This will retrieves all rows from the table and also includes additional rows that represent the sum of Total values for each unique Partno :

SELECT Partno, Total
FROM mytable
UNION ALL
SELECT Partno, SUM(Total)
FROM mytable
GROUP BY Partno
ORDER BY Partno, Total DESC

Demo here

Seems like you need to use GROUPING SETS here. Note that if you have 2 or more rows with the same value of Total they will be aggregated; if that's the case you should be using your primary key column instead (which is omitted in the sample data):

SELECT Partno,
       SUM(Total) AS Total
FROM (VALUES('0100',50),
            ('0100',200),
            ('0100',300),
            ('0450',100),
            ('0450',350),
            ('0600',250))V(Partno,Total)
GROUP BY GROUPING SETS((PartNo,Total),
                       (PartNo))
ORDER BY V.Partno,
         V.Total;

You can achieve it by using UNION ALL between two queries:

select partno,total,1 as sort_col 
 from my_table

union all 

select partno,sum(total) total,0  
 from my_table group by partno

order by partno,sort_col