我提供了三种不同的方法来解决这个问题：
- 使用枢轴
- 使用案例陈述
- 在 where 子句中使用内联查询
所有解决方案都假设我们根据orderId 列确定“最新”顺序。使用createDate 列会由于时间戳冲突而增加复杂性并严重影响性能，因为createDate 可能不是索引键的一部分。我只使用 MS SQL Server 2005 测试了这些查询，所以我不知道它们是否可以在您的服务器上运行。
解决方案 (1) 和 (2) 的性能几乎相同。事实上，它们都导致从数据库中读取相同数量的数据。
解决方案 (3) 不是在处理大型数据集时的首选方法。它始终比 (1) 和 (2) 进行数百次逻辑读取。当过滤一个特定用户时，方法（3）与其他方法相当。在单用户情况下，cpu 时间的减少有助于抵消显着增加的读取次数；但是，随着磁盘驱动器变得更加繁忙和缓存未命中的发生，这种微小的优势将消失。
结论
对于呈现的场景，如果您的 DBMS 支持，请使用枢轴方法。与 case 语句相比，它需要更少的代码，并简化了将来添加订单类型的过程。
请注意，在某些情况下，PIVOT 不够灵活，使用 case 语句的特征值函数是可行的方法。
代码
使用 PIVOT 的方法（1）：
select
ud.userId, ud.fullname,
od1.orderId as orderId1, od1.createDate as createDate1, od1.orderType as orderType1,
od2.orderId as orderId2, od2.createDate as createDate2, od2.orderType as orderType2
from userData ud
inner join (
select userId, [1] as typeOne, [2] as typeTwo
from (select
userId, orderType, orderId
from orderData) as orders
PIVOT
(
max(orderId)
FOR orderType in ([1], [2])
) as LatestOrders) as LatestOrders on
LatestOrders.userId = ud.userId
inner join orderData od1 on
od1.orderId = LatestOrders.typeOne
inner join orderData od2 on
od2.orderId = LatestOrders.typeTwo
使用案例语句的方法（2）：
select
ud.userId, ud.fullname,
od1.orderId as orderId1, od1.createDate as createDate1, od1.orderType as orderType1,
od2.orderId as orderId2, od2.createDate as createDate2, od2.orderType as orderType2
from userData ud
-- assuming not all users will have orders use outer join
inner join (
select
od.userId,
-- can be null if no orders for type
max (case when orderType = 1
then ORDERID
else null
end) as maxTypeOneOrderId,
-- can be null if no orders for type
max (case when orderType = 2
then ORDERID
else null
end) as maxTypeTwoOrderId
from orderData od
group by userId) as maxOrderKeys on
maxOrderKeys.userId = ud.userId
inner join orderData od1 on
od1.ORDERID = maxTypeTwoOrderId
inner join orderData od2 on
OD2.ORDERID = maxTypeTwoOrderId
方法 (3) 在 where 子句中使用内联查询（基于 Steve K. 的回复）：
select ud.userId,ud.fullname,
order1.orderId, order1.orderType, order1.createDate,
order2.orderId, order2.orderType, order2.createDate
from userData ud,
orderData order1,
orderData order2
where ud.userId = order1.userId
and ud.userId = order2.userId
and order1.orderId = (select max(orderId)
from orderData od1
where od1.userId = ud.userId
and od1.orderType = 1)
and order2.orderId = (select max(orderId)
from orderData od2
where od2.userId = ud.userId
and od2.orderType = 2)
生成表格和 1000 个用户的脚本，每个用户有 100 个订单：
CREATE TABLE [dbo].[orderData](
[orderId] [int] IDENTITY(1,1) NOT NULL,
[createDate] [datetime] NOT NULL,
[orderType] [tinyint] NOT NULL,
[userId] [int] NOT NULL
)
CREATE TABLE [dbo].[userData](
[userId] [int] IDENTITY(1,1) NOT NULL,
[fullname] [nvarchar](50) NOT NULL
)
-- Create 1000 users with 100 order each
declare @userId int
declare @usersAdded int
set @usersAdded = 0
while @usersAdded < 1000
begin
insert into userData (fullname) values ('Mario' + ltrim(str(@usersAdded)))
set @userId = @@identity
declare @orderSetsAdded int
set @orderSetsAdded = 0
while @orderSetsAdded < 10
begin
insert into orderData (userId, createDate, orderType)
values ( @userId, '01-06-08', 1)
insert into orderData (userId, createDate, orderType)
values ( @userId, '01-02-08', 1)
insert into orderData (userId, createDate, orderType)
values ( @userId, '01-08-08', 1)
insert into orderData (userId, createDate, orderType)
values ( @userId, '01-09-08', 1)
insert into orderData (userId, createDate, orderType)
values ( @userId, '01-01-08', 1)
insert into orderData (userId, createDate, orderType)
values ( @userId, '01-06-06', 2)
insert into orderData (userId, createDate, orderType)
values ( @userId, '01-02-02', 2)
insert into orderData (userId, createDate, orderType)
values ( @userId, '01-08-09', 2)
insert into orderData (userId, createDate, orderType)
values ( @userId, '01-09-01', 2)
insert into orderData (userId, createDate, orderType)
values ( @userId, '01-01-04', 2)
set @orderSetsAdded = @orderSetsAdded + 1
end
set @usersAdded = @usersAdded + 1
end
用于在 MS SQL Server 上测试查询性能以及 SQL Profiler 的小 sn-p：
-- Uncomment these to clear some caches
--DBCC DROPCLEANBUFFERS
--DBCC FREEPROCCACHE
set statistics io on
set statistics time on
-- INSERT TEST QUERY HERE
set statistics time off
set statistics io off