005 | b | NULL | NULL |
| 1006 | c | NULL | NULL |
| 1007 | d | NULL | NULL |
| 1008 | e | NULL | NULL |
+--------+------+--------+-------+
Step 4: WHERE condition filteringWHERE a.userid = 1003 Generate table vt4:
+--------+------+--------+-------+ | userid | name | userid | money | +--------+------+--------+-------+ | 1003 | z | 1003 | 8 | +--------+------+--------+-------+
SELECT i.name, a.money Generate vt5:
+------+-------+ | name | money | +------+-------+ | z | 8 | +------+-------+
Introduction to the end of the connection After the table process, let’s look at the differences between commonly used
JOIN4 The differences between INNER/LEFT/RIGHT/FULL JOIN
is replaced If it becomes INNER JOIN, this step will be skipped, and the generated table vt3 will be exactly the same as vt2: +--------+------+--------+-------+
| userid | name | userid | money |
+--------+------+--------+-------+
| 1001 | x | 1001 | 22 |
| 1002 | y | 1002 | 30 |
| 1003 | z | 1003 | 8 |
+--------+------+--------+-------+
RIGHT JOINIf
, the generated table vt3 is as follows: +--------+------+--------+-------+
| userid | name | userid | money |
+--------+------+--------+-------+
| 1001 | x | 1001 | 22 |
| 1002 | y | 1002 | 30 |
| 1003 | z | 1003 | 8 |
| NULL | NULL | 1009 | 11 |
+--------+------+--------+-------+
Because the row userid=1009 exists in user_account (right table), but it is not found in user_info (left table) There is no record of this row, so the following row will be inserted in the third step:
| NULL | NULL | 1009 | 11 |
上文引用的文章中提到了标准SQL定义的FULL JOIN,这在mysql里是不支持的,不过我们可以通过LEFT JOIN + UNION + RIGHT JOIN 来实现FULL JOIN:
SELECT * FROM user_info as i RIGHT JOIN user_account as a ON a.userid=i.userid union SELECT * FROM user_info as i LEFT JOIN user_account as a ON a.userid=i.userid;
他会返回如下结果:
+--------+------+--------+-------+ | userid | name | userid | money | +--------+------+--------+-------+ | 1001 | x | 1001 | 22 | | 1002 | y | 1002 | 30 | | 1003 | z | 1003 | 8 | | NULL | NULL | 1009 | 11 | | 1004 | a | NULL | NULL | | 1005 | b | NULL | NULL | | 1006 | c | NULL | NULL | | 1007 | d | NULL | NULL | | 1008 | e | NULL | NULL | +--------+------+--------+-------+
ps:其实我们从语义上就能看出LEFT JOIN和RIGHT JOIN没什么差别,两者的结果差异取决于左右表的放置顺序,以下内容摘自mysql官方文档:
RIGHT JOIN works analogously to LEFT JOIN. To keep code portable across databases, it is recommended that you use LEFT JOIN instead of RIGHT JOIN.
所以当你纠结使用LEFT JOIN还是RIGHT JOIN时,尽可能只使用LEFT JOIN吧
上文把JOIN的执行顺序了解清楚之后,ON和WHERE的区别也就很好理解了。
举例说明:
SELECT * FROM user_info as i LEFT JOIN user_account as a ON i.userid = a.userid and i.userid = 1003;
SELECT * FROM user_info as i LEFT JOIN user_account as a ON i.userid = a.userid where i.userid = 1003;
第一种情况LEFT JOIN在执行完第二步ON子句后,筛选出满足i.userid = a.userid and i.userid = 1003的行,生成表vt2,然后执行第三步JOIN子句,将外部行添加进虚拟表生成vt3即最终结果:
vt2: +--------+------+--------+-------+ | userid | name | userid | money | +--------+------+--------+-------+ | 1003 | z | 1003 | 8 | +--------+------+--------+-------+ vt3: +--------+------+--------+-------+ | userid | name | userid | money | +--------+------+--------+-------+ | 1001 | x | NULL | NULL | | 1002 | y | NULL | NULL | | 1003 | z | 1003 | 8 | | 1004 | a | NULL | NULL | | 1005 | b | NULL | NULL | | 1006 | c | NULL | NULL | | 1007 | d | NULL | NULL | | 1008 | e | NULL | NULL | +--------+------+--------+-------+
而第二种情况LEFT JOIN在执行完第二步ON子句后,筛选出满足i.userid = a.userid的行,生成表vt2;再执行第三步JOIN子句添加外部行生成表vt3;然后执行第四步WHERE子句,再对vt3表进行过滤生成vt4,得的最终结果:
vt2: +--------+------+--------+-------+ | userid | name | userid | money | +--------+------+--------+-------+ | 1001 | x | 1001 | 22 | | 1002 | y | 1002 | 30 | | 1003 | z | 1003 | 8 | +--------+------+--------+-------+ vt3: +--------+------+--------+-------+ | userid | name | userid | money | +--------+------+--------+-------+ | 1001 | x | 1001 | 22 | | 1002 | y | 1002 | 30 | | 1003 | z | 1003 | 8 | | 1004 | a | NULL | NULL | | 1005 | b | NULL | NULL | | 1006 | c | NULL | NULL | | 1007 | d | NULL | NULL | | 1008 | e | NULL | NULL | +--------+------+--------+-------+ vt4: +--------+------+--------+-------+ | userid | name | userid | money | +--------+------+--------+-------+ | 1003 | z | 1003 | 8 | +--------+------+--------+-------+
如果将上例的LEFT JOIN替换成INNER JOIN,不论将条件过滤放到ON还是WHERE里,结果都是一样的,因为INNER JOIN不会执行第三步添加外部行
SELECT * FROM user_info as i INNER JOIN user_account as a ON i.userid = a.userid and i.userid = 1003;
SELECT * FROM user_info as i INNER JOIN user_account as a ON i.userid = a.userid where i.userid = 1003;
返回结果都是:
+--------+------+--------+-------+ | userid | name | userid | money | +--------+------+--------+-------+ | 1003 | z | 1003 | 8 | +--------+------+--------+-------+
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