mysql写一个存储过程实现美团退款业务

好的，以下是一个简单的 MySQL 存储过程实现美团退款业务的例子：

DELIMITER //

CREATE PROCEDURE refund_order(IN order_id INT, IN refund_amount DECIMAL(10, 2), OUT refund_status VARCHAR(50))
BEGIN
    DECLARE order_total DECIMAL(10, 2);
    DECLARE refund_total DECIMAL(10, 2);
    DECLARE refund_count INT;
    
    SELECT total_amount INTO order_total FROM orders WHERE id = order_id;
    
    SELECT SUM(amount) INTO refund_total FROM refunds WHERE order_id = order_id;
    
    SELECT COUNT(*) INTO refund_count FROM refunds WHERE order_id = order_id;
    
    IF order_total < refund_total + refund_amount THEN
        SET refund_status = 'Refund failed: refund amount exceeds total amount';
    ELSEIF refund_count >= 3 THEN
        SET refund_status = 'Refund failed: maximum refund count reached';
    ELSE
        INSERT INTO refunds(order_id, amount) VALUES(order_id, refund_amount);
        SET refund_status = 'Refund successful';
    END IF;
    
END //
DELIMITER ;

该存储过程包括了三个输入参数：订单 ID、退款金额和一个输出参数：退款状态。它首先查询订单总金额和已经退款的金额总数，然后根据条件进行退款操作，最后将退款状态返回给调用者。

注意，这只是一个简单的例子，实际的退款业务可能会更加复杂，需要根据具体需求进行调整。