Sql 以外部变量开始的计算字段

我有一个带有字段的表Tbl1：description、Dtype、valueLeft、valueRight 数据可能如下所示：

+----------------------------------------------+
| Description | Dtype | valueLeft | valueRight |
+----------------------------------------------+
| Art1        |   1   |     5     |       0    |
| Art2        |   1   |     3     |       0    |
| Art3        |   2   |     0     |       6    |
| Art4        |   1   |     11    |       0    |
| Art5        |   2   |     0     |       1    |
| ArtN        | ....  |    ....   |      ....  |
+..............................................+

字段数据类型只能为1或2

问题：我想对新列进行选择，该列根据字段Dtype 1=SUM valueLeft，2=Decrete valueRight上的值求和valueLeft或valueRight

例如：

+----------------------------------------------+
| Description | Dtype | valueLeft | valueRight | newcol
+----------------------------------------------+
| Art1        |   1   |     5     |       0    |    5
| Art2        |   1   |     3     |       0    |    8
| Art3        |   2   |     0     |       6    |    2
| Art4        |   1   |     11    |       0    |    13
| Art5        |   2   |     0     |       1    |    10

---

我该怎么做？

因为您没有指定RDBMS。。。使用SQL Server语法：

---

为什么？您尝试过什么？您使用的是什么rdbms？

SELECT a.Description, a.Dtype, a.valueLeft, a.valueRight, 
            (SELECT SUM (CASE WHEN b.Dtype = 1 THEN b.valueLeft ELSE -b.valueRight END)
             FROM (
                    SELECT  ROW_NUMBER() OVER (ORDER BY Description) as id, Dtype, valueLeft, valueRight 
                    FROM Tbl1 ) b
             WHERE b.id <= a.id)
FROM (SELECT ROW_NUMBER() OVER (ORDER BY Description) as id,  Description, Dtype, valueLeft, valueRight 
      FROM Tbl1 ) a