Mysql 如何对子查询结果的两列进行减法

我有这个查询，它工作正常，正如预期的那样，即，它显示了所有列以及每个子查询中的2列

SELECT 
    a.group_id,
    a.code,
    a.description_en,
    c.size_code,
    (
        SELECT
            SUM(b.qty)
        FROM
            receiving_details AS b
        WHERE
            b.code = c.size_code
    ) AS in_qty,
    (
        SELECT
            SUM(d.qty)
        FROM
            requisition_details AS d
        WHERE
            d. matl_code = c.size_code
    ) AS out_qty
FROM products AS a
INNER JOIN products_sizes AS c ON c.prod_code = a.code
ORDER BY a.group_id ASC, a.code ASC, c.size_code ASC

然而，当我尝试添加这一行时

(in_qty - out_qty) AS balance,

就在'FROM'语句之前，我在'field list'中得到一个未知列'in_qty'的错误

我做错了什么

编辑：

根据公认的答案，我又做了几次修正，得到了我想要的结果

SELECT *, (e.in_qty - e.out_qty) AS balance FROM
(SELECT 
    a.group_id,
    a.code,
    a.description_en,
    c.size_code,
    (
        SELECT
            IFNULL(SUM(b.qty),0)
        FROM
            receiving_details AS b
        WHERE
            b.code = c.size_code
    ) AS in_qty,
    (
        SELECT
            IFNULL(SUM(d.qty),0)
        FROM
            requisition_details AS d
        WHERE
            d. matl_code = c.size_code
    ) AS out_qty
FROM products AS a
INNER JOIN products_sizes AS c ON c.prod_code = a.code) AS e
ORDER BY e.group_id ASC, e.code ASC, e.size_code ASC

---

不能从

select

语句中引用同一字段。一个选项是将结果移动到另一个子查询中，然后执行计算：

select *, (in_qty - out_qty) AS balance
from (
    SELECT 
        a.group_id,
        a.code,
        a.description_en,
        c.size_code,
        (
            SELECT
                SUM(b.qty)
            FROM
                receiving_details AS b
            WHERE
                b.code = c.size_code
        ) AS in_qty,
        (
            SELECT
                SUM(d.qty)
            FROM
                requisition_details AS d
            WHERE
                d. matl_code = c.size_code
        ) AS out_qty
    FROM products AS a
    INNER JOIN products_sizes AS c ON c.prod_code = a.code
) t
ORDER BY group_id, code, size_code

---

你从stmt得到了什么3@Drew，抱歉…在此之前，'FROM products AS'查看@sgeddes所说的，否则另一个包装器将使该列别名工作。这意味着另一个使用派生表的外部包装器拥有它们available@Drew，是的，我刚刚读了斯盖德斯的回复，并接受了它作为答案。谢谢你的帮助。@sgedes…太好了！经过几次修正后，我终于得到了我想要的结果。你的回答提供了途径。谢谢！