使用 mysql JOIN 到 select 数据

Question

I have two tables representing users and their friends: users and friendships

• users has id and name
• friendships has id , friend1_id and friend2_id

By searching a user's name, I want to get their associated friends' names.

Answer 1

You can try this:

SELECT u2.name from users u1
INNER JOIN users u2 ON u1.name = [your_name_to_search]
INNER JOIN friendships fs 
ON u2.id = fs.friend2_id AND u1.id = fs.friend1_id

Explanation :

I used 2 joins in this approach: the first users self join will assure you take the id corresponding to [your_name_to_search] , and then the second join with friendships will extract the names of friend_2_id matching with the friend_1_id taken from the first join.

You can test this approach on this db-fiddle

Answer 2

Join friendships to 2 copies of users .
The 1st copy will be filtered for the user name that you search for and the 2nd will return all the user's friends:

select uf.name
from users u
inner join friendships fs on u.id in (fs.friend1_id, fs.friend2_id)
inner join users uf on uf.id in (fs.friend1_id, fs.friend2_id) and uf.id <> u.id
where u.name = ?

Replace ? with the name of the user whose friends you want in the results.