sql-server – SQL Server：临时收集聚合中的值,并在同一查询中

SELECT a[1] || a[i] AS foo,a[2] || a[5] AS bar  -- assuming we have >= 5 rows for simplicity
FROM   (
    SELECT array_agg(text_col ORDER BY text_col) AS a,count(*)::int4 AS i
    FROM   tbl
    WHERE  id between 10 AND 100
    ) x

我如何用T-SQL最好地解决这个问题？
我能想出的最好的是两个CTE和子选择：

;WITH x AS (
  SELECT row_number() OVER (ORDER BY name) AS rn,name AS a
  FROM   #t
  WHERE  id between 10 AND 100
  ),i AS (
  SELECT count(*) AS i
  FROM   x
  )
SELECT (SELECT a FROM x WHERE rn = 1) + (SELECT a FROM x WHERE rn = i) AS foo,(SELECT a FROM x WHERE rn = 2) + (SELECT a FROM x WHERE rn = 5) AS bar
FROM   i

测试设置：

CREATE TABLE #t(
 id   INT PRIMARY KEY,name NVARCHAR(100))

INSERT INTO #t VALUES
 (3,'John'),(5,'Mary'),(8,'Michael'),(13,'Steve'),(21,'Jack'),(34,'Pete'),(57,'Ami'),(88,'Bob')

有更简单的方法吗？

解决方法

编辑2：对于解决方案编号4)我添加了第三种连接方法.

我不确定我是否理解你的问题.

a)不使用SQL Server中的数组,而是使用表变量或XML.

b)要连接字符串(在本例中),我将使用SELECT @var = @var Name FROM tbl语句或XML xqueries.

c)基于CTE和多个子查询的解决方案(WITH cte AS()FROM SELECT(SELECT * FROM cte.rn = 1)()…)将生成大量扫描和逻辑读取.

解决方案：
1)表变量SELECT @var = @var Name FROM tbl：

--Creating the "array"
DECLARE @Array TABLE
(
    Idx     INT PRIMARY KEY,Val     NVARCHAR(100) NOT NULL
);

WITH Base
AS
(
    SELECT  Val = t.name,Idx = ROW_NUMBER() OVER(ORDER BY t.name ASC)
    FROM    #t t
    WHERE  t.id between 10 AND 100
)
INSERT  @Array (Idx,Val)
SELECT  b.Idx,b.Val
FROM    Base b;

--Concatenating all names
DECLARE @AllNames NVARCHAR(4000);
--”Reset”/Init @AllNames
SET     @AllNames = '';
--String concatenation
SELECT  @AllNames = @AllNames + ',' + a.Val
FROM    @Array a;
--Remove first char (',')
SELECT  @AllNames = STUFF(@AllNames,1,'');
--The final result
SELECT  @AllNames [Concatenating all names - using a table variable];
/*
Concatenating all names - using a table variable
------------------------------------------------
Ami,Bob,Jack,Pete,Steve
*/

--Concatenating Idx=2 and Idx=5
--”Reset” @AllNames value
SET     @AllNames = '';
--String concatenation
SELECT  @AllNames = @AllNames + ',' + a.Val
FROM    @Array a
WHERE   a.Idx IN (2,5) --or a.Idx IN (2,(SELECT COUNT(*) FROM @Array))
ORDER BY a.Idx ASC;
--Remove first char (','');
--The final result
SELECT  @AllNames [Concatenating Idx=2 and Idx=5 - using a table variable];
/*
Concatenating Idx=2 and Idx=5 - using a table variable
------------------------------------------------------
Bob,Steve
*/

2)表变量PIVOT：

--Concatenating a finite number of elements (names)
SELECT   pvt.[1] + ',' + pvt.[0]    AS [PIVOT Concat_1_and_i(0)],pvt.[2] + ',' + pvt.[5]    AS [PIVOT Concat_2_and_5],pvt.*
FROM    
(
        SELECT  a.Idx,a.Val
        FROM    @Array a
        WHERE   a.Idx IN (1,2,5)
        UNION ALL   
        SELECT  0,a.Val --The last element has Idx=0
        FROM    @Array a
        WHERE   a.Idx = (SELECT COUNT(*) FROM @Array)
) src
PIVOT   (MAX(src.Val) FOR src.Idx IN ([1],[2],[5],[0])) pvt;
/*
PIVOT Concat_1_and_i(0) PIVOT Concat_2_and_5
----------------------- --------------------
Ami,Steve               Bob,Steve           
*/

3)XML XQuery：

SET ANSI_WARNINGS ON;
GO

DECLARE @x XML;
;WITH Base
AS
(
    SELECT  Val = t.name,Idx = ROW_NUMBER() OVER(ORDER BY t.name ASC)
    FROM    #t t
    WHERE   t.id BETWEEN 10 AND 100
)
SELECT  @x = 
(
    SELECT   b.Idx  AS [@Idx],b.Val  AS [text()]
    FROM    Base b
    FOR XML PATH('Element'),ROOT('Array')
);
/* @x content
<Array>
  <Element Idx="1">Ami</Element>
  <Element Idx="2">Bob</Element>
  <Element Idx="3">Jack</Element>
  <Element Idx="4">Pete</Element>
  <Element Idx="5">Steve</Element>
</Array>
*/

--Concatenating all names (the result is XML,so a cast is needed)
DECLARE @r XML; --XML result
SELECT  @r=@x.query('
(: $e = array element :)
for $e in (//Array/Element)
    return string($e)
');
SELECT  REPLACE(CONVERT(NVARCHAR(4000),@r),' ',',') AS [Concatenating all names - using XML];
/*
Concatenating all names - using XML
-----------------------------------
Ami,Steve
*/

--Concatenating Idx=1 and all names
SELECT  @r=@x.query('
(: $e = array element :)
for $e in (//Array/Element[@Idx=1],//Array/Element)
    return string($e)
');
SELECT  REPLACE(CONVERT(NVARCHAR(4000),') AS [Concatenating Idx=1 and all names - using XML];
/*
Concatenating Idx=1 and all names - using XML
---------------------------------------------
Ami,Ami,Steve
*/

--Concatenating Idx=1 and i(last name)
DECLARE @i INT;
SELECT  @r=@x.query('
(: $e = array element :)
for $e in (//Array/Element[@Idx=1],//Array/Element[@Idx=count(//Array/Element)])
    return string($e)
');
SELECT  REPLACE(CONVERT(NVARCHAR(4000),') AS [Concatenating Idx=1 and i(last name) - using XML];
/*
Concatenating Idx=1 and i(last name) - using XML
------------------------------------------------
Ami,Steve
*/


--Concatenating Idx=2 and Idx=5
SELECT  @r=@x.query('
(: $e = array element :)
for $e in (//Array/Element[@Idx=2],//Array/Element[@Idx=5])
    return string($e)
');
SELECT  REPLACE(CONVERT(NVARCHAR(4000),') AS [Concatenating Idx=2 and Idx=5 - using XML (method 1)];
/*
Concatenating Idx=2 and Idx=5 - using XML (method 1)
----------------------------------------------------
Bob,Steve
*/

--Concatenating Idx=2 and Idx=5
SELECT  @x.value('(//Array/Element)[@Idx=2][1]','NVARCHAR(100)')
        + ','
        + @x.value('(//Array/Element)[@Idx=5][1]','NVARCHAR(100)') AS [Concatenating Idx=2 and Idx=5 - using XML (method 2)];;
/*
Concatenating Idx=2 and Idx=5 - using XML (method 2)
----------------------------------------------------
Bob,Steve
*/

4)如果问题是如何在SQL Server上模拟ARRAY_AGG,那么一个答案可能是：使用XML.
例：

SET ANSI_WARNINGS ON;
GO

DECLARE @Test TABLE
(
     Id         INT PRIMARY KEY,GroupID    INT NOT NULL,Name       NVARCHAR(100) NOT NULL
);

INSERT INTO @Test (Id,GroupID,Name)
VALUES
 (3,'Bob');

WITH BaseQuery
AS
(
        SELECT  a.GroupID,a.Name
        FROM    @Test a
        WHERE   a.Id BETWEEN 10 AND 100 
)
SELECT  x.*,CONVERT(XML,x.SQLServer_Array_Agg).query
        ('
        for $e in (//Array/Element[@Idx=1],//Array/Element[@Idx=count(//Array/Element)])
            return string($e)
        ') AS [Concat Idx=1 and Idx=i (method 1)],x.SQLServer_Array_Agg).query('
            let $a :=  string((//Array/Element[@Idx=1])[1])
            let $b :=  string((//Array/Element[@Idx=count(//Array/Element)])[1])
            let $c :=  concat($a,",$b) (: " is used as a string delimiter :)
            return $c
        ') AS [Concat Idx=1 and Idx=i (method 2)],x.SQLServer_Array_Agg).query
        ('
        for $e in (//Array/Element[@Idx=(1,count(//Array/Element))])
            return string($e)
        ') AS [Concat Idx=1 and Idx=i (method 3)]
FROM
(
    SELECT  a.GroupID,(SELECT ROW_NUMBER() OVER(ORDER BY b.Name) AS [@Idx],b.Name AS [text()]
        FROM    BaseQuery b
        WHERE   a.GroupID = b.GroupID 
        ORDER BY b.Name
        FOR XML PATH('Element'),ROOT('Array') ) AS SQLServer_Array_Agg
    FROM    BaseQuery a
    GROUP BY a.GroupID
) x;

结果：

GroupID SQLServer_Array_Agg                                                                                        Concat Idx=1 and Idx=i (method 1) Concat Idx=1 and Idx=i (method 2) Concat Idx=1 and Idx=i (method 3)
------- ---------------------------------------------------------------------------------------------------------- --------------------------------- --------------------------------- ---------------------------------
1       <Array><Element Idx="1">Jack</Element><Element Idx="2">Steve</Element></Array>                             Jack Steve                        Jack,Steve                        Jack Steve
2       <Array><Element Idx="1">Ami</Element><Element Idx="2">Bob</Element><Element Idx="3">Pete</Element></Array> Ami Pete                          Ami,Pete                          Ami Pete

（编辑：李大同）

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