（1）select语句
select * from website
select name from website
select name country from website
（2）在表中一个列可能包含多个重复值，有时候希望列出不同（distinct）的值
select distinct country from websites
（3）where子句用于提取那些满足制定标准的记录
select * from Websites where country = ‘CN’;
select * from websites where id= 1;
SQL使用单引号来引用文本值，文本字段使用单引号，数字字段不使用
（4）如果在第一个条件第二个条件都成立，则and运算符显示一条记录，只有一个成立则是OR
select * from Websites where country = ‘CN’ and alexa >50;
select * from Websites where country = ‘USA’ or country = ‘CN’;
select * from Websites where alexa >15 and (country = ‘CN’ or country = ‘USA’);
(5)order by 关键字用于对结果集按照一个列或者多个列进行排序，默认升序，降序desc
select * from Websites order by alexa;
select* from Websites order by alexa DESC;
select *from Websites order by country, alexa;
(6)insert into用于向表中插入新记录
假如像websites表中插入一个新行
insert into websites (name, url, alexa,country) values(‘百度’，’https://www.baidu.com/’,’4’,’CN’);
在指定列插入数据
insert into Websites (name,url, country)values (‘stackoverflow’,’http://stackoverflow.com/’,’IND’);
(7)update用于更新表中存在记录
update Websites set alexa = ‘5000’,country = ‘USA’ where name=‘菜鸟教程’;
(8)用于删除表中的行
delect from Websites where name= ‘百度’ and country=‘CN’;
不删除表情况下删除所有行
delect from Websites或 delect * from Websites
