如何在UPDATE语句中使用JOIN?

我有一个这样的表:

// QandA

+----+----------------------------------------+---------+----------+-----------+

| Id | body | related | accepted | author_id |

+----+----------------------------------------+---------+----------+-----------+

| 1 | content of question1 | null | null | 12345 |

| 2 | content of first answer for question1 | 1 | 0 | 53456 |

| 3 | content of question2 | null | null | 43634 |

| 4 | content of second answer for question1 | 1 | 0 | 43665 |

| 5 | content of first answer for question2 | 3 | 1 | 43324 |

+----+----------------------------------------+---------+----------+-----------+

/* related column: Actually that's just for answers and this column is containing the id of

its question. (questions always are null) */

/* accepted column: Actually that's just for answers and specifics accepted answer.

0 means it isn't accepted answer, 1 means it is accepted answer.

(questions always are null) */


在设置问题的可接受答案之前,我正在尝试实现此 :

验证当前用户是否为OP。author_id of问题应该与相同$_SESSION['id']


这是我的查询:( 我拥有的所有数据只是接受的答案的ID:answer_id

UPDATE QandA q CROSS JOIN ( SELECT related FROM QandA WHERE id = :answer_id ) x

SET accepted = ( id = :answer_id ) -- this acts like a if statement

WHERE q.related = x.related

AND

-- validating OP

(SELECT 1 FROM QandA

WHERE id = x.related AND

author_id = $_SESSION['id']

)

1093-您无法在FROM子句中指定目标表’tbname’进行更新

我该如何解决?


实际上还有一个条件:

+----+----------------------------------------+---------+----------+-----------+------+

| Id | body | related | accepted | author_id | free |

+----+----------------------------------------+---------+----------+-----------+------+

| 1 | content of question1 | null | null | 12345 | null |

| 2 | content of first answer for question1 | 1 | 0 | 53456 | null |

| 3 | content of question2 | null | null | 43634 | 300 |

| 4 | content of second answer for question1 | 1 | 0 | 43665 | null |

| 5 | content of first answer for question2 | 3 | 1 | 43324 | null |

+----+----------------------------------------+---------+----------+-----------+------+

/* free column: Actually that's just for questions. `null` means it is a free question

and any number else means it isn't. (answers always are `null`) */

如果问题是免费的,则OP可以接受该问题的答案,然后更改其接受的答案,然后撤消其接受的答案。但是,如果问题不是免费的,那么OP可以一次接受一个问题,他不能撤消它,也不能更改已接受的答案。这是在MySQL中实现该条件的方法:

(SELECT 1 FROM QandA

WHERE id = x.related AND

(

( free IS NOT NULL AND

NOT IN ( SELECT 1 FROM QandA

WHERE related = x.related AND

accepted = 1 )

) OR free IS NULL

)

)

回答:

我认为应该这样做:

UPDATE QandA AS ans1

JOIN QandA AS ans2 ON ans2.related = ans1.related

JOIN QandA AS ques ON ans2.related = ques.id

SET ans1.accepted = (ans1.id = :answer_id)

WHERE ques.author_id = :session_id

AND ans2.id = :answer_id

第一个JOIN过滤器筛选出与接受的问题相同的问题的答案。

第二个JOIN找到了这个问题。

WHERE子句将仅将更新限制为给定作者的问题,并指定接受的答案ID。

演示

对于其他条件,请添加

AND (ques.free IS NULL or ans1.accepted IS NULL)

WHERE条款。ques.free IS NULL匹配任何免费问题,并ans1.accepted IS

NULL匹配没有接受答案的问题(因为当一个答案被接受时,该问题的所有其他答案都获得accepted = 0)。

问题的演示,没有被接受的答案

免费的问题的演示

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