Question

这是我的问题：

UPDATE reputations SET 
    type = new.type,
    score = new.score,
    qora = NOT (new.post_id = (SELECT t1.id
        FROM qanda t1 
        WHERE (EXISTS (SELECT 1 
            FROM qanda t2 
            WHERE ID = new.post_id 
                AND t1.ID = t2.related)
                OR t1.id = new.post_id)
                AND Type = 0)),
    question_id = (SELECT t1.id
        FROM qanda t1 
        WHERE (EXISTS (SELECT 1 
            FROM qanda t2 
            WHERE ID = new.post_id 
                AND t1.ID = t2.related)
                OR t1.id = new.post_id)
                AND Type = 0),
    post_id = new.post_id,
    table_code = new.table_code,
    comment_id = new.comment_id,
    owner_id = new.author_id,
    date_time = UNIX_TIMESTAMP()
WHERE events_table_id = old.id;

我要做的就是删除其中一个子查询，因为两者都是相同的。我怎么能这样做？

Answer 1

在这种情况下，我会DECLARE一个局部变量来存储子查询的结果。阅读https://dev.mysql.com/doc/refman/5.7/en/stored-program-variables.html

使用SELECT...INTO语法将查询结果存储到变量中。阅读https://dev.mysql.com/doc/refman/5.7/en/select-into.html

BEGIN
  DECLARE QANDA_ID INT;

  SELECT t1.id FROM qanda t1 
  WHERE (EXISTS (SELECT 1 FROM qanda t2 WHERE ID = new.post_id AND t1.ID = t2.related)
    OR t1.id = new.post_id) AND Type = 0
  INTO QANDA_ID;

  UPDATE reputations SET ...
    qora = not (new.post_id = QANDA_ID),
    question_id = QANDA_ID,
    ...

END

我在所有大写字母中创建了变量名，只是为了让它在这个例子中脱颖而出。但是你可以将它命名为任何你想要的名称，它遵循与其他标识符名称相同的规则。

但是，我建议您不要将本地变量命名为与您在UPDATE语句中使用的表中的任何列名相同。如果你这样做会让人感到困惑。

Answer 2

在我看来，您可以使用CROSS JOIN：

UPDATE reputations r CROSS JOIN
       (SELECT t1.id
        FROM qanda t1 
        WHERE (EXISTS (SELECT 1 
                       FROM qanda t2 
                       WHERE ID = new.post_id and t1.ID = t2.related
                      ) OR
               t1.id = new.post_id
              ) AND
              Type = 0
       ) t1
    SET r.type = new.type,
        r.score = new.score,
        r.qora = not (new.post_id = t1.id),
        r.question_id = t1.id,
        r.post_id = new.post_id,
        r.table_code = new.table_code,
        r.comment_id = new.comment_id,
        r.owner_id = new.author_id,
        r.date_time = UNIX_TIMESTAMP()
WHERE r.events_table_id = old.id;

如何在UPDATE语句中使用JOIN？

2 个答案: