Use of COALESCE not giving the desired result in SQL Server

Question

I have a procedure due to generate a JSON-like string which is composed of an array of arrays.

Here is the query I'm using (presented as a simple script):

DECLARE @_l_All_Records_Str VARCHAR(MAX)
SELECT @_l_All_Records_Str = COALESCE(@_l_All_Records_Str , ',' , '') + '["' 
                                    + ISNULL(Time_Unit                  , ''  ) + '","'
                                    + ISNULL(CAST(Periods    AS VARCHAR), ''  ) + '","'
                                    + ISNULL(Details_Present            , 'N' ) + '","'     
                                    + ISNULL(Slice_1                    , ''  ) + '","'              
                                    + ISNULL(Slice_2                    , ''  ) + '","'              
                                    + ISNULL(Slice_3                    , ''  ) + '","'              
                                    + ISNULL(Slice_4                    , ''  ) + '","'              
                                    + ISNULL(Slice_5                    , ''  ) + '","'              
                                    + ISNULL(Slice_6                    , ''  ) + '","'              
                                    + ISNULL(Slice_7                    , ''  ) + '","'              
                                    + ISNULL(Slice_8                    , ''  ) + '","'              
                                    + ISNULL(Slice_9                    , ''  ) + '","'              
                                    + ISNULL(Slice_10                   , ''  ) + '","'              
                                    + ISNULL(Slice_11                   , ''  ) + '","'              
                                    + ISNULL(Slice_12                   , ''  ) + '"],'
 FROM MyTable ;

SET @_l_All_Records_Str = '[' + @_l_All_Records_Str + ']' ;

print @_l_All_Records_Str ;

The result is ALMOST correct:

[,["M","1","N","Y","Y","Y","N","Y","Y","N","N","Y","Y","Y","N"],["D","","Y","Y","N","N","N","Y","N","N","N","N","N","N","N"],]

As can be seen in the result, there extra , characters (one at the beginning and the other at the end of the outer array).

Would anyone suggest what I'm missing here?

Answer 1

(Posted answer on behalf of the question author.)

Problem resolved!

There was a typo in:

COALESCE(@_l_All_Records_Str , ',' , '')

which should be:

COALESCE(@_l_All_Records_Str + ',' , '')

and also the last , needs to be removed (in ...) + '"],' ).