Websites数据表如下

+----+--------------+---------------------------+-------+---------+
| id | name         | url                       | alexa | country |
+----+--------------+---------------------------+-------+---------+
| 1  | Google       | https://www.google.cm/    | 1     | USA     |
| 2  | 淘宝          | https://www.taobao.com/   | 13    | CN      |
| 3  | 菜鸟教程      | http://www.runoob.com/    | 4689  | CN      |
| 4  | 微博          | http://weibo.com/         | 20    | CN      |
| 5  | Facebook     | https://www.facebook.com/ | 3     | USA     |
+----+--------------+---------------------------+-------+---------+

SELECT 语句用于从数据库中选取数据

SELECT column_name,column_name FROM table_name;
SELECT * FROM table_name;

SELECT DISTINCT 语句用于返回唯一不同的值

SELECT DISTINCT column_name, column_name FROM table_name;

WHERE 子句用于过滤记录

SELECT column_name,column_name 	FROM table_name WHERE column_name <operator> value;
# 例
SELECT * FROM Websites WHERE country='CN';

WHERE 子句中的运算符

运算符	描述
=	等于
<>	不等于。注释：在 SQL 的一些版本中，该操作符可被写成 !=
>	大于
<	小于
>=	大于等于
<=	小于等于
BETWEEN	在某个范围内
LIKE	搜索某种模式
IN	指定针对某个列的多个可能值

AND & OR 运算符用于基于一个以上的条件对记录进行过滤

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');

ORDER BY 关键字用于对结果进行排序

默认升序排列，可以使用 DESC 关键字进行降序排列

SELECT column_name,column_name FROM table_name ORDER BY column_name;
# 例
SELECT * FROM Websites ORDER BY alexa;
SELECT * FROM Websites ORDER BY alexa DESC;

INSERT INTO 语句用于向表中插入新纪录

INSERT INTO table_name (column1,column2,...)
VALUES (value1,value2,...);
# 例
INSERT INTO Websites (name, url, alexa, country)
VALUES ('百度'， 'https://www.baidu.com/', '4', 'CN');

UPDATE 语句用于更新表中的记录

UPDATE table_name 
SET column1=value1, column2=value2,...
WHERE some_column=some_value;
# 例
UPDATE Websites
SET alexa='5000',country='USA'
WHERE name='菜鸟教程'

更新记录时需要格外小心！如果省略了WHERE语句，所有的记录都会被更新。

DELETE 语句用于删除表中的记录

DELETE FROM table_name
WHERE some_column=some_value;
# 例
DELETE FROM Websites
WHERE name='Facebook' AND country='USA';

删除记录时需要格外小心！如果省略了WHERE语句，所有的记录都会被删除。