SELECT DISTINCT country FROM Websites;
SELECT TOP 50 PERCENT a.count FROM Websites AS a #别名
WHERE alexa > 15
AND alexa BETWEEN 1 AND 20
AND name IN（‘google’,‘baidu’）
AND (country='CN' OR country='USA')
AND name LIKE ‘G%’ #通配符%替代0个或多个字符，_替代一个字符，[charlist]字符列中任意,[^charlist]
ORDER BY alexa DESC，age; #DESC降序，多列时先按照第一个排序再按照第二个排序
LIMIT 10
SELECT Websites.id, Websites.name, access_log.count, access_log.date #SQL连接
FROM Websites
INNER JOIN access_log
ON Websites.id=access_log.site_id;
INSERT INTO Websites (name, url, alexa, country) #向制定的列插入一个新行
VALUES ('百度','https://www.baidu.com/','4','CN');
UPDATE Websites SET alexa='5000', country='USA' WHERE name='菜鸟教程'; #更新表中的数据
DELETE FROM Websites WHERE name='Facebook' AND country='USA'; #删除表中的行
