Data types and constraints

Reference article ： Novice tutorial https://www.runoob.com/mysql/mysql-data-types.html

• In order to store data more accurately , Make sure the data is correct and valid , It is necessary to use data types and constraints reasonably to limit the storage of data .

• MySQL The type of data field defined in is very important for database optimization .

• MySQL Support for multiple types , It can be roughly divided into three categories ： The number 、 date / Time and string ( character ) type .

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Common data types

• Integers ：int,4 Bytes ,32 position , The maximum number that can be represented is ：2^32-1=4294967296, There is a range of symbols （-2147483648 ~2147483647）, Unsigned range （0 ~ 4294967295）
• Double precision floating point ：double,8 Bytes
• decimal ：decimal, Such as decimal(5,2) It means coexistence 5 digit , Decimal share 2 position , The whole number takes up 3 position
• Small integers ：tinyint,1 Bytes
• character string ：varchar, Range （0~65533）, Such as varchar(3) At most 3 Characters , A Chinese character or a letter takes up one character
• Date time : datetime, Range （1000-01-01 00:00:00 ~ 9999-12-31 23:59:59）, Such as '2020-01-01 12:29:59'

constraint

• Primary key (primary key)： The order of physical storage , Represents the unique identifier of a record , Cannot be repeated or empty .
• Non empty (not null)： This field is not allowed to fill in null value
• Only (unique)： Duplicate values are not allowed for this field
• The default value is (default)： When this value is not filled in, the default value is used , If it is filled in, the filling shall prevail
• Foreign keys (foreign key)： Maintain the association between two tables