Mysql作业 Day1 - 爱码网

项目一：查找重复的电子邮箱

create database test;
use test;
create table email(
	ID INT NO NULL PRIMARY KEY,
	Email varchar(255)
);
insert into email values(1,'[email protected]');
insert into email values(2,'[email protected]');
insert into email values(3,'[email protected]');

查找重复的email语句:
select Email ,count(*) as count from email group by Email having count(检查字段)>1;
select 检查字段，count(检查字段) from table group by 检查字段 having count(检查字段) >1;

项目二：查找大国

use test；
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
	
);
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);

Q：如果一个国家的面积超过300万平方公里，或者(人口超过2500万并且gdp超过2000万)，那么这个国家就是大国家。编写一个SQL查询，输出表中所有大国家的名称、人口和面积。

select  name,population,area from World where area>3000000 or (population>25000000 and gdp >20000000);

Mysql作业 Day1