如何根據鍵找到特定列的值之間的差異？

Question

我有兩個表作爲purchase_details和invoice_details

我想從這兩個表的數據中存儲每個產品的庫存/庫存。

結構purchase_details。

'pid', 'int(10)' 
'product_id', 'int(10)' 
'quantity', 'float(8,2)' 
'amount', 'float(12,2)' 
'expiry_date', 'date' 

的invoice_details結構。

'invoice_id', 'int(10) unsigned' 
'product_id', 'int(10) unsigned' 
'quantity', 'float(10,2)' 
'price', 'float(12,2)' 

我想計算剩餘股票 （從purchase_details的產品和數量）的總量 - （從invoice_details的產品和數量）

PRODUCT_ID是將相同的兩張表。

產品表由產品數據，並且該結構是

'id', 'int(10) unsigned' 
'cid', 'int(10) unsigned' 
'name', 'varchar(90)' 
'selling_price', 'float(10,2)' 
'mrp', 'float(10,2)' 
'reorder_level', 'bigint(20) unsigned' 

的invoice_details可以或可以不具有用於每PRODUCT_ID條目。

我該怎麼辦？

Answer 1

select (sum(purchase_details.quantity) - sum(invoice_details.quantity)) as stock 
    from products 
    left outer join purchase_details 
    on products.product_id == purchase_details.product_id 
    left outer join invoice_details 
    on products.product_id = invoice_details.product_id 
    group by products.product_id 

這會給你的股票爲每個產品，如果你需要某些產品

select (sum(purchase_details.quantity) - sum(invoice_details.quantity)) as stock 
    from products 
    left outer join purchase_details 
    on products.product_id == purchase_details.product_id 
    left outer join invoice_details 
    on products.product_id = invoice_details.product_id 
    where products.product_id = 137 
    group by products.product_id 

Answer 2

這是我的新答案，已經在sqlserver上測試過了。我不知道關於MySQL

select pro.id, SUM(pro.quantity - inv.quantity) 
from (select sum(p.quantity) as quantity, p.id as id from product p group by p.id) as pro, 
    (select sum(i.quantity) as quantity, i.id as id from invoice i group by i.id) as inv 
where inv.id = pro.id 
group by pro.id;