在SQL中将一个查询的结果传递给另一个查询

我有两个问题：

SELECT g.id FROM "group" g INNER JOIN group_role gr on g.id = gr.group_fk
INNER JOIN role_details rd on rd.group_role_fk = gr.id
INNER JOIN mandate m on m.role_details_fk = rd.id where m.id = x

及

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我希望能够将g.id传递到第二个查询中，以便对于给定的“x”，返回p.id、p.name、g.name、g.id。我该怎么做？可以用更多的连接来完成吗

您可以像这样嵌套查询，称为子查询，假设两个查询都工作，并且子查询只返回一个值

SELECT p.id, p.name, g.name, g.id
FROM person p INNER JOIN position pos on p.id = pos.person_fk
INNER JOIN role_details rd on rd.id = pos.role_details_fk
INNER JOIN group_role gr on gr.id = rd.group_role_fk
INNER JOIN "group" g on g.id = gr.group_fk
WHERE g.id = (SELECT g.id FROM "group" g INNER JOIN group_role gr on g.id = gr.group_fk
INNER JOIN role_details rd on rd.group_role_fk = gr.id
INNER JOIN mandate m on m.role_details_fk = rd.id where m.id = x)

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最简单的方法是使用子查询，如

SELECT p.id, p.name, g.name, g.id
FROM person p INNER JOIN position pos on p.id = pos.person_fk
INNER JOIN role_details rd on rd.id = pos.role_details_fk
INNER JOIN group_role gr on gr.id = rd.group_role_fk
INNER JOIN "group" g on g.id = gr.group_fk
WHERE g.id IN (SELECT g2.id FROM "group" g2 INNER JOIN group_role gr on g2.id = gr.group_fk
INNER JOIN role_details rd on rd.group_role_fk = gr.id
INNER JOIN mandate m on m.role_details_fk = rd.id where m.id = x)

请注意在子查询中使用不同的别名，除非您希望在外部查询中查找准确的行

为了避免子查询，您可以使用更多的联接重新格式化查询-可能使用不同的别名多次联接同一个表。根据DBMS的不同，这可能会为您带来一些性能

SELECT p.id, p.name, g.name, g.id
FROM person p INNER JOIN position pos on p.id = pos.person_fk
INNER JOIN role_details rd on rd.id = pos.role_details_fk
INNER JOIN group_role gr on gr.id = rd.group_role_fk
INNER JOIN "group" g on g.id = gr.group_fk
WHERE g.id IN (SELECT g2.id FROM "group" g2 INNER JOIN group_role gr on g2.id = gr.group_fk
INNER JOIN role_details rd on rd.group_role_fk = gr.id
INNER JOIN mandate m on m.role_details_fk = rd.id where m.id = x)