SQL中的叠加--结果集合转换为字符串(小提示)
SQL中的叠加(小提示):有如下的需求,用SQL实现把结果集合转换为字符串.
如下:
可以有至少如下三种方法来做转换:
1. 在.NET中通常的做法是先取出结果集放在DataTable中,再利用foreach循环取出每个字段串联起来. 亦如这样,在SQL中也有相同的做法,就是用游标(CURSOR)做循环,示例代码如下:
Cursor
declare
@mytable
table
(
Col1
varchar
(
10
)
)
insert
into
@mytable
values
(
'
AA
'
),
(
'
BB
'
),
(
'
CC
'
)
select
SUBSTRING
(
@Colstring
,
2
,
LEN
(
@Colstring
)
-
1
)
as
Colstring
declare mytype cursor local for
select Col1 from @mytable
open mytype
fetch next from mytype into @Col1
while @@FETCH_STATUS=0
begin
set @Colstring+=','+@Col1
fetch next from mytype into @Col1
end
deallocate mytypedeclare @Colstring varchar(50)=''
declare @Col1 varchar(10)
转换之后的字符串:
CURSOR在SQL中的效率很差,当数据量比较大时(>1 Million)会严重影响性能,不建议使用.
2. SQL查询中直接赋值,这种方式CODE比较简捷.
代码
declare
@mytable
table
(
Col1
varchar
(
10
)
)
insert
into
@mytable
values
(
'
AA
'
),
(
'
BB
'
),
(
'
CC
'
)
declare @Colstring varchar(50)
select @Colstring = isnull(@Colstring + ',' , '' ) + isnull( Col1 , '' )
from @mytable
select
@Colstring
as
Colstring
运行结果: 
3. FOR XML PATH
首先转换为XML的数据库类型.
declare
@mytable
table
(
Col1
varchar
(
10
)
)
insert
into
@mytable
values
(
'
AA
'
),
(
'
BB
'
),
(
'
CC
'
)
SELECT
'
,
'
+
Col1 --No alias
FROM
@mytable
ORDER
BY
Col1
FOR
XML PATH(
'TYPE'
)
可以看到输入结果是XML类型的,TYPE为节点
<
TYPE
>
,AA
</
TYPE
>
<
TYPE
>
,BB
</
TYPE
>
<
TYPE
>
,CC
</
TYPE
>
假如没有TYPE节点,就会接近想要的结果,修改CODE如下:
代码
declare
@mytable
table
(
Col1
varchar
(
10
)
)
insert
into
@mytable
values
(
'
AA
'
),
(
'
BB
'
),
(
'
CC
'
)
SELECT
'
,
'
+
Col1
--
No alias
FROM
@mytable
ORDER
BY
Col1
FOR
XML PATH(
''
)
太好了,只需要把前置逗号(,)取消就OK了,接续修改CODE:
FOR XML PATH
declare
@mytable
table
(
Col1
varchar
(
10
)
)
insert
into
@mytable
values
(
'
AA
'
),
(
'
BB
'
),
(
'
CC
'
)
select
STUFF
((
SELECT
'
,
'
+
Col1
--
No alias
FROM
@mytable
ORDER
BY
Col1
FOR
XML PATH(
''
)) ,
1
,
1
,
space
(
0
))
as
Colstring
得到需求的结果:
扩展如下:
代码
declare
@mytable
table
(
id
int
,
potype
varchar
(
10
)
)
insert
into
@mytable
values
(
1
,
'
A>A
'
)
insert
into
@mytable
values
(
1
,
'
B&B
'
)
insert
into
@mytable
values
(
1
,
'
C<C
'
)
SELECT
p1.id,
STUFF
( (
SELECT
'
,
'
+
potype
FROM
@mytable
p2
WHERE
p2.id
=
p1.id
ORDER
BY
potype
FOR
XML PATH(
''
),TYPE
).value(
'
.
'
,
'
VARCHAR(MAX)
'
) ,
1
,
1
,
SPACE
(
0
))
AS
Concat_Values
FROM
@mytable
p1
GROUP
BY
p1.id ;
代码
DECLARE
@mytable
TABLE
(
id
INTEGER
NOT
NULL
,
potype
VARCHAR
(
10
)
NOT
NULL
);
INSERT
@mytable
VALUES
(
1
,
'
A>A
'
);
INSERT
@mytable
VALUES
(
1
,
'
B&B
'
);
INSERT
@mytable
VALUES
(
1
,
'
C<C
'
);
SELECT
P1.id,
csv
=
STUFF
(
(
SELECT
'
,
'
+
P2.potype
FROM
@mytable
P2
WHERE
P2.id
=
P1.id
ORDER
BY
P2.potype
ASC
FOR
XML PATH(
''
), TYPE
).value(
'
./text()[1]
'
,
'
VARCHAR(MAX)
'
)
,
1
,
1
,
SPACE
(
0
)
)
FROM
@mytable
P1
GROUP
BY
P1.id;
推荐采用第二种,或是第三种方法.