Question

我有以下查询：

SELECT
    *,
    (SELECT locked FROM forum_topics t WHERE t.id = topicid) as topiclocked,
    forum_subtopics.locked as stlocked
FROM forum_subtopics
WHERE topicid = 1;

我想使用字段topiclocked和stlocked来创建名为locked的第三个自定义列。我尝试了以下内容，但MySQL告诉我topiclocked不存在：

SELECT
    *,
    (SELECT locked FROM forum_topics t WHERE t.id = topicid) as topiclocked,
    forum_subtopics.locked as stlocked,
    (topiclocked || stlocked) as locked
FROM forum_subtopics
WHERE topicid = 1;

未知专栏＆＃39; topiclocked＆＃39;在＆＃39;字段列表＆＃39;

如何以实际工作的方式创建第三个coloumn？

Answer 1

您不能在同一alias name (topiclocked)声明中使用select。将查询设为sub-select并在outer query中进行连接。试试这个。

SELECT *,
       ( topiclocked || stlocked ) AS locked
FROM   (SELECT *,
               (SELECT locked
                FROM   forum_topics t
                WHERE  t.id = topicid) AS topiclocked,
               forum_subtopics.locked  AS stlocked
        FROM   forum_subtopics
        WHERE  topicid = 1) a;

Answer 2

您可以在select - 部分中分配变量，就像这样

SELECT
    *,
    @topiclocked:=(SELECT locked FROM forum_topics t WHERE t.id = topicid) as topiclocked,
    @stlocked:=forum_subtopics.locked as stlocked,
    (@topiclocked || @stlocked) as locked
FROM forum_subtopics
WHERE topicid = 1;

http://sqlfiddle.com/#!2/572453/1

我会用JOIN重写该查询，这样您就不必执行子查询，并且您可以从两个表中获得locked。

SELECT s.*,
    t.locked as topiclocked,
    s.locked as stlocked,
    (t.locked || s.locked) as locked
FROM forum_subtopics s
JOIN forum_topics t on (s.topicid = t.id)
WHERE topicid = 1;

Answer 3

select中定义的别名不能用于select或where子句。通常，您将使用子查询或公用表表达式（但不在MySQL中）来解决此问题。但是，MySQL实现了子查询，因此这会对性能产生影响。

在您的情况下，您可以将查询重写为：

SELECT s.*, t.locked as topiclocked, s.locked as stlocked,
       (t.locked || s.locked) as locked
FROM forum_subtopics s JOIN
     forum_topics t
     ON t.id = s.topicid
WHERE s.topicid = 1;

如果您的子主题没有主题，则可能需要left join。我猜你的数据不是这种情况，所以我只是使用内连接。

MySQL SELECT使用2个新创建的＆＃34; coloumns＆＃34;在第三个

3 个答案: