请求SQL 2左外部联接

Question

i tried to create a request with double LETF OUTER JOIN with sequelize without success i have 2 table : Users

And : friends

my userid and idfriend are related to my table user

if i have on my table user Id = X , userId = 23 , idFriend = 34 ,...

i have a query who provide me all row who have for example 23 (userID or Idfriend) how can i get the all the data of userId (username, email ...) and all the data idFriend(username, email ...) in the same request

thx

Answer 1

you should left join the user two time eg:

  select a.userId, a.Idfriend, b.username as  username, c.username as friendname 
  from  friends a
  left join user b on b.id = a.userId 
  left join user c on c.id = a.idfriend 

Answer 2

If I understand you right, you are trying to get data for userId and friendId with one query? I think this must work for you, but you need to test it

First way:

SELECT user.*, friend.* FROM users
LEFT JOIN users as user ON user.id=23
LEFT JOIN users as friend ON friend.id=34
LIMIT 1

Second way using UNION:

(SELECT user.*, friend.* FROM users as user
LEFT JOIN users as friend ON friend.id=23
WHERE user.id=34 OR user.id IS NULL)
UNION
(SELECT user.*, friend.* FROM users as user
RIGHT JOIN users as friend ON friend.id=34
WHERE user.id=23 OR user.id IS NULL)

Another way (this way have risk to return you empty if WHERE clause is null):

SELECT user.*, friend.* FROM users as user
JOIN users as friend ON friend.id=34
WHERE user.id=23

I hope this will help you