适用于Microsoft SQL Server 2008 R2。
如果我们有几十个Outer Apply（30），那么他们开始工作得相当慢。在Outer Apply的中间，我有一个比简单选择更复杂的东西，一个视图。
我正在编写一种分配给表的属性（在数据库中）。通常，一些表包含对属性表（键，值）的引用。
伪结构看起来像这样：
DECLARE @Lot TABLE (
LotId INT PRIMARY KEY IDENTITY,
SomeText VARCHAR(8))
INSERT INTO @Lot
OUTPUT INSERTED.*
VALUES ('Hello'), ('World')
DECLARE @Attribute TABLE(
AttributeId INT PRIMARY KEY IDENTITY,
LotId INT,
Val VARCHAR(8),
Kind VARCHAR(8))
INSERT INTO @Attribute
OUTPUT INSERTED.* VALUES
(1, 'Foo1', 'Kind1'), (1, 'Foo2', 'Kind2'),
(2, 'Bar1', 'Kind1'), (2, 'Bar2', 'Kind2'), (2, 'Bar3', 'Kind3')
LotId SomeText
----------- --------
1 Hello
2 World
AttributeId LotId Val Kind
----------- ----------- -------- --------
1 1 Foo1 Kind1
2 1 Foo2 Kind2
3 2 Bar1 Kind1
4 2 Bar2 Kind2
5 2 Bar3 Kind3
我现在可以运行查询，例如：
SELECT
[l].[LotId]
, [SomeText]
, [Oa1].[AttributeId]
, [Oa1].[LotId]
, 'Kind1Val' = [Oa1].[Val]
, [Oa1].[Kind]
, [Oa2].[AttributeId]
, [Oa2].[LotId]
, 'Kind2Val' = [Oa2].[Val]
, [Oa2].[Kind]
, [Oa3].[AttributeId]
, [Oa3].[LotId]
, 'Kind3Val' = [Oa3].[Val]
, [Oa3].[Kind]
FROM @Lot AS l
OUTER APPLY(SELECT * FROM @Attribute AS la WHERE la.[LotId] = l.[LotId] AND la.[Kind] = 'Kind1') AS Oa1
OUTER APPLY(SELECT * FROM @Attribute AS la WHERE la.[LotId] = l.[LotId] AND la.[Kind] = 'Kind2') AS Oa2
OUTER APPLY(SELECT * FROM @Attribute AS la WHERE la.[LotId] = l.[LotId] AND la.[Kind] = 'Kind3') AS Oa3
LotId SomeText AttributeId LotId Kind1Val Kind AttributeId LotId Kind2Val Kind AttributeId LotId Kind3Val Kind
----------- -------- ----------- ----------- -------- -------- ----------- ----------- -------- -------- ----------- ----------- -------- --------
1 Hello 1 1 Foo1 Kind1 2 1 Foo2 Kind2 NULL NULL NULL NULL
2 World 3 2 Bar1 Kind1 4 2 Bar2 Kind2 5 2 Bar3 Kind3
简单方式获取属性值的数据透视表以及没有属性如Kind3的Lot行的结果。 我知道微软PIVOT并不简单，不适合这里。
最后，什么会更快，并会给出相同的结果？
答案 0 :(得分：5)
为了获得结果，您可以取消隐藏，然后转移数据。
有两种方法可以执行此操作。首先，您可以使用UNPIVOT和PIVOT函数：
select *
from
(
select LotId,
SomeText,
col+'_'+CAST(rn as varchar(10)) col,
value
from
(
select l.LotId,
l.SomeText,
cast(a.AttributeId as varchar(8)) attributeid,
cast(a.LotId as varchar(8)) a_LotId,
a.Val,
a.Kind,
ROW_NUMBER() over(partition by l.lotid order by a.attributeid) rn
from @Lot l
left join @Attribute a
on l.LotId = a.LotId
) src
unpivot
(
value
for col in (attributeid, a_Lotid, val, kind)
) unpiv
) d
pivot
(
max(value)
for col in (attributeid_1, a_LotId_1, Val_1, Kind_1,
attributeid_2, a_LotId_2, Val_2, Kind_2,
attributeid_3, a_LotId_3, Val_3, Kind_3)
) piv
或者从SQL Server 2008+开始，您可以使用带有CROSS APPLY子句的VALUES来取消数据的转出：
select *
from
(
select LotId,
SomeText,
col+'_'+CAST(rn as varchar(10)) col,
value
from
(
select l.LotId,
l.SomeText,
cast(a.AttributeId as varchar(8)) attributeid,
cast(a.LotId as varchar(8)) a_LotId,
a.Val,
a.Kind,
ROW_NUMBER() over(partition by l.lotid order by a.attributeid) rn
from @Lot l
left join @Attribute a
on l.LotId = a.LotId
) src
cross apply
(
values ('attributeid', attributeid),('LotId', a_LotId), ('Value', Val), ('Kind', Kind)
) c (col, value)
) d
pivot
(
max(value)
for col in (attributeid_1, LotId_1, Value_1, Kind_1,
attributeid_2, LotId_2, Value_2, Kind_2,
attributeid_3, LotId_3, Value_3, Kind_3)
) piv
unpivot进程为每个LotID和SomeText获取多列，并将其转换为包含结果的行：
| LOTID | SOMETEXT | COL | VALUE |
--------------------------------------------
| 1 | Hello | attributeid_1 | 1 |
| 1 | Hello | LotId_1 | 1 |
| 1 | Hello | Value_1 | Foo1 |
| 1 | Hello | Kind_1 | Kind1 |
| 1 | Hello | attributeid_2 | 2 |
我在内部子查询中添加了row_number()，用于创建要旋转的新列名。创建名称后，可以将数据透视表应用于新列，以显示最终结果
这也可以使用动态SQL来完成：
DECLARE @cols AS NVARCHAR(MAX),
@query AS NVARCHAR(MAX)
select @cols = STUFF((SELECT ',' + QUOTENAME(col+'_'+rn)
from
(
select
cast(ROW_NUMBER() over(partition by l.lotid order by a.attributeid) as varchar(10)) rn
from Lot l
left join Attribute a
on l.LotId = a.LotId
) t
cross apply (values ('attributeid', 1),
('LotId', 2),
('Value', 3),
('Kind', 4)) c (col, so)
group by col, rn, so
order by rn, so
FOR XML PATH(''), TYPE
).value('.', 'NVARCHAR(MAX)')
,1,1,'')
set @query = 'SELECT LotId,
SomeText,' + @cols + '
from
(
select LotId,
SomeText,
col+''_''+CAST(rn as varchar(10)) col,
value
from
(
select l.LotId,
l.SomeText,
cast(a.AttributeId as varchar(8)) attributeid,
cast(a.LotId as varchar(8)) a_LotId,
a.Val,
a.Kind,
ROW_NUMBER() over(partition by l.lotid order by a.attributeid) rn
from Lot l
left join Attribute a
on l.LotId = a.LotId
) src
cross apply
(
values (''attributeid'', attributeid),(''LotId'', a_LotId), (''Value'', Val), (''Kind'', Kind)
) c (col, value)
) x
pivot
(
max(value)
for col in (' + @cols + ')
) p '
execute(@query)
所有三个版本都会给出相同的结果：
| LOTID | SOMETEXT | ATTRIBUTEID_1 | LOTID_1 | VALUE_1 | KIND_1 | ATTRIBUTEID_2 | LOTID_2 | VALUE_2 | KIND_2 | ATTRIBUTEID_3 | LOTID_3 | VALUE_3 | KIND_3 |
-----------------------------------------------------------------------------------------------------------------------------------------------------------
| 1 | Hello | 1 | 1 | Foo1 | Kind1 | 2 | 1 | Foo2 | Kind2 | (null) | (null) | (null) | (null) |
| 2 | World | 3 | 2 | Bar1 | Kind1 | 4 | 2 | Bar2 | Kind2 | 5 | 2 | Bar3 | Kind3 |