mysql仅当另一行中的字段等于

尝试连接account表两次:

SELECT *
FROM tAccounts a1
JOIN tFriends f ON a1.id = f.id_lo
JOIN tAccounts a2 ON a2.id = f.id_hi
WHERE a1.id = 4 AND a2.server_id != -1
   OR a2.id = 4 AND a1.server_id != -1

SELECT *
FROM tAccounts a
JOIN tFriends f ON a.id = f.id_lo
WHERE f.id_hi = 4 AND a.server_id != -1
UNION
SELECT *
FROM tAccounts a
JOIN tFriends f ON a.id = f.id_hi
WHERE f.id_lo = 4 AND a.server_id != -1

你在哪里加入条件?应该是这样的:(我想朋友。身份证号码是朋友的帐户id)

SELECT FR.id_lo AS id_friend
FROM tFriends FR 
INNER JOIN tAccounts AC ON (AC.id = FR.id_lo AND AC.server_id != -1)
WHERE FR.id_hi = 4 ;

您需要连接这两个表;否则,您将得到一个笛卡尔积(将id\u hi=4的所有tFriends行与server\u id不为-1的所有tAccounts行组合在一起)。

试试这个:

SELECT FR.id_lo AS id_friend
FROM tFriends FR
INNER JOIN tAccounts AC ON AC.id = FR.id_hi
WHERE (FR.id_hi = 4 && AC.server_id != -1)