Evaluate a string value as a computed expression in an SQL statement (sthg like EVAL)
for my specific problem, an answer could be : 

    DROP FUNCTION stringExp2Int;
    DELIMITER $$
    CREATE FUNCTION stringExp2Int (expression VARCHAR(20))
        RETURNS INT
        NO SQL
        BEGIN
            DECLARE val INT DEFAULT 0;
            DECLARE ct INT DEFAULT 1;
            WHILE ct &lt;= LENGTH(expression) do
                SET val = val + SUBSTR(expression,ct,1);
                SET ct = ct + 2;
            END WHILE;
             RETURN val;
        END
    $$
    DELIMITER ;


Using LOCATE to find the next separator, could be more secure :

    DROP FUNCTION stringExp2Int;
    DELIMITER $$
    CREATE FUNCTION stringExp2Int (expression VARCHAR(20))
        RETURNS INT
        NO SQL
    BEGIN
      DECLARE val INT DEFAULT 0;
      DECLARE ct INT DEFAULT 1;
      DECLARE pos INT DEFAULT 0;
      DECLARE sep CHAR(1) DEFAULT "+";

      WHILE ct &lt;= LENGTH(expression) DO
         SET pos = LOCATE(sep,SUBSTR(expression,ct,LENGTH(expression)-ct+1));
         IF pos &gt;= 1 THEN 
            BEGIN -- when pos = 1 : add a null value
                SET val = val + SUBSTR(expression,ct,pos-1);
                SET ct = ct + pos;
            END;
        ELSE 
            BEGIN
                SET val = val + SUBSTR(expression,ct,LENGTH(expression)-ct+1);
                SET ct = ct + LENGTH(expression)-ct+1;
            END;
        END IF;
     END WHILE;
     RETURN val;
    END
    $$
    DELIMITER ;


Trials :

mysql&gt; SELECT stringExp2Int("");
+-------------------+
| stringExp2Int("") |
+-------------------+
|                 0 |
+-------------------+

mysql&gt; SELECT stringExp2Int("+");
+--------------------+
| stringExp2Int("+") |
+--------------------+
|                  0 |
+--------------------+

mysql&gt; SELECT stringExp2Int("1");
+--------------------+
| stringExp2Int("1") |
+--------------------+
|                  1 |
+--------------------+

mysql&gt; SELECT stringExp2Int("+1");
+---------------------+
| stringExp2Int("+1") |
+---------------------+
|                   1 |
+---------------------+

mysql&gt; SELECT stringExp2Int("1+");
+---------------------+
| stringExp2Int("1+") |
+---------------------+
|                   1 |
+---------------------+

mysql&gt; SELECT stringExp2Int("1+1");
+----------------------+
| stringExp2Int("1+1") |
+----------------------+
|                    2 |
+----------------------+

mysql&gt; SELECT stringExp2Int("+1+1+");
+------------------------+
| stringExp2Int("+1+1+") |
+------------------------+
|                      2 |
+------------------------+

mysql&gt; SELECT stringExp2Int("+++1+++1+++");
+------------------------------+
| stringExp2Int("+++1+++1+++") |
+------------------------------+
|                            2 |
+------------------------------+

mysql&gt; SELECT stringExp2Int("+++001+++001+++");
+----------------------------------+
| stringExp2Int("+++001+++001+++") |
+----------------------------------+
|                                2 |
+----------------------------------+


and :

mysql&gt; SELECT stringExp2Int(detailNote) FROM A;
+---------------------------+
| stringExp2Int(detailNote) |
+---------------------------+
|                        16 |
+---------------------------+
1 row in set (0.00 sec)


I close this question...hope it could be useful for someone...