String

1、Char

Fixed-length character : After the specified length, the system will allocate space for storing the specified data

The basic syntax:

char (L), L represents the number of characters (letters as Chinese and English), L the length of 0 to 255

2、Varchar

Variable-length character : after a specified length, the system calculates the length of the data actually stored, assign the appropriate length (the data does not exceed the length)

The basic syntax:

Varchar (L), L represents the number of characters, L is the length of the theoretical value bits 0 to 65535

Because varchar length data to be recorded (data length according to the system automatically allocate space), so that after each data generation varchar, the system will increase 1-2 bytes of overhead data behind: it is used to save space occupied by the data length

If the data itself is less than 127 characters: an overhead byte; if more than 127, on the overhead bytes of two

(1) Char and varchar comparison data storage (utf8, a character will occupy 3 bytes)

Storing data	Char(2)	Varchar(2)	Char share of bytes	Varchar of Bytes
A	A	A	2 * 3 = 6	1 * 3 + 1 = 4
FROM	FROM	FROM	2 * 3 = 6	2 * 3 + 1 = 7

The difference (2) Char and the varchar

1, char will use the specified space, it varchar based on data given space

2, char data query efficiency is higher than varchar: varchar is calculated by the number of records required behind

Description:

If determined that the data must be accounted for a specified length, then the use of char type;

If you are unsure how much data there is in the end, then use the varchar type;

If the data is longer than 255 characters, whether or not a fixed length, will use the text, using no char and varchar

3、Text

Text types: mysql provides two types of essentially text

Text: storing plain text characters

Blob: storing binary text (pictures, documents) are not normally use blob to store the file itself, usually using a link pointing to the corresponding file itself.

(1) Text : The system provides four text

(2) TINYTEXT : The system uses one byte to store, the actual data can be stored as follows: 1 + 2 ^ 8

(. 3) the Text : saving two bytes actually stored as: 2 + 16 ^ 2

(. 4) MEDIUMTEXT : using three bytes of storage, real storage is: 2 ^ 24 + 3

(. 5) LONGTEXT : stored using four bytes, actual storage of: 2 ^ 32 + 4

note:

(1) corresponding to the text in the selected memory when not deliberately to choose the type of text, the system will automatically select an appropriate type based on the data length of the text stored.

(2) when the selected character is stored, if the data exceeds 255 characters, then select the stored text

4、Enum

Enumerated type: Before data is inserted, to set up some items, these items may eventually result data appear.

If the data to determine a field of only a few values: such as gender, male and female, confidentiality, the system can be specified when setting field of the current field can hold only a fixed number of values: We'll use enumeration

The basic syntax:

enum (a data value, the data value 2, ...)

The system provides one or two bytes to store data enumerated: selects the actual storage space specifically recited enum value calculation: if the data value within the list 255, then a byte is enough, if more than 255 but less than 65535, the system uses two bytes saved.

Create a table ①

② insert data: valid data, corresponding to the field value must be set to the table when the determined value

③ Data Error: enum functional specification data, to ensure that the data must be inserted into the set range, other types can not

Enumeration enum ④ storage principle: in fact the value of the field is not really stored string, but the string corresponding index: when the system is set enumeration type, each element will enumerate the definition of an index, the index rules from the beginning

Enum (1 => 'M', 2 => 'F', 3 => 'Confidential')

特性：在mysql中系统是自动进行类型转换的：如果数据碰到“+、-、*、/”系统就会自动将数据转换成数值，而普通字符串转换成数值为0

范例：查看enum元素的值

Select 字段名 + 0 from 表名;