¿Cómo obtener solo dígitos de String en mysql?

Solución:

Si la cadena comienza con un número y luego contiene caracteres no numéricos, puede usar el CAST() función o convertirlo a un numérico implícitamente agregando un 0:

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

Para extraer números de un arbitrario cadena, podría agregar una función personalizada como esta:

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 ;

Una vez definida la función, puede utilizarla en su consulta:

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

Para quien todavía esté mirando, use regex:

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

Basado en la respuesta de Eugene Yarmash. Aquí hay una versión de la función personalizada que extrae un decimal con dos lugares decimales. Bueno para la extracción de precios.

DELIMITER $$

CREATE FUNCTION `ExtractDecimal`(in_string VARCHAR(255)) 
RETURNS decimal(15,2)
NO SQL
BEGIN
    DECLARE ctrNumber VARCHAR(255);
    DECLARE in_string_parsed VARCHAR(255);
    DECLARE digitsAndDotsNumber VARCHAR(255) DEFAULT '';
    DECLARE finalNumber VARCHAR(255) DEFAULT '';
    DECLARE sChar VARCHAR(1);
    DECLARE inti INTEGER DEFAULT 1;
    DECLARE digitSequenceStarted boolean DEFAULT false;
    DECLARE negativeNumber boolean DEFAULT false;

    -- FIX FIND_IN_SET cannot find a comma ","
    SET in_string_parsed = replace(in_string,',','.');

    IF LENGTH(in_string_parsed) > 0 THEN
        -- extract digits and dots
        WHILE(inti <= LENGTH(in_string_parsed)) DO
            SET sChar = SUBSTRING(in_string_parsed, inti, 1);
            SET ctrNumber = FIND_IN_SET(sChar, '0,1,2,3,4,5,6,7,8,9,.'); 
            IF ctrNumber > 0 AND (sChar != '.' OR LENGTH(digitsAndDotsNumber) > 0) THEN
                -- add first minus if needed
                IF digitSequenceStarted = false AND inti > 1 AND SUBSTRING(in_string_parsed, inti-1, 1) = '-' THEN
                    SET negativeNumber = true;
                END IF;

                SET digitSequenceStarted = true;
                SET digitsAndDotsNumber = CONCAT(digitsAndDotsNumber, sChar);
            ELSEIF digitSequenceStarted = true THEN
                SET inti = LENGTH(in_string_parsed);
            END IF;
            SET inti = inti + 1;
        END WHILE;

        -- remove dots from the end of number list
        SET inti = LENGTH(digitsAndDotsNumber);
        WHILE(inti > 0) DO
            IF(SUBSTRING(digitsAndDotsNumber, inti, 1) = '.') THEN
                SET digitsAndDotsNumber = SUBSTRING(digitsAndDotsNumber, 1, inti-1);
                SET inti = inti - 1;
            ELSE
                SET inti = 0;
            END IF;
        END WHILE;

        -- extract decimal
        SET inti = 1;
        WHILE(inti <= LENGTH(digitsAndDotsNumber)-3) DO
            SET sChar = SUBSTRING(digitsAndDotsNumber, inti, 1);
            SET ctrNumber = FIND_IN_SET(sChar, '0,1,2,3,4,5,6,7,8,9'); 
            IF ctrNumber > 0 THEN
                SET finalNumber = CONCAT(finalNumber, sChar);
            END IF;
            SET inti = inti + 1;
        END WHILE;

        SET finalNumber = CONCAT(finalNumber, RIGHT(digitsAndDotsNumber, 3));
        IF negativeNumber = true AND LENGTH(finalNumber) > 0 THEN
            SET finalNumber = CONCAT('-', finalNumber);
        END IF;

        IF LENGTH(finalNumber) = 0 THEN
            RETURN 0;
        END IF;

        RETURN CAST(finalNumber AS decimal(15,2));
    ELSE
        RETURN 0;
    END IF;    
END$$

DELIMITER ;

Pruebas:

select ExtractDecimal("1234"); -- 1234.00
select ExtractDecimal("12.34"); -- 12.34
select ExtractDecimal("1.234"); -- 1234.00
select ExtractDecimal("1,234"); -- 1234.00
select ExtractDecimal("1,111,234"); -- 11111234.00
select ExtractDecimal("11,112,34"); -- 11112.34
select ExtractDecimal("11,112,34 and 123123"); -- 11112.34
select ExtractDecimal("-1"); -- -1.00
select ExtractDecimal("hello. price is 123"); -- 123.00
select ExtractDecimal("123,45,-"); -- 123.45

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