给出下表和示例数据:
create table Orders (
ID int not null primary key,
Customer nvarchar(100) not null);
create table OrderItems (
ID int not null primary key,
OrderID int not null foreign key references Orders(ID),
Product nvarchar(100) not null);
insert into Orders values (1, 'John');
insert into Orders values (2, 'James');
insert into OrderItems values (1, 1, 'Guitar');
insert into OrderItems values (2, 1, 'Bass');
insert into OrderItems values (3, 2, 'Guitar');
insert into OrderItems values (4, 2, 'Drums');
我想知道是否可以查询父 Orders
表,并获取子 OrderItems
表作为父结果中的嵌套结果集。像这样的事情:
| ORDER.ID | ORDER.CUSTOMER | ORDER.ORDERITEMS |
------------------------------------------------------------------
| | | ORDERITEMS.ID | ORDERITEMS.PRODUCT |
| | |-------------------------------------
| 1 | John | 1 | Guitar |
| | | 2 | Bass |
| 2 | James | 3 | Guitar |
| | | 4 | Drums |
我想到的查询(在 SQL Server 中不起作用)是这样的:
-- doesn't work, but shows the intent to have nested result sets
select
o.OrderID [Order.ID], o.Customer [Order.Customer],
(select
oi.ID [OrderItems.ID], oi.Product [OrderItems.Product]
from OrderItems oi where o.ID = oi.OrderID
) [Order.OrderItems]
from Orders o;
这只是一个概念性问题,因为我正在尝试寻找以最少重复的方式获取相关数据的方法(例如,与 join
所发生的情况相反)。
SQL fiddle here .
更新
我从this answer得知Oracle 通过游标表达式支持它:
select
o.*,
cursor(select oi.* from OrderItems oi where o.ID = oi.OrderID) as OrderItems
from Orders o;
最佳答案
没有。这实际上是不可能的。
SQL Server 不支持嵌套关系和 NF²
尽管您可以使用FOR XML PATH
以分层方式将其恢复。
SELECT ID AS [@ID],
Customer AS [@Customer],
(SELECT ID AS [@ID],
OrderID AS [@OrderID],
Product AS [@Product]
FROM OrderItems
WHERE OrderItems.OrderID = o.ID
FOR XML PATH('OrderItems'), TYPE)
FROM Orders o
FOR XML PATH('Order'), ROOT('Orders')
返回
<Orders>
<Order ID="1" Customer="John">
<OrderItems ID="1" OrderID="1" Product="Guitar" />
<OrderItems ID="2" OrderID="1" Product="Bass" />
</Order>
<Order ID="2" Customer="James">
<OrderItems ID="3" OrderID="2" Product="Guitar" />
<OrderItems ID="4" OrderID="2" Product="Drums" />
</Order>
</Orders>
这不会重复父订单
关于sql - select 语句可以包含嵌套结果集吗?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/15748497/