如果id更改，则SQL重置变量值

Question

SELECT * FROM quantity;
+----+-----------+------------+-------------+
| id | productid | inquantity | outquantity |
+----+-----------+------------+-------------+
|  1 |         1 |        100 |           0 |
|  2 |         1 |         10 |           0 |
|  3 |         1 |          0 |          50 |
|  4 |         1 |          0 |         100 |
|  5 |         2 |        200 |           0 |
|  6 |         2 |          0 |         200 |
|  7 |         2 |         15 |           0 |
|  8 |         3 |        100 |           0 |
|  9 |         3 |         50 |           0 |
| 10 |         3 |          0 |           5 |
+----+-----------+------------+-------------+

计算特定产品的数量我这样做

SELECT 
id , productid,
inquantity,
outquantity,
@qty:= (@qty+inquantity)-outquantity as qty 
from quantity,(select @qty:= 0 )r 
WHERE productid=1;
+----+-----------+------------+-------------+-----+
| id | productid | inquantity | outquantity | qty |
+----+-----------+------------+-------------+-----+
|  1 |         1 |        100 |           0 | 100 |
|  2 |         1 |         10 |           0 | 110 |
|  3 |         1 |          0 |          50 |  60 |
|  4 |         1 |          0 |         100 | -40 |
+----+-----------+------------+-------------+-----+

是否有可能为所有产品计算这个？ 如果productid与privious不同，如何将变量@qty重置为从0开始？

要得到这个：

+----+-----------+------------+-------------+-----+
| id | productid | inquantity | outquantity | qty |
+----+-----------+------------+-------------+-----+
|  1 |         1 |        100 |           0 | 100 |
|  2 |         1 |         10 |           0 | 110 |
|  3 |         1 |          0 |          50 |  60 |
|  4 |         1 |          0 |         100 | -40 |
|  5 |         2 |        200 |           0 | 200 |
|  6 |         2 |          0 |         200 |   0 |
|  7 |         2 |         15 |           0 |  15 |
|  8 |         3 |        100 |           0 | 100 |
|  9 |         3 |         50 |           0 | 150 |
| 10 |         3 |          0 |           5 | 145 |
+----+-----------+------------+-------------+-----+

Answer 1

您可以使用case语句和其他变量来检查相同的产品ID

SELECT 
id , productid,
inquantity,
outquantity,
@qty := case when @g = productid then @qty else 0 end,
@qty:= (@qty+inquantity)-outquantity as qty ,
@g := productid
from quantity,
(select @qty:= 0 ,@g=null)r 
order by productid

DEMO

Answer 2

我认为

| 7 | 2 | 15 | 0 | -15 |

数量为15而不是-15

你可以得到结果

select
id,
productid,
inquantity,
outquantity,
qty from (
 SELECT 
 id , 
 productid,
 inquantity,
 outquantity,
 @qty:= if(@prev_prod = productid, ((@qty+inquantity)-outquantity),(inquantity-outquantity)) as qty,
 @prev_prod:=  productid 
 from quantity,(select @qty:= 0,@prev_prod:=0 )r
 order by productid,id 
)x;