SQL Server: collect values in an aggregation temporarily and re-use in the same query

Question

How do I accumulate values in T-SQL? AFAIK there is no ARRAY type.
I want to re-use the values in the same query like demonstrated in this PostgreSQL example using array

Answer 1

Edit 1: I have added another solution that shows how to simulate ARRAY_AGG on SQL Server (the last answer).

Edit 2: For the solution number 4) I have added the third method for concatenation.

I'm not sure I have I understood correctly your question.

a) Instead of using arrays in SQL Server I would use table variables or XML.

b) To concatenate strings (in this case) I would use SELECT @var = @var + Name FROM tbl statements or XML xqueries.

c) The solution based on CTEs and multiple subqueries (WITH cte AS () FROM SELECT (SELECT * FROM cte.rn=1) + ()...) will generates a lot of scans and logical reads.

Solutions: 1) Table variable + 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, 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 (',')
SELECT  @AllNames = STUFF(@AllNames, 1, 1, '');
--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) Table variable + 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

  Ami
  Bob
  Jack
  Pete
  Steve

*/

--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,Bob,Jack,Pete,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), @r), ' ', ',') AS [Concatenating Idx=1 and all names - using XML];
/*
Concatenating Idx=1 and all names - using XML
---------------------------------------------
Ami,Ami,Bob,Jack,Pete,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), @r), ' ', ',') 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), @r), ' ', ',') 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) If the question is how to simulate ARRAY_AGG on SQL Server then, one answer might be: by using XML. Example:

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 , 1, 'John')
,(5 , 1, 'Mary')
,(8 , 1, 'Michael')
,(13, 1, 'Steve')
,(21, 1, 'Jack')
,(34, 2, 'Pete')
,(57, 2, 'Ami')
,(88, 2, '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)]
        , CONVERT(XML,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)]
        , CONVERT(XML,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;

Results:

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       JackSteve                             Jack Steve                        Jack,Steve                        Jack Steve
2       AmiBobPete Ami Pete                          Ami,Pete                          Ami Pete