Detailed description of mysq-based field selection

Mysql supports many field types, including numeric, date/time, and string (character). storage space and storage efficiency must be taken into account during use;
The following conventions are used for column type descriptions:
· M
The maximum display width. The maximum valid display width is 255.
· D
Time type: datetime (8 bytes), timestamp (4 bytes)
Character Type:
VARCHAR (M), L + 1 byte, where L <= M and 0 <= M <= 65535, it is not recommended to set M too large, because the actual length is calculated in the memory, this computing consumes resources;
CHAR (M) M bytes, 0 <= M <= 255
BINARY (M), MB bytes, 0 <= M <= 255
VARBINARY (M), L + 1 byte, where L <= M and 0 <= M <= 255
ENUM ('value1 ', 'value2 ',...), 1 or 2 bytes, depending on the number of enumerated values (up to 65,535 values). The adjustment is complex and needs to be dropped in create first; when the enumerated value is smaller than 255, it occupies 1 byte, when the value is greater than 255, it occupies 2 bytes;
SET ('value1', 'value2 ',...) 1, 2, 3, 4, or 8 bytes, depending on the number of set members (up to 64 members), 1-8 occupies 1 byte, 9-16 occupies 2 bytes, 17-24 occupies 3 bytes, 25-32 occupies 4 bytes, and 33-64 occupies 8 bytes;
Integer type:
Tinyint (8), one byte
Smallint (16), 2 bytes
Mediumint (24), 3 bytes
Int (32), 4 bytes
Bigint (64), 8 bytes
Real Type:
Float (4 bytes)
Float (M, D) is a single-precision Float, accurate to 6 digits after the decimal point, and double (M, D) is precise to 13 digits after the decimal point;
How to store float memory

Type
Storage bits
Total digits
Offset Value

Digit (S)
Level Code (E)
Tail (M)
(Offset)

Short real number (float)
1
8
23
32
127

Long real number (double)
1
11
52
64
1023

How to store Float
N (10) = 123.456,
Converted to binary representation:
N (2) = 1111011. 01110100101111001
= 1. 11101101110100101111001 (...) * 2 ^ 6
Then E = 127 + 6 = 133 (10) = 10000101 (2)
M = 111 0110 1110 1001 0111 1001 (maximum digit 1 is omitted, total 23bit)
The combination is:
S E M
0 10000101 111 0110 1110 1001 0111

Integer type:
Decimal (M, D) occupies M + 2 bytes, Decimal point and symbol need
Each occupies one byte;
The maximum possible value range of DECIMAL is the same as that of DOUBLE.
But the valid value range is determined by the values of M and D.
Is a string;
Decimal (M, D) occupies M + 2 bytes, And the Decimal point and symbol respectively occupy one byte;
The maximum possible value range of DECIMAL is the same as that of DOUBLE, but its valid value range is determined by the values of M and D. The storage method is string;