Mysql 如何获取产品名称、数量和余额？

我有三张桌子

DMZ:

Ndm int - number of Document
Ddm date - date of supply
Pr int - 1(income), 2(expense)

Id int
Kol decimal - quantity of product
Price decimal
Ndm int - number of document foreign key with DMZ
Ktov int - id of product foreign key with TOV

Ktov - id if product
Ntov - name of product

DMS:

Ndm int - number of Document
Ddm date - date of supply
Pr int - 1(income), 2(expense)

Id int
Kol decimal - quantity of product
Price decimal
Ndm int - number of document foreign key with DMZ
Ktov int - id of product foreign key with TOV

Ktov - id if product
Ntov - name of product

TOV:

Ndm int - number of Document
Ddm date - date of supply
Pr int - 1(income), 2(expense)

Id int
Kol decimal - quantity of product
Price decimal
Ndm int - number of document foreign key with DMZ
Ktov int - id of product foreign key with TOV

Ktov - id if product
Ntov - name of product

我需要得到 余额是收入和支出之间的差额。

结构：

产品名称（Ntov）

残留量

余额的金额精确到分（差额 在销售过程中花费的和收到的之间）

按Ntov分类， 每种产品， 对于每个有收入也可能有支出的产品，报告中有一个汇总行

现在我有了这个sql请求，但我被卡住了：/ 请帮帮我，我做错了什么

SELECT
 Ntov AS Product,
 SUM(CASE WHEN Pr = 1 then Kol*Price ELSE 0 END) AS Income,
 SUM(CASE WHEN Pr = 2 then Kol*Price ELSE 0 END) AS Expense,
 COUNT(DMS.Kol) AS LeftProducts
FROM DMS LEFT JOIN TOV
on DMS.Ktov = Tov.Ktov 
LEFT JOIN DMZ on DMS.Ndm = DMZ.Ndm
GROUP BY Ntov, Kol, Price
ORDER BY Product

DMS数据：
DMZ数据：
TOV数据：

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结果：

您可以聚合和执行条件和。考虑：

select
    t.ntov,
    sum(case z.pr when 1 then s.kol when 2 then - s.kol else 0 end) residue,
    sum(case z.pr when 1 then s.price when 2 then - s.price else 0 end) balance
from tov t
left join dms s on t.ktov = s.ktov
left join dmz z on s.ndm = z.ndm
group by t.ntov, t.ktov
order by t.ntov

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示例数据和期望的结果将是一个很大的帮助，特别是因为列名不是很有意义。抱歉，添加了带有数据的屏幕截图谢谢，我不知道我可以使用2次“when”