Multiple rows to one comma-separated value in Sql Server

Question

I want to create a table-valued function in the SQL Server, which I want to return data in comma-separated values.

For example table: tbl

ID | Value
---+-------
 1 | 100
 1 | 200
 1 | 300     
 1 | 400

Now when I execute the query using the function Func1(value) 

SELECT Func1(Value) 
FROM tbl 
WHERE ID = 1

Output that I want is: 100,200,300,400 

Answer 1

Test Data

DECLARE @Table1 TABLE(ID INT, Value INT)
INSERT INTO @Table1 VALUES (1,100),(1,200),(1,300),(1,400)

Query

SELECT  ID
       ,STUFF((SELECT ', ' + CAST(Value AS VARCHAR(10)) [text()]
         FROM @Table1 
         WHERE ID = t.ID
         FOR XML PATH(''), TYPE)
        .value('.','NVARCHAR(MAX)'),1,2,' ') List_Output
FROM @Table1 t
GROUP BY ID

Result Set

╔════╦═════════════════════╗
║ ID ║     List_Output     ║
╠════╬═════════════════════╣
║  1 ║  100, 200, 300, 400 ║
╚════╩═════════════════════╝

SQL Server 2017 and Later Versions

If you are working on the SQL Server 2017 or any of the later version, you can use the built-in SQL Server Function STRING_AGG to create the comma delimited list:

DECLARE @Table1 TABLE(ID INT, Value INT);
INSERT INTO @Table1 VALUES (1,100),(1,200),(1,300),(1,400);


SELECT ID , STRING_AGG([Value], ', ') AS List_Output
FROM @Table1
GROUP BY ID;

Result Set

╔════╦═════════════════════╗
║ ID ║     List_Output     ║
╠════╬═════════════════════╣
║  1 ║  100, 200, 300, 400 ║
╚════╩═════════════════════╝

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Answer 2

 In SQL to combine multiple rows into a comma separated single value, try the following 

SELECT STRING_AGG(column_name, ',') AS CommaSeparatedVal

FROM table_name;

To concatenate the values of columns, STRING_AFF() will help in that.