Question

I came across some very cool t-sql to generate a comma delimited list of column values from selected rows in one t-sql query:

SELECT @MyList = ISNULL(@MyList,'')  + Title + ', ' FROM Titles

But I can’t figure out how it works. Somehow it must be doing a recursive call but I don’t know how.
Can anyone explain it to me or send me a link that explains it?
To see it work, use the following script:

CREATE TABLE  Titles(
    Title varchar(50)  
    )

    insert Titles  values ( 'Doctor')
    insert Titles   values ( 'Nurse')
    insert Titles   values ( 'Administrator')
    insert Titles   values ( 'CMA')

    select * from Titles

    DECLARE @MyList VARCHAR(1000)
    SET @MyList = ''
    SELECT @MyList = ISNULL(@MyList,'')  + Title + ', ' FROM Titles
    SELECT @MyList

Answer 1

The assignment:

@MyList = ISNULL(@MyList,'')  + Title + ', ' 

is evaluated for every row of the Titles table. It concatenates each row’s Title column value to @MyList.

The test ISNULL(@MyList,'') is only needed, so that @MyList starts with an empty string if it is NULL. In your example the ISNULL is unnecessary, because @MyList is explicitly set to an empty string.