MySQL UPDATE statement using a table created on the fly
UPDATE syntax in MySQL is different than SQL-Server. This will do:

UPDATE Table1 AS t
  JOIN
      ( SELECT A, SUM(UNO) AS ctemp 
        FROM Table1
        GROUP BY A                     -- no need for `ORDER BY` in either DBMS
      ) AS temp
    ON temp.A = t.A  
SET 
    t.COUNT = temp.ctemp ;