局部变量上的MySQL海象运算符？

Question

所以我有一个看起来像这样的过程

CREATE PROCEDURE slct()
        BEGIN
        DECLARE ttl INT DEFAULT 0;
        SELECT NULL AS price, NULL AS account_id, NULL as max_amount, NULL as total
        FROM sellers
        UNION ALL
        SELECT price, account_id, max_amount,  ttl := ttl + max_amount
        FROM (SELECT * FROM sellers ORDER BY price ASC) C where selling_currency_id = :buying and buying_currency_id = :buying_with and ttl < :quantity
        END

它给了我一个语法错误，因为这里的海象操作符：ttl := ttl + max_amount。如果这是一个用户变量(@ttl)，海象操作符就可以工作，但是@ttl没有限定作用域的事实会在我的查询中引入一些but。

所以我想知道:我该如何让这个:=起作用，或者用类似的东西来取代它呢？

编辑：

我发现了INTO关键字，现在我的查询如下所示。仍然说我有一个语法错误...

CREATE PROCEDURE select_buy(
            IN buying BIGINT UNSIGNED,
            IN buying_with BIGINT UNSIGNED,
            IN quantity BIGINT UNSIGNED,
            INOUT ttl BIGINT UNSIGNED
        )
        BEGIN
        SELECT NULL AS price, NULL AS account_id, NULL as max_amount, NULL as total
        FROM sellers
        UNION ALL
        SELECT price, account_id, max_amount,  (SELECT (ttl + max_amount) INTO ttl) AS total
        FROM (SELECT * FROM sellers ORDER BY price ASC) C where selling_currency_id = buying and buying_currency_id = buying_with and ttl < quantity
        END

Answer 1

我的代码不能与@total一起工作，因为我没有重置@total (尽管它看起来很像)。我添加了SET @total = 0，现在一切都正常了。