Operation database

notes

Show databases; -- show all databases, single line comments
/*
hello
world
Multi line comment of SQL
*/

Operation database > operation tables in Database > operation data of tables in database
MySQL is case insensitive

create database [if not exists] school;

drop database [if exists] school;

--Tab key, if the table name or field name is a special character, you need to add``
use `school`;

show database;

numerical value

type	describe	Byte size
tinyint	Very small data	1 byte
smallint	Smaller data	2 bytes
mediumint	Medium sized data	3 bytes
int	Standard integer	4 bytes
bigint	Big data	8 bytes
float	Floating point number	4 bytes
double	Double precision	8 bytes (accuracy problem)
decimal	Floating point number in string form	2 bytes (half used in financial calculation)

character string

type	describe	Byte size
char	The string has a fixed size, char (3) represents 0 filling bits, independent of storage bits	0～255
varchar	Variable string	0 ～ 65535 (commonly used string)
tinytext	Micro text	28-1
text	Text string	216-1 (save large text)

Time date

java.util.Date

type	describe	Byte size
date	yyyy-MM-dd	Date format
time	HH : mm : ss	Time format
datetime	yyyy-MM-dd HH : mm : ss	The most commonly used time format
timestamp	time stamp	Time milliseconds from 1970.1.1 to now
year	Year representation

null

Unsigned：

zerofill：

auto_increment：

It is usually understood as self incrementing and automatically gives + 1 on the basis of the previous record (default)
It is usually used to design a unique primary key index, which must be an integer type
You can customize the starting value and step size of the primary key

Not null and null:

If it is set to not null, an error will be reported if it is not assigned a value
Null, but do not fill in, the default is null

defalut：

Set default
Sex, the default value is male, if you do not specify the change column value, there will be a default value!

Each table must have the following five fields to indicate the meaning of a record!

ID primary key
`Version ` optimistic lock
is_ Delete pseudo delete
gmt_ Create time
gmt_ Update modification time

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