[英]SQL: selecting the set of A not B
LEFT JOIN
的解决方案:
SELECT
*
FROM
projects p LEFT JOIN users_projects up ON (p.p_ID = up.p_ID AND up.u_ID = 3)
WHERE
up.u_ID IS NULL
基本上选择所有项目,然后将它们与所需用户的user_projects
加入。 左users_projects
使project
表中的所有行都出现,即使users_projects
表中没有对应的行也是users_projects
。 这些行将users_projects
所有字段users_projects
设置为NULL
,因此我们可以选择它们。
这不是JOIN
查询,而是具有不相关子选择且带有NOT IN()
谓词的查询。
我希望projects
表的列足够了...
SELECT
*
FROM
( SELECT 1,'Apple' -- input data, don't use in 'real' query
UNION ALL SELECT 5,'Banna' -- input data, don't use in 'real' query
UNION ALL SELECT 7,'Carrot' -- input data, don't use in 'real' query
UNION ALL SELECT 8,'Durian') -- input data, don't use in 'real' query
projects(p_id,p_name)
WHERE p_id NOT IN (
SELECT
p_id
FROM
( SELECT 2,1 -- input data, don't use in 'real' query
UNION ALL SELECT 2,5 -- input data, don't use in 'real' query
UNION ALL SELECT 2,7 -- input data, don't use in 'real' query
UNION ALL SELECT 3,1 -- input data, don't use in 'real' query
UNION ALL SELECT 3,8) -- input data, don't use in 'real' query
users_projects(u_id,p_id)
WHERE u_id=3
)
;
p_id|p_name
7|Carrot
5|Banna
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