MySQL round half

Question

Is it possible, in MySQL, to round half a specific way like PHP would do?

• PHP_ROUND_HALF_UP
• PHP_ROUND_HALF_DOWN
• P

Answer 1

1. From Oracle documentation:

Based on MySQL official documentation, the rounding function act as the following:

For exact-value numbers, ROUND() uses the “round half away from zero” or “round toward nearest” rule: A value with a fractional part of .5 or greater is rounded up to the next integer if positive or down to the next integer if negative. (In other words, it is rounded away from zero.) A value with a fractional part less than .5 is rounded down to the next integer if positive or up to the next integer if negative.

http://dev.mysql.com/doc/refman/5.7/en/mathematical-functions.html#function_round

which means that we can only use the rounding function to round up. I have written the following UDF to work around this limitation.

2. The code (tested in MySQL 5.6):

CREATE FUNCTION roundHalf (
    numberToRound DECIMAL(20,6),
    roundingPrecision TINYINT(2),
    roundingType ENUM (
        'ROUND_HALF_UP',
        'ROUND_HALF_DOWN',
        'ROUND_HALF_EVEN',
        'ROUND_HALF_ODD'
    )
)
    RETURNS DECIMAL(20,6)
BEGIN
    DECLARE digitEvenOdd TINYINT (2) UNSIGNED DEFAULT 255;
    DECLARE digitPosition TINYINT (2) UNSIGNED DEFAULT 0;
    DECLARE digitToRound TINYINT (2) DEFAULT -1;
    DECLARE roundedNumber DECIMAL(20,6) DEFAULT 0;

    SET digitPosition = INSTR(numberToRound, '.');

    IF (roundingPrecision < 1) THEN

        SET digitPosition = digitPosition + roundingPrecision;
    ELSE

        SET digitPosition = digitPosition + roundingPrecision + 1;
    END IF;

    IF (digitPosition > 0 AND 
        digitPosition <= CHAR_LENGTH(numberToRound)
    ) THEN

        SET digitToRound = CAST(
                SUBSTR(
                numberToRound, 
                digitPosition, 
                1
            ) AS UNSIGNED
        );

        SET digitPosition = digitPosition - 1;

        IF (digitPosition > 0 AND 
            digitPosition <= CHAR_LENGTH(numberToRound)
        ) THEN    

            SET digitEvenOdd = CAST(
                SUBSTR(
                    numberToRound, 
                    digitPosition, 
                        1
                ) AS UNSIGNED
            );
        END IF;
    END IF;

    IF (digitToRound > -1) THEN

        CASE roundingType

            WHEN 'ROUND_HALF_UP' THEN

                IF (digitToRound >= 5) THEN

                    SET roundedNumber = ROUND(numberToRound, roundingPrecision);
                ELSE
                    SET roundedNumber = TRUNCATE(numberToRound, roundingPrecision);
                END IF;

            WHEN 'ROUND_HALF_DOWN' THEN

                IF (digitToRound > 5) THEN

                    SET roundedNumber = ROUND(numberToRound, roundingPrecision);
                ELSE

                    SET roundedNumber = TRUNCATE(numberToRound, roundingPrecision);
                END IF;

            WHEN 'ROUND_HALF_EVEN' THEN

                IF (digitToRound >= 5 AND 
                    digitEvenOdd IN (1,3,5,7,9)
                ) THEN

                    SET roundedNumber = ROUND(numberToRound, roundingPrecision);
                ELSE

                    SET roundedNumber = TRUNCATE(numberToRound, roundingPrecision);
                END IF;

            WHEN 'ROUND_HALF_ODD' THEN

                IF (digitToRound >= 5 AND
                    digitEvenOdd IN (0,2,4,6,8)
                ) THEN

                    SET roundedNumber = ROUND(numberToRound, roundingPrecision);
                ELSE

                    SET roundedNumber = TRUNCATE(numberToRound, roundingPrecision);
                END IF;
        END CASE;

    ELSEIF (roundingPrecision > 0) THEN

        SET roundedNumber = numberToRound;
    END IF;

    RETURN roundedNumber;
END //

Note: The value the number to round can be increased based on the need of your project up to 65 digits in total (in that case, do not forget to change all the instances of DECIMAL(20,6) accordingly).

https://stackoverflow.com/a/19201329/4949388

3. Rounding results in PHP:

http://sandbox.onlinephpfunctions.com/code/054de06b074c2b3ece5fb6e5d4180524cd2207e2

4. Unit tests (in SQL):

/* round number not enough digits */

IF (roundHalf(1.455, 7, 'ROUND_HALF_UP') <> 1.455) THEN

  SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = 'INVALID_ROUND_HALF_ODD_TEST_1';

ELSEIF (roundHalf(1.455, -5, 'ROUND_HALF_UP') <> 0) THEN

  SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = 'INVALID_ROUND_HALF_ODD_TEST_2';

ELSEIF (roundHalf(555, -1, 'ROUND_HALF_UP') <> 560) THEN

  SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = 'INVALID_ROUND_HALF_ODD_TEST_3';

END IF;


/* round half up */

IF (roundHalf(1.541, 2, 'ROUND_HALF_UP') <> 1.54) THEN

  SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = 'INVALID_ROUND_HALF_UP_TEST_1';

ELSEIF (roundHalf(1.545, 2, 'ROUND_HALF_UP') <> 1.55) THEN

  SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = 'INVALID_ROUND_HALF_UP_TEST_2';

ELSEIF (roundHalf(555, 0, 'ROUND_HALF_UP') <> 555) THEN

  SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = 'INVALID_ROUND_HALF_UP_TEST_3';

ELSEIF (roundHalf(1000999, -2, 'ROUND_HALF_UP') <> 1001000) THEN

  SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = 'INVALID_ROUND_HALF_UP_TEST_4';

ELSEIF (roundHalf(1000999, -3, 'ROUND_HALF_UP') <> 1001000) THEN

  SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = 'INVALID_ROUND_HALF_UP_TEST_5';

ELSEIF (roundHalf(1000999, -4, 'ROUND_HALF_UP') <> 1000000) THEN

  SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = 'INVALID_ROUND_HALF_UP_TEST_6';
END IF;


/* round half down */

IF (roundHalf(1.541, 2, 'ROUND_HALF_DOWN') <> 1.54) THEN

  SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = 'INVALID_ROUND_HALF_DOWN_TEST_1';

ELSEIF (roundHalf(1.545, 2, 'ROUND_HALF_DOWN') <> 1.54) THEN

  SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = 'INVALID_ROUND_HALF_DOWN_TEST_2';

ELSEIF (roundHalf(555, 0, 'ROUND_HALF_DOWN') <> 555) THEN

  SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = 'INVALID_ROUND_HALF_DOWN_TEST_3';

ELSEIF (roundHalf(1000999, -2, 'ROUND_HALF_DOWN') <> 1001000) THEN

  SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = 'INVALID_ROUND_HALF_UP_TEST_4';

ELSEIF (roundHalf(1000999, -3, 'ROUND_HALF_DOWN') <> 1001000) THEN

  SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = 'INVALID_ROUND_HALF_DOWN_TEST_5';

ELSEIF (roundHalf(1000999, -4, 'ROUND_HALF_DOWN') <> 1000000) THEN

  SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = 'INVALID_ROUND_HALF_DOWN_TEST_6';
END IF;


/* round half even */

IF (roundHalf(1.541, 2, 'ROUND_HALF_EVEN') <> 1.54) THEN

  SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = 'INVALID_ROUND_HALF_EVEN_TEST_1';

ELSEIF (roundHalf(1.544, 2, 'ROUND_HALF_EVEN') <> 1.54) THEN

  SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = 'INVALID_ROUND_HALF_EVEN_TEST_2';

ELSEIF (roundHalf(1.455, 2, 'ROUND_HALF_EVEN') <> 1.46) THEN

  SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = 'INVALID_ROUND_HALF_EVEN_TEST_3';
END IF;

/* round half odd */

IF (roundHalf(1.544, 2, 'ROUND_HALF_ODD') <> 1.54) THEN

  SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = 'INVALID_ROUND_HALF_ODD_TEST_1';

ELSEIF (roundHalf(1.545, 2, 'ROUND_HALF_ODD') <> 1.55) THEN

  SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = 'INVALID_ROUND_HALF_ODD_TEST_2';

ELSEIF (roundHalf(1.455, 2, 'ROUND_HALF_ODD') <> 1.45) THEN

  SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = 'INVALID_ROUND_HALF_ODD_TEST_3';
END IF;

Use that coding as you please, but don't forget to like my post. Thank you all for your comments and suggestions.