MySQL将查询值赋给变量

我制定了以下程序：

delimiter // 
CREATE PROCEDURE processPayment(in id int(11), status int(11)) 
BEGIN 
declare msg_a text DEFAULT "Payment was marked as sent";
declare msg_d text DEFAULT "Payment was marked as denied and user balance was updated";
declare msg_w text DEFAULT "Wrong payment status";
declare msg_u text DEFAULT "Wrong payment ID";
declare amount varchar(255) DEFAULT 0;
amount = "SELECT amount FROM payments WHERE id = id";
user_id = "SELECT userid FROM payments WHERE id = id";

IF(SELECT COUNT(*) FROM payments WHERE id = id AND status = status) = 0
THEN
SELECT msg_u;
END IF;

IF(status = 0)
THEN
SELECT msg_w;
END IF;

IF(status = 1)
THEN
UPDATE payments SET status = status WHERE id = id;
SELECT msg_a;
END IF;

IF(status = -1)
THEN
UPDATE payments SET status = status WHERE id = id;
UPDATE balances SET actual = actual+amount WHERE user_id = user;
SELECT msg_d;
END IF;

END 

但当我尝试将其添加到数据库中时，出现错误： “=”SELECT amount FROM payments WHERE id=id”附近的语法有问题；user\u id=“从pa”第8行选择userid 我需要将查询值赋给变量。 我该怎么做？

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提前感谢您的帮助。

您有两种方法可以实现此目的。首先是使用SET：

SET amount  = (SELECT amount FROM payments WHERE id = id);
SET user_id = (SELECT userid FROM payments WHERE id = id);

或使用SELECT INTO语法：

SELECT amount INTO amount FROM payments WHERE id = id;
SELECT userid INTO user_id FROM payments WHERE id = id;

您甚至可以使用第二种方法在一行中实现：

SELECT amount, userid INTO amount, user_id FROM payments WHERE id = id;