Question

我有以下查询：

UPDATE lessonstatus
INNER JOIN user ON lessonstatus.user_id = user.user_id
SET user_id = (SELECT user_id FROM user WHERE username = 'too_many_accounts')
WHERE last_name = 'stupid' 
AND first_name = 'user'
AND username != 'too_many_accounts'
AND lessonstatus.lesson_id NOT IN (SELECT lesson_id FROM lessonstatus WHERE user_id = 1);

但是，尝试执行时会出现以下错误：

Error Code : 1093
You can't specify target table 'lessonstatus_rtab' for update in FROM clause

如何修复此查询以使其有效？

Answer 1

您不能在表中（即使在子查询中）SELECT在同一查询中更新。这就是错误“无法指定目标表”的意思。

但您可以在user语句中多次加入lessonstatus和UPDATE，并创造性地使用加入条件来挑选您想要的各个行。

使用联接模拟NOT IN的方法是LEFT OUTER JOIN。如果该联接的右侧不匹配，那么NOT IN将成立。

UPDATE lessonstatus l1
  INNER JOIN user u1 ON (l1.user_id = u1.user_id)
  INNER JOIN user u2 ON (u2.username = 'too_many_accounts')
  LEFT OUTER JOIN lessonstatus l2 
    ON (l1.lesson_id = l2.lesson_id AND l2.user_id = 1)
SET l1.user_id = u2.user_id
WHERE u1.last_name = 'stupid' AND u1.first_name = 'user'
  AND u1.username != 'too_many_accounts'
  AND l2.lesson_id IS NULL; -- equivalent to "l NOT IN l2"

nb：我已经测试了此查询的语法，但没有测试真实数据。无论如何，它应该让你开始。

Answer 2

还有更多错误（“user”表和“user_rtab”别名不匹配，建议不要使用非限定字段名称），但UPDATE语法本身应该类似：

UPDATE lessonstatus
SET user_id = (SELECT TOP 1 user_id FROM user WHERE username = 'too_many_accounts')
FROM lessonstatus
    INNER JOIN user ON lessonstatus.user_id = user_rtab.user_id
WHERE last_name = 'stupid' 
    AND first_name = 'user'
    AND username != 'too_many_accounts'
    AND lessonstatus.lesson_id NOT IN (
        SELECT lesson_id FROM lessonstatus WHERE user_id = 1
    );

如何修复此MySQL查询以便它可以工作？

2 个答案: