SQL数据库的一些操作

//登陆
mysql -u root -h localhost -P 3306 -p 

//创建一个名为test_lib的数据库
CREATE DATABASE test_lib


//查看有多少个database
show databases;

//查看某个database有多少个table
show tables;


//删除一个名为test_lib的数据库
DROP DATABASE test_lib

//选择test_lib数据库
USE test_lib

//添加一个包含3列的表（table）[参考](http://www.runoob.com/sql/sql-create-table.html)
CREATE TABLE table1
    (
        column_name1 data_type(size),
        column_name2 data_type(size),
        column_name3 data_type(size)
    );


//创建带有主键的表
//主键必须包含唯一的值，主键列不能包含 NULL 值。
CREATE TABLE Persons (
	Id_P INT NOT NULL,
	LastName VARCHAR ( 255 ) NOT NULL,
	FirstName VARCHAR ( 255 ),
	Address VARCHAR ( 255 ),
	City VARCHAR ( 255 ),
PRIMARY KEY ( Id_P ) 
)


//外键
//https://www.w3school.com.cn/sql/sql_foreignkey.asp
CREATE TABLE Orders (
	Id_O INT NOT NULL,
	OrderNo INT NOT NULL,
	Id_P INT,
	PRIMARY KEY ( Id_O ),
FOREIGN KEY ( Id_P ) REFERENCES Persons ( Id_P ) 
)


//删除一个表（table）
DROP TABLE table_name

//添加一个列（表头）
ALTER TABLE table_name ADD COLUMN column_name VARCHAR(45);

//删除一列
ALTER TABLE table1 DROP COLUMN column1;

//添加一行（两种方式，指定列，或者不指定列）
INSERT INTO table_name (column1,column2,column3,...) VALUES ('value1','value2','value3',...);
INSERT INTO table_name VALUES ('value1','value2','value3',...);

//删除一行
DELETE FROM table_name WHERE column5='CCC';

//显示test_lib的所有数据
SECLET * FROM test_lib

//查看当前使用的数据库
select database();

//查看数据库使用端口
show variables  like 'port';

//查看数据库编码
show variables like 'character%';

//查看数据库的所有用户信息
select distinct concat('user: ''',user,'''@''',host,''';') as query from mysql.user;

//查看某个具体用户的权限
show grants for 'root'@'localhost';

//查看数据库当前连接数，并发数
show status like 'Threads%';

//查看数据文件存放路径
show variables like '%datadir%';

//where与and使用
SELECT * FROM `person` WHERE `Name` LIKE "%kun%"  AND PersonType IS NOT NULL

//排序，默认asc（顺序）
SELECT * FROM `person` WHERE `Name` LIKE "%王%"  ORDER BY `IdCardNumber`

//先按照PersonType排序（逆序），然后按照IdCardNumber排序（顺序）
SELECT * FROM `person`  ORDER BY `PersonType` DESC,`IdCardNumber` ASC

//两表联查https://www.w3school.com.cn/sql/sql_join.asp
SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo
FROM Persons, Orders
WHERE Persons.Id_P = Orders.Id_P 
    

//Left Join 
SELECT
	person.`Name`,
	person.IdCardNumber,
	customer_conference_person.SeatNumber 
FROM
	person
	LEFT JOIN customer_conference_person ON person.IdCardNumber = customer_conference_person.IdNum 
	AND person.`Name` LIKE "%于%" 
ORDER BY
	person.`Name` DESC

多个表的join查询

//多表 https://www.cnblogs.com/xiaofeng028/p/4098965.html
SELECT
	person.Age,
	person.IdNum,
	person.`Name`,
	gun.GunNum,
	gun.`Status` 
FROM
	person_gun
	JOIN person ON person_gun.IdNum = person.IdNum
	JOIN gun ON person_gun.GunNum = gun.GunNum