Question

考虑三个表，让他们称之为groups，subgroups，another_groups和表subgroups_another_groups，指定{{{{{}之间的多对多关系1}}和subgroups。 another_groups和subgroups属于一对多关系，因此groups具有外键subgroups。

如何选择group_id another_groups中的所有subgroups与group有关系？

Answer 1

我认为你说的是这样的设置：

CREATE TABLE groups (
   id integer PRIMARY KEY
);

CREATE TABLE subgroups (
   id integer PRIMARY KEY,
   group_id integer REFERENCES groups NOT NULL
);
CREATE INDEX ON subgroups(group_id);

CREATE TABLE another_groups (
   id integer PRIMARY KEY
);

CREATE TABLE subgroups_another_groups (
   subgroup_id integer REFERENCES subgroups NOT NULL,
   another_groups_id integer REFERENCES another_groups NOT NULL,
   PRIMARY KEY(subgroup_id, another_groups_id)
);
CREATE INDEX ON subgroups_another_groups(another_groups_id);

然后，您想知道通过其他两个表连接到another_groups的所有groups，除了那些存在与此another_groups没有连接的子组的表，对吗？

在SQL中，这将是：

SELECT DISTINCT g.id, a.id
FROM another_groups a
   JOIN subgroups_another_groups sag ON a.id = sag.another_groups_id
   JOIN subgroups s ON sag.subgroup_id = s.id
   JOIN groups g ON s.group_id = g.id
WHERE NOT EXISTS
         (SELECT 1 FROM subgroups s1
          WHERE s1.group_id = g.id
            AND NOT EXISTS
                   (SELECT 1 FROM subgroups_another_groups sag1
                    WHERE sag1.subgroup_id = s1.id
                      AND sag1.another_groups_id = a.id
                   )
         );

Postgres多对多关系：查找所有与相关表

1 个答案: