Is it possible to cast a DECIMAL to DOUBLE in MySQL?

问题

I know all the numerical implications, that is, the possible rounding issues inherent to floating point formats, but in my case I have DECIMAL columns in MySQL that I want to convert to DOUBLE straight in the MySQL query rather than down stream.

Could anyone help?

回答1:

SELECT my_decimal_field + 0E0 FROM my_table

回答2:

It seems not possible to cast it to DOUBLE which brings problems if you do calculations and for example want to ROUND() a DECIMAL 12,2 in the third digit. Using ROUND(foo * bar,2) will just ignore the additional digits if your foo and bar are DECIMAL 12,2 fields.

That said you can do something like this to still make it work:

ROUND(CAST(foo AS DECIMAL(30,15)*CAST(bar AS DECIMAL(30,15)),2)

回答3:

Because of the limitations of the built in CAST function in MySQL, it is only possible to convert DECIMAL to DOUBLE with your own user defined cast function.

Sample use case:

SELECT castDecimalAsDouble(0.000000000000000000100000000000);

Result: 1e-23

CREATE DEFINER=`root`@`localhost` FUNCTION `castDecimalAsDouble`(
decimalInput DECIMAL(65,30) ) RETURNS double
DETERMINISTIC
BEGIN
DECLARE doubleOutput DOUBLE;

SET doubleOutput = decimalInput;

RETURN doubleOutput;
END

回答4:

DECIMAL may save space. For example, DECIMAL(4,2) occupies only 2 bytes. FLOAT takes 4; DOUBLE takes 8.

As for the original question, simply do:

ALTER TABLE t MODIFY COLUMN c DOUBLE ...;

(The "..." should include the other stuff you already had, such as NOT NULL.)

来源：https://stackoverflow.com/questions/12742322/is-it-possible-to-cast-a-decimal-to-double-in-mysql

标签

mysql

casting

double

decimal