SQL JOIN查询编写

Question

我正在尝试编写一个涉及两个表的简单查询."person"表具有唯一的person_id和a name,并且"friends"表具有a person_id和a friend_id,其是person_id人员表中的a的FK .

person:
<PK> int person_id 
varchar[45] name

friends: 
<PK> int person_id
<PK> int friend_id

我想选择所有人1的朋友的名字.

我可以使用一个IN声明轻松地做到这一点:

SELECT p.name FROM person p WHERE p.person_id IN (SELECT f.friend_id FROM friends f WHERE f.person_id = 1);

但是,我并不擅长撰写JOIN陈述.有人可以帮我写等效的连接吗？

显然这是一个人为的例子,但我尝试过我的真实数据并且在概念上遗漏了一些东西.谢谢.

Answer 1

你想要这样的东西:

SELECT p.name, f.friend_id
FROM person AS p
INNER JOIN friends AS f ON p.person_id = f.person_id
WHERE p.person_id = 1

这使用两个表连接在一起 p.person_id = f.person_id

如果一个人没有朋友,你将不会获得任何行 - 如果你不想这样,那么使用LEFT JOIN,你将获得一行NULL friend_id.

编辑:如果你想加入朋友回到人:

SELECT p.name AS person_name, friend.name AS friend_name
FROM person AS p                                         -- Our person
INNER JOIN friends AS f ON p.person_id = f.person_id     -- the join table
INNER JOIN person AS friend on f.friend_id = friend.id   -- Join back on person again
WHERE p.person_id = 1

也许你需要为你的应用程序提供这样的3方式连接,但更常见的是你只需要如上所述的双向连接,或者像这样:

SELECT p.name, f.friend_id
FROM person AS p
INNER JOIN friends AS f ON p.person_id = f.friend_id
WHERE f.person_id = 1

这将为您提供所有与person_id 1成为朋友的人的姓名(但不是person_id 1的名字)

Answer 2

select 
    p.name,
    p2.name as friend_name,
from
    person p 
    inner join friends f on p.person_id = f.person_id
    inner join person p2 on f.friend_id = p2.person_id -- friends
where
    p.person_id = <your criteria>