SQL查询优化 [英] SQL query optimize

问题描述

在幻灯片中 http://www.slideshare.net/billkarwin/models-for-hierarchical-data，我看到了一个 sql 代码(第 22 页):

In a slide http://www.slideshare.net/billkarwin/models-for-hierarchical-data, i saw a sql code (page 22):

INSERT INTO Comments (author, comment)
VALUES (‘Ollie’, ‘Good job!’);

SELECT path FROM Comments
WHERE comment_id = 7;

UPDATE Comments
SET path = $parent_path || LAST_INSERT_ID() || ‘/’
WHERE comment_id = LAST_INSERT_ID();

我认为我们可以优化它:

I think we can optimize it:

SELECT path FROM Comments
WHERE comment_id = 7;

INSERT INTO Comments (author, comment, path )
VALUES (‘Ollie’, ‘Good job!’,  $parent_path || LAST_INSERT_ID() || ‘/’);

我说得对吗?

推荐答案

是的，单个插入比插入后更新更好，但在这种情况下，自动生成的 id(?) 被用作列值也是如此.路径中使用的 LAST_INSERT_ID() 不是当前正在执行的插入语句的 ID，而是最后(上一个)插入的 ID.

Yes, a single insert would be better than an insert followed by an update, but in this case the auto-generated id(?) is being used as a column value as well. The LAST_INSERT_ID() used in the path would not be the id of the current insert statement being performed, but that of the last (previous) insert.

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