How to get only Digits from String in mysql?

To whoever is still looking, use regex:

select REGEXP_SUBSTR(name,"[0-9]+") as amount from `subscriptions`

If the string starts with a number, then contains non-numeric characters, you can use the CAST() function or convert it to a numeric implicitly by adding a 0:

SELECT CAST('1234abc' AS UNSIGNED); -- 1234
SELECT '1234abc'+0; -- 1234

To extract numbers out of an arbitrary string you could add a custom function like this:

DELIMITER $$

CREATE FUNCTION `ExtractNumber`(in_string VARCHAR(50)) 
RETURNS INT
NO SQL
BEGIN
    DECLARE ctrNumber VARCHAR(50);
    DECLARE finNumber VARCHAR(50) DEFAULT '';
    DECLARE sChar VARCHAR(1);
    DECLARE inti INTEGER DEFAULT 1;

    IF LENGTH(in_string) > 0 THEN
        WHILE(inti <= LENGTH(in_string)) DO
            SET sChar = SUBSTRING(in_string, inti, 1);
            SET ctrNumber = FIND_IN_SET(sChar, '0,1,2,3,4,5,6,7,8,9'); 
            IF ctrNumber > 0 THEN
                SET finNumber = CONCAT(finNumber, sChar);
            END IF;
            SET inti = inti + 1;
        END WHILE;
        RETURN CAST(finNumber AS UNSIGNED);
    ELSE
        RETURN 0;
    END IF;    
END$$

DELIMITER ;

Once the function is defined, you can use it in your query:

SELECT ExtractNumber("abc1234def") AS number; -- 1234