MySQL - data type

Catalog

Data type classification

bit type

Decimal type

String type

Time type

 enum and set

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Data type classification

There are four types

• value type
• Text binary type
• Time date
• string type

  We need to understand value type What are the different allowable maximum and minimum values

  Let's do some cross-border tests ：

  We can find out , Shaping is signed by default . So how to create unsigned ？

bit type

//bit Basic grammar 
bit[(M)]:  Bit field type .M Represents the number of digits in each value , Range from 1 To 64. If M Be ignored , The default is 1.

ask ：  Why? a The corresponding value is not displayed ？

because bit When the field is displayed , Is in accordance with the ASCII The value corresponding to the code displays .

Decimal type

In numeric types , among float The type and decimal Type belongs to decimal type .

//float grammar 
float[(m, d)] [unsigned] : M Specify the display length ,d Specify the number of decimal places , Occupancy space 4 Bytes 

such as float（4,2） The range of representation is -99.9~99.99.

We can see from the inserted values ,float It will be rounded when saving .

When float Is unsigned ,float（4,2）unsigned The scope is 0~99.99

 

//decimal grammar 
decimal(m, d) [unsigned] :  Fixed-point number m Specify the length ,d Number of decimal places 

such as decimal（5,2） The range of representation is -999.99~999.99

Above ,decimal(5,2) unsigned Range of representation 0 ~ 999.99

look decimal and float Very similar , Let's see the difference between the two

String type

String types are char and varchar

//char grammar 
char(L):  Fixed length string ,L Is the length that can be stored , Unit as character , The maximum length value can be 255

//varchar grammar 
varchar(L):  Variable length string ,L Represents character length , Maximum length 65535 Bytes 

 

char and varchar Compare two string types

How to choose which one to use ？ Please follow the following rules

• If the data is determined to be the same length , Just use a fixed length （char）, such as ： Id card , cell-phone number ,md5
• If the data length changes , Just use lengthening (varchar), such as ： name , Address , But you have to make sure that the longest can be saved .
• Fixed length disk space is a waste , But it's efficient .
• Longer disk space is more economical , But it's inefficient .
• The meaning of fixed length is , Directly open up the corresponding space
• The meaning of lengthening is , Without exceeding the custom scope , How much to use , How much .

Time type

There are three common dates ：

• date : date 'yyyy-mm-dd' , Take up three bytes
• datetime Time date format 'yyyy-mm-dd HH:ii:ss' Indicates a range from 1000 To 9999 , Take up the octet
• timestamp ： Time stamp , from 1970 Year begins yyyy-mm-dd HH:ii:ss Format and datetime Exactly the same , Occupy four bytes

We can see , The timestamp will automatically generate the current time . Next let's revise t1 Time for .

 enum and set

// Both syntax 
enum(' Options 1',' Options 2',' Options 3',...);
set(' Option value 1',' Option value 2',' Option value 3', ...);