Function to Get Digits from Strings in MySQL Version 8.0.3 or Below

extract_digits.sql

DELIMITER $$

CREATE FUNCTION extract_digits (string_mixed VARCHAR(100)) RETURNS VARCHAR(100) NO SQL
BEGIN
    DECLARE find_digit_position VARCHAR(100);
    DECLARE string_digits VARCHAR(100) DEFAULT '';
    DECLARE search_char VARCHAR(1);
    DECLARE i INTEGER DEFAULT 1;

    IF LENGTH(string_mixed) > 0 THEN
        WHILE(i <= LENGTH(string_mixed)) DO
            SET search_char = SUBSTRING(string_mixed, i, 1);
            SET find_digit_position = FIND_IN_SET(search_char, '0,1,2,3,4,5,6,7,8,9'); 
            IF find_digit_position > 0 THEN
                SET string_digits = CONCAT(string_digits, search_char);
            END IF;
            SET i = i + 1;
        END WHILE;
        RETURN string_digits;
    ELSE
        RETURN '';
    END IF;    
END$$

DELIMITER ;