Mysql database Basics: introduction to data types

mysql Database foundation

1、 integer

classification

Integer types	byte	Range
Tinyint	1	A signed ：-128~ 127, Unsigned ：0~255
Smallint	2	A signed ：-32768~32767, Unsigned
Mediumint	3	A signed ：-8388608~ 8388607
Int/integer	4	A signed ：-2147483648 ~ 2147483647
Bigint	8	A signed ：-9223372036854775808 ~ 9223372036854775807, Unsigned ：0~ 9223372036854775807*2+1

characteristic

1、 How to set up unsigned and signed

create table  Table name (
	t1 INT, #  Set signed （ Default ）
	t2 INT UNSIGNED #  Set up unsigned 
	#  If you add a negative number , The result will be displayed as 0
)

2、 If the inserted data exceeds the value range of the integer , Will be submitted to the out of range abnormal , And insert the critical value .

3、 If the length is not set , There will be a default length

2、 decimal

2.1 classification

floating-point

Floating point type	byte	Range
float(M,D)	4	±1.75494351E^ 38~±3.402823466EE^38
double(M,D)	8	It's big ~~~

Fixed point type

Fixed point number type	byte	Range
DEC(M,D),DECIMAL(M,D)	M+2	The maximum value range is related to double identical , Given decimal The valid value range of is determined by M and d decision

2.2 characteristic

（1）M and D The meaning of
M： Integer part digit + Number of decimal places
D： The number of decimal places reserved （ rounding ）
（2）M and D All can be omitted
（3） If the time is decimal Fixed point type , be M The default is 10,D The default is 0
If it is float and double type , The precision is determined by the precision of the inserted value
（4） The accuracy of fixed-point type is higher , If the precision of the inserted value is required to be high , For example, the fixed-point type is considered for currency operation .

3、 Character

3.1 Shorter text

String type	The maximum number of characters	Description and storage requirements	difference
char(M)	M	M by 0~255 Integer between	Fixed length characters , It's more space consuming , High efficiency
varchar(M)	M	M by 0~65535 Integer between	Variable length characters , Not that space consuming , Low efficiency

3.2 Longer text

String type	Description and storage requirements
text	Store long text types
blob	Store picture type

4、 Date type

4.1 classification

The date type	Number of bytes	minimum value	Maximum
date	4	100-1-01	9999-12-31
time	3	-838:59:59	838:59:59
year	1	1901	2155
datetime	8	1000-01-01 00:00:00	9999-12-31 23:59:59
timestamp	4	1970	2038 Some time in the year

Add ：
datetime and timestamp The difference between
（1）timestamp The time range supported is small ,datetime The value range of is relatively large
（2）timestamp Related to the actual time zone , Better reflect the actual date , and datetime It can only reflect the local time zone at the time of insertion
（3）timestamp Attribute Collection of mysql Version and sqlMode The impact of

5、 Enumeration type （Enum)

The value to be inserted must belong to one of the values specified in the list .
for example ：

#  Build table , Define data of an enumeration type 
create table tab_char(
	c1 ENUM('a','b','c')
);

#  Insert value 
insert into tob_char values('a'),('b'),('c');
#  If you insert an unexpected value in the list , May be an error 
insert into tob_char values('m'); #  Report errors 

6、set（ type ）

set Used to hold collections
and Enum Similar type , It can hold 0~64 Members .
and Enum The biggest difference in type is ：set Type can select more than one member at a time , and Enum Only one... Can be chosen . According to the number of members , The bytes occupied by storage are also different

• for example

create table tab_set(
	s1 SET('a','b','c','d')
)

insert into tab_set values('a');
insert into tab_set values('a,b');
insert into tab_set values('a,b,c');

result