MySQL: data types and operators

Preface

MySQL The data type defines the size range of the data , Therefore, choose the appropriate type when using , It will not only reduce the disk space occupied by the table , Indirectly reduces the number of disks I/O The number of times , Improved table access efficiency , And the efficiency of index is also closely related to the type of data .

data type

value type

When creating a table, it should be based on the scene Choose the appropriate numerical type , Although the type looks similar in byte size , But if there are thousands of pieces of data , It will waste a lot of space .

String type

Be careful ： String type CHAR(M), The number in this bracket represents the number of bytes , But sometimes we see this kind of problem ,
Parentheses after integer type ：

The memory size occupied by shaping is fixed , Is strongly related to specific types , This 9 Represents the width of integer display .

age INT(9)

Date and time type

The date type is also in the process of doing the project , Frequently used type information , In especial TIMESTAMP and DATETIME Two types , But notice TIMESTAMP Will automatically update the time , It is very suitable for those scenarios that need to record the latest update time , and DATETIME Need to update manually .

Timestamp example ：

enum and set

These two types , It is restricted that this field can only take a fixed value , But the enumeration field can only take a unique value , The set field can take any value .

Example ： Set gender to a fixed value , If you enter others, you will report an error .

sex enum(‘M’, ‘W’) default ‘M’

Operator

Arithmetic operator

Logical operators

Comparison operator

Wildcard match like： There are two wildcards

%
_

Example ：
There are three names ：
zhang san
zhang yao
zhangx

select* from where name like ‘zhang%’; // Can match all zhang At the beginning
select* from where name like ‘zhang_’; // Means only match zhang The last character