Introduction and advanced level of MySQL (6)

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1、 constraint

When we create database tables
CREATE TABLE Table name (
Name type [ constraint ],
Name type [ constraint ]
)

2、 Constraint classification

2.1、primary key

Primary key constraint When a column is created add to primary key here The primary key constraint is added to this column , This column is called Primary key column . for example
Create table student (
id int primary key
)
here id This column adds a primary key constraint , This column is called Primary key column .
When added After the primary key constraint , At this time, this column is not empty （ This column must have a value ） And only （ The value of each row of this column cannot be repeated ） The limit of .

doubt ： Can I use unique + not null Instead of primary key ？
Can not be , because primary key Except for non empty and unique restrictions , Another function is Functions of primary key index .

I'm interested in studying binary trees （ No need to know the code ）
therefore Our database tables all have one id Column Set as primary key constraint

2.2、AUTO_INCREMENT( Self growth )

2.3、DEFAULT( Default )

2.4、unique Represents the only constraint .

The only constraint is to specify table Column or column combination of cannot be repeated , Guarantee the uniqueness of data

2.5、not null Is a non empty constraint ,

The value of a specified row cannot be null.

3、SQL Import and export of files

export ： Select database –》 Right click —》 Transfer to storage SQL --》 preservation
Import ：

4、MYSQL data type

TINYINT 1 Bytes (-128,127) (0,255) Small integer value
SMALLINT 2 Bytes (-32 768,32 767) (0,65 535) Large integer value
MEDIUMINT 3 Bytes (-8 388 608,8 388 607) (0,16 777 215) Large integer value
INT or INTEGER 4 Bytes (-2 147 483 648,2 147 483 647) (0,4 294 967 295) Large integer value
BIGINT 8 Bytes (-9,223,372,036,854,775,808,9 223 372 036 854 775 807) (0,18 446 744 073 709 551 615) Maximum integer value
FLOAT 4 Bytes (-3.402 823 466 E+38,-1.175 494 351 E-38),0,(1.175 494 351 E-38,3.402 823 466 351 E+38) 0,(1.175 494 351 E-38,3.402 823 466 E+38) Single precision floating point values
DOUBLE 8 Bytes (-1.797 693 134 862 315 7 E+308,-2.225 073 858 507 201 4 E-308),0,(2.225 073 858 507 201 4 E-308,1.797 693 134 862 315 7 E+308) 0,(2.225 073 858 507 201 4 E-308,1.797 693 134 862 315 7 E+308) Double precision floating point value
DECIMAL Yes DECIMAL(M,D) , If M>D, by M+2 Otherwise D+2 Depend on M and D Value Depend on M and D Value Small value