SQL UPDATE SET一列是否等于另一列引用的相关表中的值？

Question

我希望这是有道理的,让我详细说明:

有一个测验程序跟踪数据表,其中每一行都有...

QuestionID和AnswerID(每个都有一个表).因此,由于一个错误,有一堆QuestionID设置为NULL,但相关AnswerID的QuestionID在Answers表中.

所以说QuestionID是NULL,AnswerID是500,如果我们转到Answers表并找到AnswerID 500,那么有一个带有QuestionID的列应该是NULL值所在的列.

所以基本上我想将每个NULL QuestionID设置为等于在跟踪表中的AnswerID的Answer行上的Answers表中找到的QuestionID(与正在写入的NULL QuestionID相同的行).

我该怎么做？

UPDATE QuestionTrackings
SET QuestionID = (need some select query that will get the QuestionID from the AnswerID in this row)
WHERE QuestionID is NULL AND ... ?

不知道我怎么能够从匹配的AnswerID中将QuestionID分配给QuestionID ...

Answer 1

update q
set q.QuestionID = a.QuestionID
from QuestionTrackings q
inner join QuestionAnswers a
on q.AnswerID = a.AnswerID
where q.QuestionID is null -- and other conditions you might want

我建议在运行更新之前检查要更新的结果集(相同的查询,只需选择):

select *
from QuestionTrackings q
inner join QuestionAnswers a
on q.AnswerID = a.AnswerID
where q.QuestionID is null -- and other conditions you might want

特别是每个答案id是否肯定只有1个相关的问题ID.

Answer 2

如果没有update-and-join表示法(并非所有DBMS支持),请使用:

UPDATE QuestionTrackings
   SET QuestionID = (SELECT QuestionID
                        FROM AnswerTrackings
                        WHERE AnswerTrackings.AnswerID = QuestionTrackings.AnswerID)
   WHERE QuestionID IS NULL
     AND EXISTS(SELECT QuestionID
                        FROM AnswerTrackings
                        WHERE AnswerTrackings.AnswerID = QuestionTrackings.AnswerID)

通常在这样的查询中,您需要使用包含子查询的EXISTS子句限定WHERE子句.这可以防止UPDATE践踏没有匹配的行(通常使所有值都为空).在这种情况下,由于缺少的问题ID会将NULL更改为NULL,因此可以说无关紧要.

Answer 3

我不知道你是否在MySQL Workbench上遇到了与我相同的问题,但是INNER JOIN在FROM声明之后运行查询对我来说不起作用.我无法运行查询,因为该程序抱怨该FROM语句.

因此,为了使查询工作,我将其更改为

UPDATE table1 INNER JOIN table2 on table1.column1 = table2.column1
SET table1.column2 = table2.column4
WHERE table1.column3 = 'randomCondition';

代替

UPDATE a
FROM table1 a INNER JOIN table2 b on a.column1 = b.column1
SET a.column2 = b.column4
WHERE a.column3 = 'randomCondition';

我想我的解决方案是MySQL的正确语法.

Answer 4

UPDATE
    "QuestionTrackings"
SET
    "QuestionID" = (SELECT "QuestionID" FROM "Answers" WHERE "AnswerID"="QuestionTrackings"."AnswerID")
WHERE
    "QuestionID" is NULL
AND ...

Answer 5

我有同样的问题.这是一个类似于eglasius的工作解决方案.我正在使用postgresql.

UPDATE QuestionTrackings
SET QuestionID = a.QuestionID
FROM QuestionTrackings q, QuestionAnswers a
WHERE q.QuestionID IS NULL

它抱怨如果在第1行使用q代替表名,并且在第2行的QuestionID之前不应该有任何内容.