mysql从两个表

Question

我有这个查询从两个表调用信息。

DB::get("SELECT friends. * , (SELECT `login` FROM `users` WHERE `users`.`id` = friends.`user_id`) AS `login` FROM `friends` WHERE `id_user`='" . $this->user['id'] . "' ORDER BY `id` DESC LIMIT ")

因此，如果用户在朋友列表中显示用户名，我想获得用户名和头像。头像行是avatar。 我试着这个。

DB::get("SELECT friends. * , (SELECT `login`, `*avatar*` FROM `users` WHERE `users`.`id` = friends.`user_id`) AS `login` FROM `friends` WHERE `id_user`='" . $this->user['id'] . "' ORDER BY `id` DESC LIMIT ")

并给我错误

SQLSTATE[21000]: Cardinality violation: 1241 Operand should contain 1 column(s)

哪里出错？

Answer 1

您需要使用JOIN，例如：

SELECT f.*, u.*
FROM friends f JOIN users u ON f.user_id = u.id
WHERE f.id_user = <your_id>
ORDER BY id DESC LIMIT <your_limit>;

Answer 2

首先你应该使用Prepared Statement，其次，你不能写内联视图，它有两列

SELECT friends. * , (SELECT `login`, `*avatar*` FROM ..

相反，您应该使用JOIN，这可能比当前方法更有效且更具可读性。