使用 OR 在多个列上通过左连接创建视图

Question

我正在尝试通过左连接两列来从两个表创建视图：t1.recipient_email = t2.username 或 t1.created_by = t2.id。如下面的伪代码所示，我希望第一个 t2.name 是收件人姓名，第二个 t2.name 是发件人姓名。我想不出实现这一目标的正确方法。

CREATE VIEW  emailsent_log_view
(id_email_que_log, date_sent, recipent_email, recipient_name, send_status, sender_name)
 AS
SELECT
    t1.id,
    t1.date_send,
    t1.recipient_email,
    t2.name, --recipient_name: corresponds with t1.recipient_email = t2.username
    t1.send_status,
    t2.name --sender_name: correspond with t1.created_by = t2.id

    FROM email_que_log AS t1
    LEFT JOIN user_account as t2
    ON  t1.recipient_email = t2.username
    OR t1.created_by = t2.id

Answer 1

正如您所猜想的，您无法选择哪一行连接到具有or 条件的哪一行。解决此类问题的方法是加入表两次，每次需要一次：

CREATE VIEW  emailsent_log_view
(id_email_que_log, date_sent, recipent_email, recipient_name, send_status, sender_name)
AS
SELECT
    eql.id,
    eql.date_send,
    eql.recipient_email,
    res.name AS reciever, -- From the first join
    eql.send_status,
    snd.name AS sender -- From the second join
FROM
    email_que_log AS eql
LEFT JOIN 
    user_account AS res ON eql.recipient_email = res.username
LEFT JOIN 
    user_account AS snd ON eql.created_by = snd.id