Question

我试图找到仅存在于一个表中的行。大表名（包括所有行）是kwf_uploads，小表存在于许多名称中。我的目标是找到存在于kwf_uploads中但不存在于其他行中的行（此示例是没有关系的表中的主键和外键）。

我在sql中做了什么：

select id from
kwf_uploads
left join

(SELECT picture_id as id
FROM documents where picture_id is not null
UNION 
SELECT file_id as id
FROM books where file_id is not null
UNION 
SELECT picture_id as id
FROM employee where picture_id is not null
UNION 
SELECT file_id as id
FROM flightFiles where file_id is not null
UNION 
SELECT picture_id as id
FROM tasks where picture_id is not null
UNION 
SELECT picture_id as id
FROM trainingContentQuestions where picture_id is not null
UNION 
SELECT picture_id as id
FROM trainingQuestions where picture_id is not null) foo  

ON kwf_uploads.id = foo.id

在SQL: cascade UNION and JOIN

上找到它

但它失败了，错误：SQL (1052): Column "id" in field list is ambiguous。

我不想在每个表中使用join，因为我的联接不好，而且sql变得非常庞大且难以理解。我也尝试not exists没有任何结果。我想我们可以找到更好的解决方案=）

来自select id from kwf_uploads查询的结果（9690行）：

联合查询的结果（6096行）：

结果我想看3594（9690减去6096）行。

Answer 1

UNION（单独使用，没有ALL）是一种“昂贵的”＃34;操作

有另一种选择，不存在。这个结构是一个＆＃34;半连接＆＃34;我怀疑可能比工会方法便宜。

SELECT
      id
FROM kwf_uploads AS u
WHERE NOT EXISTS     (SELECT NULL FROM documents WHERE u.id = picture_id )
      AND NOT EXISTS (SELECT NULL FROM books WHERE u.id = file_id )
      AND NOT EXISTS (SELECT NULL FROM employee WHERE u.id = picture_id )
      AND NOT EXISTS (SELECT NULL FROM flightFiles WHERE u.id = file_id )
      AND NOT EXISTS (SELECT NULL FROM tasks WHERE u.id = picture_id )
      AND NOT EXISTS (SELECT NULL FROM trainingContentQuestions WHERE u.id = picture_id )
      AND NOT EXISTS (SELECT NULL FROM trainingQuestions WHERE u.id = picture_id )

Answer 2

修复当前错误

带有正确结果集别名的前缀标识列。在这种情况下kwf_uploads.id。因为Id存在于两个结果集中。

获得所需的输出

添加Where foo.id is null。这将获取那些在连接中失败的ID，这些是你想要的ID。

  select kwf_uploads.id from
        kwf_uploads
        left join

        (SELECT picture_id as id
        FROM documents where picture_id is not null
        UNION 
        SELECT file_id as id
        FROM books where file_id is not null
        UNION 
        SELECT picture_id as id
        FROM employee where picture_id is not null
        UNION 
        SELECT file_id as id
        FROM flightFiles where file_id is not null
        UNION 
        SELECT picture_id as id
        FROM tasks where picture_id is not null
        UNION 
        SELECT picture_id as id
        FROM trainingContentQuestions where picture_id is not null
        UNION 
        SELECT picture_id as id
        FROM trainingQuestions where picture_id is not null) foo  

        ON kwf_uploads.id = foo.id
        where foo.id is null

Answer 3

在DarkKnight旁边回答：

您不需要在每个where上添加union select。最糟糕的情况是，您从NULL获得一个UNION，但不会出现在LEFT JOIN上

select kwf_uploads.id 
from kwf_uploads
left join
    (
    SELECT picture_id as id FROM documents
    UNION 
    SELECT file_id as id FROM books
    UNION 
    SELECT picture_id as id FROM employee
    UNION 
    SELECT file_id as id FROM flightFiles
    UNION 
    SELECT picture_id as id FROM tasks
    UNION 
    SELECT picture_id as id FROM trainingContentQuestions 
    UNION 
    SELECT picture_id as id FROM trainingQuestions
    ) foo      
ON kwf_uploads.id = foo.id
where foo.id is null

如何将表与多个工会进行比较并加入？

3 个答案: