MySQL仅为ID 1返回值

Question

我收到了以下问题：

    SELECT      ur.repair_id, ur.repair_complete, ur.repair_noted_by_client, ur.repair_problem, ur.added_at, ur.added_by, ur.repaired_at, ur.repaired_by,
                aa.account_fullname AS added_by_name,
                ar.account_fullname AS repaired_by_name
    FROM        units_repairs AS ur
    LEFT JOIN   (SELECT account_id, account_fullname FROM accounts LIMIT 1) AS aa ON aa.account_id = ur.added_by
    LEFT JOIN   (SELECT account_id, account_fullname FROM accounts LIMIT 1) AS ar ON ar.account_id = ur.repaired_by
    WHERE       ur.unit_id = 1

仅在account_id = 1时返回全名。如果说repaired_by = 2，则说NULL ...

谢谢，我不知道我错过了什么。

Answer 1

您正在加入内部查询：

SELECT account_id, account_fullname FROM accounts LIMIT 1

你有LIMIT 1，它只给你一行（可能有repaired_by = 1），然后你想要过滤并只得到repaired_by为2的行...而你没有那一个。

Answer 2

可能没有记录满足行中的条件repaired_by = 2，

 LEFT JOIN   (SELECT account_id, account_fullname FROM accounts LIMIT 1) AS ar ON ar.account_id = ur.repaired_by

因此，ar.account_id = ur.repaired_by返回NULL。