mysql> select sum(importo) from tabella where value="A";
+--------------------+
|       sum(importo) |
+--------------------+
|            4922.28 |
+--------------------+
1 row in set (0.00 sec)

mysql> select sum(importo) from tabella where value="B";
+--------------------+
|       sum(importo) |
+--------------------+
|              10.55 |
+--------------------+
1 row in set (0.00 sec)