Question

我正在尝试做这样的事情：

SELECT COUNT(topic.topic_id) + COUNT(post.post_id)
FROM topic, post WHERE author_id = ?

两个表都有列author_id。

我收到column reference "author_id" is ambiguous错误。

我怎么能告诉它{两个表中都有author_id？

Answer 1

虽然可以，但您很可能不想加入这两个表，因为这可能导致不同的计数。本相关答案的解释：
Two SQL LEFT JOINS produce incorrect result

两个子查询最快：

SELECT (SELECT COUNT(topic_id) FROM topic WHERE author_id = ?)
     + (SELECT COUNT(post_id)  FROM post  WHERE author_id = ?) AS result

如果topic_id和post_id在各自的表格中定义为NOT NULL，则可以略微简化：

SELECT (SELECT COUNT(*) FROM topic WHERE author_id = ?)
     + (SELECT COUNT(*) FROM post  WHERE author_id = ?) AS result

如果两个author_id列中至少有一列唯一， JOIN 也可以使用在这种情况下（但速度较慢，我不会使用它）：

SELECT COUNT(t.topic_id) + COUNT(p.post_id) AS result
FROM   topic t
LEFT   post  p USING (author_id)
WHERE  t.author_id = ?;

如果您只想输入一次值，请使用CTE：

WITH x AS (SELECT ? AS author_id)  -- enter value here
SELECT (SELECT COUNT(*) FROM topic JOIN x USING (author_id))
     + (SELECT COUNT(*) FROM post  JOIN x USING (author_id)) AS result

但一定要了解联接是如何工作的。阅读有关Joined Tables in the manual。

的章节

根据单一标准从两个表计数

1 个答案: