window平台：
（管理员）启动数据库：net start mysql
登陆： mysql -u root -p
create database test；
use test；
项目一：查找重复的电子邮箱
CREATE TABLE email (
ID INT NOT NULL PRIMARY KEY,
Email VARCHAR(255)
)
INSERT INTO email VALUES('1','a@b.com');
INSERT INTO email VALUES('2','c@d.com');
INSERT INTO email VALUES('3','a@b.com');
语句：
select Email from email
group by
having
count(Email) > 1;
项目二：查找大国
-- 创建表
CREATE TABLE World (
name VARCHAR(50) NOT NULL,
continent VARCHAR(50) NOT NULL,
area INT NOT NULL,
population INT NOT NULL,
gdp INT NOT NULL
);
-- 插入数据
INSERT INTO World VALUES( 'Afghanistan', 'Asia',652230,25500100,20343000);
INSERT INTO World VALUES( 'Albania', 'Europe' ,28748,2831741,12960000);
INSERT INTO World VALUES( 'Algeria', 'Africa' ,2381741,37100000,188681000);
INSERT INTO World VALUES( 'Andorra' , 'Europe' ,468,78115,3712000);
INSERT INTO World VALUES( 'Angola' , 'Africa' ,1246700,20609294,100990000);
语句：
select name population area from
World where
population > 25000000 or
area > 3000000 or
gdp > 20000000;