MySQL - 连接两个不同表的特定列

Question

My database has two tables, one containing information about users and one containing a list of transactions between users. I want to select rows in the second table substituting the email with the username.

ACCOUNTS
----------------------------------------------
     email      | password | username | token
----------------------------------------------
 joe@mail.com   | xxxxxxxx | joe      | abcde
----------------------------------------------
 bob@mail.com   | xxxxxxxx | bob      | edcba
----------------------------------------------

TRANSACTIONS
------------------------------------------------------
     sender     |   receiver   | amount   | timestamp
------------------------------------------------------
 joe@mail.com   | bob@mail.com | 20       | 123456789
------------------------------------------------------

EXPECTED RESULT
------------------------------------------------------
     sender     |   receiver   | amount   | timestamp
------------------------------------------------------
 joe            | bob          | 20       | 123456789
------------------------------------------------------

What I tried to do is using JOIN and UNION using the following query:

SELECT amount, timestamp, accounts.username AS sender, accounts.username FROM tx JOIN accounts ON sender=accounts.email 
UNION 
SELECT amount, timestamp, accounts.username AS receiver, accounts.username FROM tx JOIN accounts ON receiver=accounts.email

Of course it doesn't work, instead returns:

-----------------------------------------------------
     sender     |   receiver   | amount   | timestamp
-----------------------------------------------------
 joe            | joe          | 20       | 123456789
-----------------------------------------------------
 bob            | bob          | 20       | 123456789
-----------------------------------------------------

My question is, how do you join specific fields of two different tables, selecting rows in the second table on the basis of the value of a column? Thanks!

PS: I know this question probably has an answer somewhere in the internet, but I already tried several solutions and none worked.

Answer 1

One solution would be to JOIN the accounts table twice, once for the sender and once for the receiver:

SELECT
    c1.username sender,
    c2.username receiver,
    t.amount,
    t.timestamp
FROM transactions t
INNER JOIN accounts c1 ON c1.email = t.sender
INNER JOIN accounts c2 ON c2.email = t.receiver