[英]Order of precedence of UPDATE on Multiple-table update
如果我有一個UPDATE語句,我在做SET p.old_email = u.email, u.email = NULL
, p.old_email = u.email
總是會在u.email = NULL
之前發生嗎? 我問的原因是我沒有目睹這種行為。
mysql> SHOW TRIGGERS;
UPDATE
Empty set (0.01 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 = NULL, p.username = 'myusername',
-> p.old_email = NULL, u.email = 'myemail@example.com',
-> 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 |
+--------------+------------+-----------+---------------------+
| 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
-> WHERE u.id =1753671666 AND u.id !=506836355 AND e.sites_id =2846702735;
WHERE u.id =1753671666 AND u.id !=506836355 AND e.sites_id =2846702735Query OK, 1 row affected (0.00 sec);
Rows matched: 1 Changed: 1 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 | 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_email = u.email, u.email = NULL
-> WHERE u.id =1753671666 AND u.id !=506836355 AND e.sites_id =2846702735;
Query OK, 1 row affected (0.00 sec)
Rows matched: 2 Changed: 1 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>
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 = NULL, p.username = 'myusername',
-> p.old_email = NULL, u.email = 'myemail@example.com',
-> 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, 2 rows affected (0.00 sec)
Rows matched: 3 Changed: 2 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 |
+--------------+------------+-----------+---------------------+
| 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, 2 rows affected (0.00 sec)
Rows matched: 3 Changed: 2 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>
mysql>
從MySQL文檔 :
以下語句中的第二個賦值將col2設置為當前(更新的)col1值,而不是原始col1值。 結果是col1和col2具有相同的值。 此行為與標准SQL不同。
UPDATE t1 SET col1 = col1 + 1, col2 = col1;
單表UPDATE分配通常從左到右進行評估。 對於多表更新,無法保證以任何特定順序執行分配。
您正在執行多表更新,並且列文檔分配不會按照文檔說明的特定順序進行。
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