sql - select 语句可以包含嵌套结果集吗？

Question

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;

Answer 1

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