How Stuff and 'For Xml Path' work in SQL Server?

Here is how it works:

1. Get XML element string with FOR XML

Adding FOR XML PATH to the end of a query allows you to output the results of the query as XML elements, with the element name contained in the PATH argument. For example, if we were to run the following statement:

SELECT ',' + name 
              FROM temp1
              FOR XML PATH ('')

By passing in a blank string (FOR XML PATH('')), we get the following instead:

,aaa,bbb,ccc,ddd,eee

2. Remove leading comma with STUFF

The STUFF statement literally "stuffs” one string into another, replacing characters within the first string. We, however, are using it simply to remove the first character of the resultant list of values.

SELECT abc = STUFF((
            SELECT ',' + NAME
            FROM temp1
            FOR XML PATH('')
            ), 1, 1, '')
FROM temp1

The parameters of STUFF are:

The string to be “stuffed” (in our case the full list of name with a leading comma)

The location to start deleting and inserting characters (1, we’re stuffing into a blank string)

The number of characters to delete (1, being the leading comma)

So we end up with:

aaa,bbb,ccc,ddd,eee

3. Join on id to get full list

Next we just join this on the list of id in the temp table, to get a list of IDs with name:

SELECT ID,  abc = STUFF(
             (SELECT ',' + name 
              FROM temp1 t1
              WHERE t1.id = t2.id
              FOR XML PATH (''))
             , 1, 1, '') from temp1 t2
group by id;

And we have our result:

Id Name 1 aaa,bbb,ccc,ddd,eee

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This article covers various ways of concatenating strings in SQL, including an improved version of your code which doesn't XML-encode the concatenated values.

SELECT ID, abc = STUFF
(
    (
        SELECT ',' + name
        FROM temp1 As T2
        -- You only want to combine rows for a single ID here:
        WHERE T2.ID = T1.ID
        ORDER BY name
        FOR XML PATH (''), TYPE
    ).value('.', 'varchar(max)')
, 1, 1, '')
FROM temp1 As T1
GROUP BY id

To understand what's happening, start with the inner query:

SELECT ',' + name
FROM temp1 As T2
WHERE T2.ID = 42 -- Pick a random ID from the table
ORDER BY name
FOR XML PATH (''), TYPE

Because you're specifying FOR XML, you'll get a single row containing an XML fragment representing all of the rows.

Because you haven't specified a column alias for the first column, each row would be wrapped in an XML element with the name specified in brackets after the FOR XML PATH. For example, if you had FOR XML PATH ('X'), you'd get an XML document that looked like:

<X>,aaa</X>
<X>,bbb</X>
...

But, since you haven't specified an element name, you just get a list of values:

,aaa,bbb,...

The .value('.', 'varchar(max)') simply retrieves the value from the resulting XML fragment, without XML-encoding any "special" characters. You now have a string that looks like:

',aaa,bbb,...'

The STUFF function then removes the leading comma, giving you a final result that looks like:

'aaa,bbb,...'

It looks quite confusing at first glance, but it does tend to perform quite well compared to some of the other options.

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PATH mode is used in generating XML from a SELECT query

1. SELECT   
       ID,  
       Name  
FROM temp1
FOR XML PATH;  

Ouput:
<row>
<ID>1</ID>
<Name>aaa</Name>
</row>

<row>
<ID>1</ID>
<Name>bbb</Name>
</row>

<row>
<ID>1</ID>
<Name>ccc</Name>
</row>

<row>
<ID>1</ID>
<Name>ddd</Name>
</row>

<row>
<ID>1</ID>
<Name>eee</Name>
</row>

The Output is element-centric XML where each column value in the resulting rowset is wrapped in an row element. Because the SELECT clause does not specify any aliases for the column names, the child element names generated are the same as the corresponding column names in the SELECT clause.

For each row in the rowset a tag is added.

2.
SELECT   
       ID,  
       Name  
FROM temp1
FOR XML PATH('');

Ouput:
<ID>1</ID>
<Name>aaa</Name>
<ID>1</ID>
<Name>bbb</Name>
<ID>1</ID>
<Name>ccc</Name>
<ID>1</ID>
<Name>ddd</Name>
<ID>1</ID>
<Name>eee</Name>

For Step 2: If you specify a zero-length string, the wrapping element is not produced.

3. 

    SELECT   

           Name  
    FROM temp1
    FOR XML PATH('');

    Ouput:
    <Name>aaa</Name>
    <Name>bbb</Name>
    <Name>ccc</Name>
    <Name>ddd</Name>
    <Name>eee</Name>

4. SELECT   
        ',' +Name  
FROM temp1
FOR XML PATH('')

Ouput:
,aaa,bbb,ccc,ddd,eee

In Step 4 we are concatenating the values.

5. SELECT ID,
    abc = (SELECT   
            ',' +Name  
    FROM temp1
    FOR XML PATH('') )
FROM temp1

Ouput:
1   ,aaa,bbb,ccc,ddd,eee
1   ,aaa,bbb,ccc,ddd,eee
1   ,aaa,bbb,ccc,ddd,eee
1   ,aaa,bbb,ccc,ddd,eee
1   ,aaa,bbb,ccc,ddd,eee


6. SELECT ID,
    abc = (SELECT   
            ',' +Name  
    FROM temp1
    FOR XML PATH('') )
FROM temp1 GROUP by iD

Ouput:
ID  abc
1   ,aaa,bbb,ccc,ddd,eee

In Step 6 we are grouping the date by ID.

STUFF( source_string, start, length, add_string ) Parameters or Arguments source_string The source string to modify. start The position in the source_string to delete length characters and then insert add_string. length The number of characters to delete from source_string. add_string The sequence of characters to insert into the source_string at the start position.

SELECT ID,
    abc = 
    STUFF (
        (SELECT   
                ',' +Name  
        FROM temp1
        FOR XML PATH('')), 1, 1, ''
    )
FROM temp1 GROUP by iD

Output:
-----------------------------------
| Id        | Name                |
|---------------------------------|
| 1         | aaa,bbb,ccc,ddd,eee |
-----------------------------------

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There is very new functionality in Azure SQL Database and SQL Server (starting with 2017) to handle this exact scenario. I believe this would serve as a native official method for what you are trying to accomplish with the XML/STUFF method. Example:

select id, STRING_AGG(name, ',') as abc
from temp1
group by id

STRING_AGG - https://msdn.microsoft.com/en-us/library/mt790580.aspx

EDIT: When I originally posted this I made mention of SQL Server 2016 as I thought I saw that on a potential feature that was to be included. Either I remembered that incorrectly or something changed, thanks for the suggested edit fixing the version. Also, pretty impressed and wasn't fully aware of the multi-step review process that just pulled me in for a final option.

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In for xml path, if we define any value like [ for xml path('ENVLOPE') ] then these tags will be added with each row:

<ENVLOPE>
</ENVLOPE>

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SELECT ID, 
    abc = STUFF(
                 (SELECT ',' + name FROM temp1 FOR XML PATH ('')), 1, 1, ''
               ) 
FROM temp1 GROUP BY id

Here in the above query STUFF function is used to just remove the first comma (,) from the generated xml string (,aaa,bbb,ccc,ddd,eee) then it will become (aaa,bbb,ccc,ddd,eee).

And FOR XML PATH('') simply converts column data into (,aaa,bbb,ccc,ddd,eee) string but in PATH we are passing '' so it will not create a XML tag.

And at the end we have grouped records using ID column.

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I did debugging and finally returned my 'stuffed' query to it it's normal way.

Simply

select * from myTable for xml path('myTable')

gives me contents of the table to write to a log table from a trigger I debug.

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Declare @Temp As Table (Id Int,Name Varchar(100))
Insert Into @Temp values(1,'A'),(1,'B'),(1,'C'),(2,'D'),(2,'E'),(3,'F'),(3,'G'),(3,'H'),(4,'I'),(5,'J'),(5,'K')
Select X.ID,
stuff((Select ','+ Z.Name from @Temp Z Where X.Id =Z.Id For XML Path('')),1,1,'')
from @Temp X
Group by X.ID

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