如何从一个表中获取两个外键值

Question

I have two tables:

Users

table.increments('user_id').primary()
table.string('fname').notNull()
table.string('mname').notNull()
table.string('lname').notNull()
table.integer('level_id').notNull()
table.string('username').notNull()
table.string('password').notNull()
table.boolean('is_deleted').defaultTo(false)
table.unique('username')

Messages

table.increments('message_id').primary()
table.integer('sender_id')
table.integer('receiver_id')
table.boolean('is_private').defaultTo(true)
table.text('message')
table.boolean('is_deleted').defaultTo(false)

So now the problem is, I want to join the tables and get the name of the receiver and the sender.

At first, I tried to get only the messages and loop the data to gather the receiver and the sender's name, so another query which is very bad so I wanted to get everything in just a single query.

I tried AND on the JOIN but it resulted to nothing but empty.

SELECT * 
FROM messages m 
JOIN users u ON u.user_id = m.sender_id 
    AND u.user_id = m.receiver_id

To be honest, I am just going round and round, I don't really know the logic behind a query like this.

Expected code:

[
   {  
      "message_id":54,
      "sender_name": 'Ernie Jeash Vill',
      "receiver_name": 'Karma Morningstar Blackshaw',
      "is_private":1,
      "message":"q",
      "is_deleted":0,
      "created_at":"2019-09-03T08:45:09.000Z",
      "updated_at":null,
   },
   {  
      "message_id":53,
      "sender_name":'Karma Morningstar Blackshaw',
      "receiver_name": 'Ernie Jeash Vill',
      "is_private":1,
      "message":"e",
      "is_deleted":0,
      "created_at":"2019-09-03T08:45:09.000Z",
      "updated_at":null,
   }
]

Answer 1

You need to join messages to users twice , one for each name of the receiver and sender:

SELECT
    m.*,
    u1.lname AS sender_last,
    u1.fname AS sender_first,
    u2.lname AS receiver_last,
    u2.fname AS receiver_first
FROM messages m 
INNER JOIN users u1
    ON u1.user_id = m.sender_id
INNER JOIN users u2
    ON u2.user_id = m.receiver_id;

Answer 2

You can have multiple foreign keys from the same table.

It does not require any special treatment or syntax. Just declare two separate foreign keys on the table.

Read more here: https://dev.mysql.com/doc/refman/5.5/en/innodb-foreign-key-constraints.html