How MySQL does the math calculation of floating point addition?

问题

I tested with SELECT 0.1 + 0.2;, queried with MySQL (MariaDB), and it returned the right answer

MariaDB [(none)]> SELECT 0.1 + 0.2;
+-----------+
| 0.1 + 0.2 |
+-----------+
|       0.3 |
+-----------+
1 row in set (0.000 sec)

Floating point calculation is inaccurate in most programming languages because of IEEE 754 as explained here.

How MySQL does the floating point calculation that makes it return the right answer?

回答1:

I know SQL 92 is old standard but iám pretty sure this is not changed in the newer SQL standard versions.

SQL 92 defines

73)Subclause 6.12, "<numeric value expression>": When the data type of both operands of the addition. subtraction, multiplication, or division operator is exact numeric, the precision of the result is implementation-defined."*

75)Subclause 6.12, "<numeric value expression>": When the data type of either operand of an arithmetic operator is approximate numeric, the precision of the result is implementation-defined."*

The question is: 0.1 and 0.2 in the query SELECT 0.1 + 0.2 a approximate or is it exact?
The answer is: you don't know also the database can't know.
So the database will run the implemention defined for MySQL and MariaDB engines this seams to be handled as DECIMAL(1,1) datatypes

Why does Nick's answer return the correct values or expected ones with a table definition

SQL 92 also defines

Implicit type conversion can occur in expressions, fetch opera-
tions, single row select operations, inserts, deletes, and updates.
Explicit type conversions can be specified by the use of the CAST
operator.

Which Nick has done by defining the datatype in the table.

Edited this answer because i found something in the MySQL's manual today.

The query

SELECT (0.1 + 0.2) = 0.3

Results into 1 in MySQL which means MySQL uses exact numeric calculation and uses Precision Math where possible. So the MySQL does know that 0.1, 0.2 and 0.3 are exact datatypes here and needs to calculate exact, like i was expecting before this edit.

Meaning the query

SELECT (0.1 + 0.2) = 0.3 

will run under the hood more or less like

SELECT CAST((0.1 + 0.2) AS DECIMAL(1, 1)) = CAST((0.3) AS DECIMAL(1, 1));

回答2:

That is just MySQL being intelligent in data type/formatting choice because you haven't specified that those values are floating point. Try this:

create table test (f float);
insert into test values (0.1), (0.2);
select sum(f) from test

Output:

sum(f)
0.30000000447034836

Demo on dbfiddle

If you use double precision you get the classic 0.30000000000000004 result. Demo on dbfiddle

来源：https://stackoverflow.com/questions/55309494/how-mysql-does-the-math-calculation-of-floating-point-addition