在这个联合声明中将 ORDER BY 放在哪里？

Question

我的工会声明似乎有效：

SELECT q.id, q.hits, q.type, q.title, q.date, q.author, q.wfurl
FROM (SELECT id AS id, hits AS hits, type AS type, title AS title, date AS date, author AS author, wfurl AS wfurl, status AS status
FROM articles WHERE status = 1
UNION SELECT id, hits, type, title, published, author_id, url, status
FROM new_articles WHERE status = 1) AS q
GROUP BY q.id

我正在尝试按ORDER BY type 的类型订购整个东西，但无论我把它放在哪里似乎都会引发错误。我把它放在第一行，在 AS 之后和两个选择里面，没有运气。

Answer 1

order by 会跟在group by 之后：

SELECT q.id, q.hits, q.type, q.title, q.date, q.author, q.wfurl
FROM ((SELECT id AS id, hits AS hits, type AS type, title AS title, date AS date, author AS author, wfurl AS wfurl, status AS status
       FROM articles
       WHERE status = 1
      ) UNION
      (SELECT id, hits, type, title, published, author_id, url, status
       FROM new_articles 
       WHERE status = 1
      )) q
GROUP BY q.id
ORDER BY type;

如果您知道这两个表没有重复，那么您应该使用UNION ALL 而不是UNION。 UNION 通过删除重复项会产生开销。分配与名称相同的表别名也是多余的，因此hits as hits 是不必要的（等等）。

编辑：

如果您想要一个高效的查询，以下可能会更快，并且可能会满足您的需求：

select a.*
from articles a
where status = 1
union all
select na.*
from new_articles na
where status = 1 and
      not exists (select 1 from articles a where a.id = na.id)
order by type;

这消除了union 的开销。如果两个表中都存在id，则它会从第一个表中获取值（您可以颠倒逻辑顺序以从第二个表中获取值）。唯一真正的开销是末尾的 order by，而您的版本有 union、group by 和 order by 的开销。