Php 需要一个update语句，将mysql中不同表中的两个列值相乘

我有两张桌子，顾客和牛奶，如下所示

顾客

牛奶

我需要一个更新查询，将价格、牛奶字母相乘，并将其存储在mprice中，其中customer\u id=cid

这是错误的insert语句，我需要将其更改为update语句

$cid = $_POST['cid'];
$milk_letters = $_POST['milk_letters'];
$sql = "INSERT INTO milk (customer_id, milk_letters, milk_date)
        VALUES ('$cid','$milk_letters', NOW())";
dbQuery($sql);  
$sql2 = "INSERT INTO milk SELECT SUM(price * milk_letters) as mprice FROM customers , milk WHERE customers.cid = '$cid' and milk.customer_id = '$cid'";
    $result2 = dbQuery($sql2);

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您可以在一个查询而不是两个查询中完成此操作

INSERT INTO milk (customer_id, milk_letters, milk_date, mprice)
SELECT {$cid}, {$milk_letters}, NOW(), customers.price * {$milk_letters}
FROM customers
WHERE customers.cid = {$cid}

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请看一下：如果要修改现有行，请使用UPDATE，不插入。好的，下一步我要做的是添加特定客户的所有mprice，即在customers中再添加一列合计，并添加mprice，其中cusoter_id=cid类似于UPDATE customers SET total=SELECT SUMmprice FROM milk where milk.cid=customers.cid-尽管从编程的角度来看，计算更好此值不是存储它-您可以创建一个视图创建视图customertotals作为SELECT customer.cid，milk.milk_letters*customers.price作为milktotal组BY customer.cid-然后只使用SELECT cid，milktotal FROM customertotals

$cid = $_POST['cid'];
$milk_letters = $_POST['milk_letters'];
$sql = "INSERT INTO milk (customer_id, milk_letters, milk_date)
        VALUES ('$cid','$milk_letters', NOW())";
dbQuery($sql);  
$sql2 = "INSERT INTO milk SELECT SUM(price * milk_letters) as mprice FROM customers , milk WHERE customers.cid = '$cid' and milk.customer_id = '$cid'";
    $result2 = dbQuery($sql2);

INSERT INTO milk (customer_id, milk_letters, milk_date, mprice)
SELECT {$cid}, {$milk_letters}, NOW(), customers.price * {$milk_letters}
FROM customers
WHERE customers.cid = {$cid}