Group by to Combine/Concat a Column

GROUP BY to combine/concat a column

SELECT
     [User], Activity,
     STUFF(
         (SELECT DISTINCT ',' + PageURL
          FROM TableName
          WHERE [User] = a.[User] AND Activity = a.Activity
          FOR XML PATH (''))
          , 1, 1, '')  AS URLList
FROM TableName AS a
GROUP BY [User], Activity

SQLFiddle Demo

How to use GROUP BY to concatenate strings in SQL Server?

No CURSOR, WHILE loop, or User-Defined Function needed.

Just need to be creative with FOR XML and PATH.

[Note: This solution only works on SQL 2005 and later. Original question didn't specify the version in use.]

CREATE TABLE #YourTable ([ID] INT, [Name] CHAR(1), [Value] INT)

INSERT INTO #YourTable ([ID],[Name],[Value]) VALUES (1,'A',4)
INSERT INTO #YourTable ([ID],[Name],[Value]) VALUES (1,'B',8)
INSERT INTO #YourTable ([ID],[Name],[Value]) VALUES (2,'C',9)

SELECT 
  [ID],
  STUFF((
    SELECT ', ' + [Name] + ':' + CAST([Value] AS VARCHAR(MAX)) 
    FROM #YourTable 
    WHERE (ID = Results.ID) 
    FOR XML PATH(''),TYPE).value('(./text())[1]','VARCHAR(MAX)')
  ,1,2,'') AS NameValues
FROM #YourTable Results
GROUP BY ID

DROP TABLE #YourTable

Concatenate values that are grouped by a column

You have nothing linking your inner and outer references to [Table], and you also need to make the outer reference distinct. Finally you need to either have no column name within your subquery, or it needs to be [text()]

SELECT  [Code]
        ,[Ref]
        ,STUFF((SELECT DISTINCT [Value] AS [text()]
                FROM [Table] AS T2
                WHERE T1.Code = T2.Code -- LINK HERE
                AND T2.Ref = T2.Ref     -- AND HERE
                FOR XML PATH ('')
                ),1, 1,'') AS [Values]
FROM    [Table] AS T1
GROUP BY T1.Code, T1.Ref; -- GROUP BY HERE

As an aside, you do not need to use STUFF as you have no delimiter, STUFF is typically used to remove the chosen delimiter from the start of the string. So when you have a string like ,value1,value2,value3, STUFF(string, 1, 1, '') will replace the first character with '' leaving you with value1,value2,value3.

You should also use the value xquery method to ensure you are not tripped up by special characters, if you don't and you try an concatenate ">>" and "<<" you would not end up with ">><<" as you might want, you would get ">><<", so a better query would be:

SELECT  t1.Code,
        t1.Ref,
        [Values] = (SELECT  DISTINCT [text()] = [Value]
                    FROM    [Table] AS t2
                    WHERE   T1.Code = T2.Code
                    AND     T2.Ref = T2.Ref
                    FOR XML PATH (''), TYPE
                ).value('.', 'NVARCHAR(MAX)')
FROM    [Table] AS T1
GROUP BY t1.Code, t1.Ref;

ADDENDUM

Based on the latest edit to the question it appears as though your Value column is coming from another table, linked to the first table by Code. If anything this makes your query simpler. You don't need the JOIN, but you still need to make sure that there is an expression to link the outer table to the inner table your subquery. I am assuming that the rows are unique in the first table, so you probably don't need the group by either:

SELECT  t1.Code,
        t1.Ref,
        [Values] = (SELECT  DISTINCT [text()] = t2.[Value]
                    FROM    [Table2] AS t2
                    WHERE   T1.Code = T2.Code
                    FOR XML PATH (''), TYPE
                ).value('.', 'NVARCHAR(MAX)')
FROM    [Table] AS T1;

WORKING EXAMPLE

CREATE TABLE #Table1 (Code CHAR(2), Ref VARCHAR(10));
INSERT #Table1 VALUES ('A1', 'Car'), ('B2', 'Truck'), ('C3', 'Van');

CREATE TABLE #Table2 (Code CHAR(2), Value VARCHAR(2));
INSERT #Table2
VALUES ('A1', 'A'), ('A1', '-'), ('A1', 'B'),
        ('B2', 'CC'), ('B2', 'D'), ('B2', '-'),
        ('C3', 'F'), ('C3', '-'), ('C3', 'G');

SELECT  t1.Code,
        t1.Ref,
        [Values] = (SELECT  DISTINCT [text()] = t2.[Value]
                    FROM    #Table2 AS t2
                    WHERE   T1.Code = T2.Code
                    FOR XML PATH (''), TYPE
                ).value('.', 'NVARCHAR(MAX)')
FROM    #Table1 AS T1;

How to group-concatenate multiple columns?

Just an idea:

DECLARE @t TABLE (
    Customer VARCHAR(50),
    Product VARCHAR(50),
    Method VARCHAR(50),
    INDEX ix CLUSTERED (Customer)
)

INSERT INTO @t (Customer, Product, Method)
VALUES
    ('John', 'Computer', 'Credit'),
    ('John', 'Mouse', 'Cash'),
    ('Will', 'Computer', 'Credit'),
    ('Will', 'Mouse', 'Cash'),
    ('Will', 'Speaker', 'Cash'),
    ('Todd', 'Computer', 'Credit')

SELECT t.Customer
     , STUFF(CAST(x.query('a/text()') AS NVARCHAR(MAX)), 1, 2, '')
     , STUFF(CAST(x.query('b/text()') AS NVARCHAR(MAX)), 1, 2, '')
FROM (
    SELECT DISTINCT Customer
    FROM @t
) t
OUTER APPLY (
    SELECT DISTINCT [a] = CASE WHEN id = 'a' THEN ', ' + val END
                  , [b] = CASE WHEN id = 'b' THEN ', ' + val END
    FROM @t t2
    CROSS APPLY (
        VALUES ('a', t2.Product)
             , ('b', t2.Method)
    ) t3 (id, val)
    WHERE t2.Customer = t.Customer
    FOR XML PATH(''), TYPE
) t2 (x)

Output:

Customer   Product                    Method     
---------- -------------------------- ------------------
John       Computer, Mouse            Cash, Credit
Todd       Computer                   Credit
Will       Computer, Mouse, Speaker   Cash, Credit

Another idea with more performance benefits:

IF OBJECT_ID('tempdb.dbo.#EntityValues') IS NOT NULL
    DROP TABLE #EntityValues

DECLARE @Values1 VARCHAR(MAX)
      , @Values2 VARCHAR(MAX)

SELECT Customer
     , Product
     , Method
     , RowNum = ROW_NUMBER() OVER (PARTITION BY Customer ORDER BY 1/0)
     , Values1 = CAST(NULL AS VARCHAR(MAX))
     , Values2 = CAST(NULL AS VARCHAR(MAX))
INTO #EntityValues
FROM @t

UPDATE #EntityValues
SET 
      @Values1 = Values1 =
        CASE WHEN RowNum = 1 
            THEN Product
            ELSE @Values1 + ', ' + Product 
        END
    , @Values2 = Values2 = 
        CASE WHEN RowNum = 1 
            THEN Method
            ELSE @Values2 + ', ' + Method
        END

SELECT Customer
      , Values1 = MAX(Values1) 
      , Values2 = MAX(Values2)
FROM #EntityValues
GROUP BY Customer

But with some limitations:

Customer      Values1                       Values2
------------- ----------------------------- ----------------------
John          Computer, Mouse               Credit, Cash
Todd          Computer                      Credit
Will          Computer, Mouse, Speaker      Credit, Cash, Cash

Also check my old post about string aggregation:

http://www.codeproject.com/Articles/691102/String-Aggregation-in-the-World-of-SQL-Server

How to concatenate many rows with same id in sql?

In SQL-Server you can do it in the following:

QUERY

SELECT id, displayname = 
    STUFF((SELECT DISTINCT ', ' + displayname
           FROM #t b 
           WHERE b.id = a.id 
          FOR XML PATH('')), 1, 2, '')
FROM #t a
GROUP BY id

TEST DATA

create table #t 
(
id int,
displayname nvarchar(max)
)

insert into #t values    
 (1 ,'Editor')
,(1 ,'Reviewer')
,(7 ,'EIC')
,(7 ,'Editor')
,(7 ,'Reviewer')
,(7 ,'Editor')
,(19,'EIC')
,(19,'Editor')
,(19,'Reviewer')

OUTPUT

id  displayname
1   Editor, Reviewer
7   Editor, EIC, Reviewer
19  Editor, EIC, Reviewer

How to use GROUP BY to concatenate strings in MySQL?

SELECT id, GROUP_CONCAT(name SEPARATOR ' ') FROM table GROUP BY id;

https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html#function_group-concat

From the link above, GROUP_CONCAT: This function returns a string result with the concatenated non-NULL values from a group. It returns NULL if there are no non-NULL values.

Group Concatenate Strings (Rows) in BigQuery

Do each unnest separately:
Does aggregation work?

SELECT STRING_AGG(item.abc, ',')
       uniqueid, variable2, cust.variable1, purch.variable2
FROM mydata.order LEFT JOIN
     UNNEST(purchases) as purch
     ON true LEFT JOIN
     UNNEST(codes_abs) as cod
     ON true LEFT JOIN
     UNNEST(cod.try_items) as item
     ON true
GROUP BY uniqueid, variable2, cust.variable1, purch.variable2;

Concat single column fields using GROUP BY

Suppose you have a table test as follows:

select id, val from test order by id, val;     
2   aa
2   bb
1   bb
1   aa

You can use the HIVE function collect_set:

select id, collect_set(val) from test group by id;
1   ["aa","bb"]
2   ["bb","aa"]

But note that collect_set returns a set of objects with duplicate elements eliminated.

You can find more details at the Language Manual Wiki.

Concatenate across columns and rows using group and listagg in Oracle SQL

Use the following query where you will directly get concatenated column values:

SELECT
    "GROUP",
    LISTAGG(VAL_1 || VAL_2 || VAL_3)  
        WITHIN GROUP(ORDER BY VAL_1) AS "TEXT"
FROM DATA
GROUP BY "GROUP";

Note: Do not use oracle reserved keywords as the column names. Here GROUP is the oracle reserved keyword.

Cheers!!

Group by to Combine/Concat a Column