SQL查询，将值设置为自身加上另一个字段的值

我有一个客户表，其中

customerid

作为

PK

和

currentordertotal

字段。我想合并两个客户记录的

currentordertotal

例如：

Customer C1, Current Order Total = 2

Customer C2, Current Order Total = 4

我想将

C1的当前订单总额

更新为自身（2）加上

C2的当前订单总额

（so 6）

比如：

UPDATE customers SET currentordertotal = (itself + currentordertotal  WHERE customerid = C2) WHERE customerid = C1;

我在网上搜索过类似的查询，但找不到

这是一个Oracle数据库

您可以执行子查询

UPDATE customers SET currentordertotal = currentordertotal  + (select currentordertotal from customers WHERE customerid = C2) WHERE customerid = C1

您可以执行子查询

UPDATE customers SET currentordertotal = currentordertotal  + (select currentordertotal from customers WHERE customerid = C2) WHERE customerid = C1

---

子查询是正确的方法，但我们要小心：

update customers
    set currentordertotal = (coalesce(currentordertotal, 0) +
                             coalesce((select c2.currentordertotal
                                       from customers c2
                                       where c2.customerid = 'C2'
                                      ), 0)
                            )
    where customerid = 'C1';

---

合并是为了防止第二个客户丢失或值为

NULL

子查询是正确的方法，但我们要小心：

update customers
    set currentordertotal = (coalesce(currentordertotal, 0) +
                             coalesce((select c2.currentordertotal
                                       from customers c2
                                       where c2.customerid = 'C2'
                                      ), 0)
                            )
    where customerid = 'C1';

合并是为了防止第二个客户丢失或值为

NULL