使用Postgres中的左外连接删除

Question

我正在将数据库从MySQL切换到Postgres SQL.在MySQL中工作的选择查询在Postgres中有效,但类似的删除查询却没有.

我有两个数据表,列出了某些备份文件所在的位置.现有数据(ed)和新数据(nd).此语法将挑选出现有数据,这些数据可能表明文件在现有数据表中的位置,将其与相等的文件名和路径进行匹配,但没有关于它在新数据中的位置的信息:

SELECT ed.id, ed.file_name, ed.cd_name, ed.path, nd.cd_name
FROM tv_episodes AS ed
LEFT OUTER JOIN data AS nd ON
ed.file_name = nd.file_name AND 
ed.path = nd.path
WHERE ed.cd_name = 'MediaLibraryDrive' AND nd.cd_name IS NULL;

我希望使用以下语法运行删除查询:

DELETE ed
FROM tv_episodes AS ed
LEFT OUTER JOIN data AS nd ON
ed.file_name = nd.file_name AND 
ed.path = nd.path
WHERE ed.cd_name = 'MediaLibraryDrive' AND nd.cd_name IS NULL;

我试过DELETE ed,DELETE ed.*两者都渲染syntax error at or near "ed".如果我尝试没有别名的类似错误ed.如果我尝试

DELETE FROM tv_episodes AS ed
LEFT  JOIN data AS nd.....

Postgres发回来syntax error at or near "LEFT".

我很困难,并且使用特定于psql的连接在删除查询上找不到太多.

Answer 1

正如其他人所说,你不能直接在DELETE语句中LEFT JOIN.但是,您可以使用USING语句自行连接目标表的主键,然后对该自联接表保持连接.

DELETE FROM tv_episodes
USING tv_episodes AS ed
LEFT OUTER JOIN data AS nd ON
   ed.file_name = nd.file_name AND 
   ed.path = nd.path
WHERE
   tv_episodes.id = ed.id AND
   ed.cd_name = 'MediaLibraryDrive' AND nd.cd_name IS NULL;

请注意WHERE子句中tv_episodes.id上的自连接.这避免了上面提供的子查询路由.

Answer 2

正如 bf2020 所指出的，postgres 在执行 DELETE 查询时不支持 JOIN。子查询的建议解决方案让我想到了解决方案。从上面优化 SELECT 查询并将其用作 DELETE 查询语句的子查询：

DELETE FROM tv_episodes 
WHERE id in (
    SELECT ed.id
    FROM tv_episodes AS ed
    LEFT OUTER JOIN data AS nd ON
    ed.file_name = nd.file_name AND 
    ed.path = nd.path
    WHERE ed.cd_name = 'MediaLibraryDrive' AND nd.cd_name IS NULL
);

对于某些数据库系统，尤其是 MySQL，子查询通常会消耗时间和 CPU 资源，效率低下。根据我的经验，我尽量避免使用子查询，因为它效率低下，而且此类查询有时是磨练自己的技能（例如学习 JOIN 语法）的简单方法。

由于 postgre 不允许使用 join 进行删除查询，因此以上是有效的解决方案。

Answer 3

使用DELETE... USING语法：

DELETE FROM tv_episodes USING data WHERE 
tv_episodes.file_name = data.file_name AND 
tv_episodes.path = data.path AND 
tv_episodes.cd_name = 'MediaLibraryDrive' AND 
data.cd_name IS NULL;

Answer 4

代替

DELETE ed
FROM tv_episodes AS ed
LEFT OUTER JOIN data AS nd ON
ed.file_name = nd.file_name AND 
ed.path = nd.path
WHERE ed.cd_name = 'MediaLibraryDrive' AND nd.cd_name IS NULL;

请尝试

DELETE FROM tv_episodes
WHERE cd_name = 'MediaLibraryDrive' AND 
  (tv_episodes.filename, tv_episodes.path IN
    (SELECT ed.filename, 
    ed.path 
    FROM tv_episodes AS ed 
    INNER JOIN data AS nd 
      ON ed.file_name = nd.file_name 
        AND ed.path = nd.path
    WHERE nd.cd_name IS NULL)
  )
  ;

JOINDELETE根据 postgresql 文档，在查询中无效。您可能需要连接 IN 表达式的左右部分。