mysql - SQL UPDATE如何匹配比类似SELECT更多的行？

Question

下面的 SELECT 查询和 UPDATE 查询具有相同的 FROM 子句和 WHERE 子句。因此，我希望 SELECT 查询返回与 UPDATE 查询匹配和影响的相同行数，但正如所见，情况并非如此。

我不知道它是否相关，但正如更新所见，我试图将记录的用户名和电子邮件保存在另一列中，但正如上次查询所见，我没有保存电子邮件，只是用户名。

为什么会这样？

mysql> SELECT p.old_username, p.username, p.old_email, u.email
    -> FROM users u INNER JOIN people p ON p.id = u.id INNER JOIN entities e ON e.id = p.id
    -> WHERE u.id =1753671666 AND u.id !=506836355 AND e.sites_id =2846702735;
+--------------+------------+-----------+---------------------+
| old_username | username   | old_email | email               |
+--------------+------------+-----------+---------------------+
| NULL         | myusername | NULL      | myemail@example.com |
+--------------+------------+-----------+---------------------+
1 row in set (0.00 sec)

mysql>
mysql> UPDATE
    -> users u INNER JOIN people p ON p.id = u.id INNER JOIN entities e ON e.id = p.id
    -> SET p.old_username = p.username, p.username = NULL,
    -> p.old_email = u.email, u.email = NULL,
    -> e.record_status = 'inactive', e.date_modified = NOW( ), e.modified_by_id =506836355
    -> WHERE u.id =1753671666 AND u.id !=506836355 AND e.sites_id =2846702735;
Query OK, 3 rows affected (0.00 sec)
Rows matched: 3  Changed: 3  Warnings: 0

mysql>
mysql> SELECT p.old_username, p.username, p.old_email, u.email
    -> FROM users u INNER JOIN people p ON p.id = u.id INNER JOIN entities e ON e.id = p.id
    -> WHERE u.id =1753671666 AND u.id !=506836355 AND e.sites_id =2846702735;
+--------------+----------+-----------+-------+
| old_username | username | old_email | email |
+--------------+----------+-----------+-------+
| myusername   | NULL     | NULL      | NULL  |
+--------------+----------+-----------+-------+
1 row in set (0.00 sec)

mysql>

Answer 1

SELECT 语句报告单行是连接查询的结果。

但是你正在更新 3 表:

p.old_username (and several others)
e.date_modified (and others)
u.email

因此有 3 个更新。