mysql: column type float, double_mysql double_Ordinary netizens' blog-CSDN blog
environment:
window10
vs2022
.net 6
mysql 8.0.25
DBeaver
Reference:
"MSDN: Floating-Point Types (C# Reference)"
"mysql: 11.1.4 Floating-Point Types (Approximate Value) - FLOAT, DOUBLE"
1. float and double types
In mysql, float and double are both floating point number types:
float occupies 4 bytes and has a precision of 6 bits;
double occupies 8 bytes and has a precision of 16 bits;
Their properties are the same as those of floating point numbers in C#, except that the precision requirements are more strict and clear;
Reference: The precision requirements for floating point types in C# are Insert picture description here
Test float and double in mysql:
create table test(
t_float float,
t_double double
)
insert into test.test (t_float) values(123456);-- Normal
insert into test.test (t_float) values(1234567);-- Four settings and five inputs 1234570
insert into test.test (t_float) values(12345678901);- - Distortion12345700352
insert into test.test (t_float) values(12345.6); -- Normal
insert into test.test (t_float) values(12345.62); -- Set four and five into 12345.6
insert into test.test (t_float) values(123.123); - - Normal
insert into test.test (t_float) values(12.1234); -- Normal
insert into test.test (t_float) values(1.12345); -- Normal
insert into test.test (t_float) values(1234.1234); -- Four Let five inputs be 1234.12
-- double
insert into test.test (t_double) values(1234567890123456);-- Normal
insert into test.test (t_double) values(12345678901234567);-- Four settings and five inputs 12345678901234568
insert into test.test (t_double) values(123.12345678 901 );-- normal
insert into test.test (t_double) values(1234567890.123456);-- normal
select * from test
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The output is as follows:
Insert image description here
We can think of it like this:
when we insert data into the float type, mysql will give priority to retaining 6 significant digits and round the numbers on the right;
when we insert data into the double type, mysql will give priority to retaining 16 significant digits. , set four to five numbers on the right;
Note: mysql will not report an error because the data exceeds the range, for example: 12345678901 is far beyond the accuracy, but it will only distort the data and will not report an error.
2. float and double syntax
The recommended usage methods are: t_float float, t_double double, that is, no other parameters are added.
However, in order to be compatible with SQL standards, mysql provides: float(p). The value range of p here is: [0,53]. If it exceeds the range, an error will be reported. In addition: p here is only used to determine the internal use of float in mysql. It is still double storage. When p∈[0,23], mysql uses float storage. When p∈[24,53], mysql uses double storage.
3. Non-recommended usage of float and double
What is mentioned above is the recommended usage. In the recommended usage, the behavior of MySQL's float and C#'s float are basically the same.
There are also non-standard usages, such as: float(M,D), but this way of writing is about to be deprecated.
Although it is deprecated, it seems to be not useless:
M in float(M,D) represents the total number of significant digits, and D represents the number of decimal places. This is a bit like decimal(M,D) with M and D.
If we define: float(5,2), it means: a maximum of 2 digits after the decimal point and a maximum of 3 digits before the decimal point. If there are more than 3 digits before the decimal point, an error will be reported. If there are more than 2 digits after the decimal point, it will be rounded off;
The test is as follows:
create table test(
t_float5_2 float(5,2)
)
insert into test.test (t_float5_2) values(123.12);-- Normal
insert into test.test (t_float5_2) values(1234.1);-- Error: Out of range value
insert into test.test (t_float5_2) values(1.1234); -- Four settings and five inputs 1.12
select * from test
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The output is as follows:
Insert image description here
You can see that this non-standard usage is more restrictive, but because it is about to be enabled, we won’t explore it too much.
In addition, there is no non-standard double(M,D) writing method for double type!
In addition, in addition to float(M,D), there are also real(M,D) and double precision(M,D).
Here is a summary of their usage and differences:
Insert picture description here
4. How to use it in c#
Since float and double in mysql are floating-point data, they basically correspond to float and double in c#, so in c# we can use float and double to replace them respectively, but generally we will use them uniformly in c# double, as follows:
public class Model
{
public double? t_float { get; set; }
public double? t_double { get; set; }
}
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5. Finally, take a look at their metadata
create table test(
t_float float,
t_float7 float(7),
t_float10 float(10),
t_float35 float(35),
t_float40 float(40),
t_double double,
-- t_double double(5), -- 报错
t_float7_5 float(7,5), -- 弃用
t_real real,
t_real7_5 real(7,5),
t_double_precision8_5 double precision(8,5)
)
select c.TABLE_SCHEMA ,c.TABLE_NAME ,c.COLUMN_NAME ,c.ORDINAL_POSITION,c.DATA_TYPE,c.NUMERIC_PRECISION ,c.COLUMN_TYPE ,c.NUMERIC_SCALE
from information_schema.`COLUMNS` c where TABLE_SCHEMA ='test' and TABLE_NAME ='test' order by ORDINAL_POSITION
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Original link: https://blog.csdn.net/m0_67392661/article/details/126803834