Java题目解析-day01

问题1：查询出Person表中重复的email

Person表

id      email

1       [e-mail]

2       [e-mail]

3       [e-mail]

4       [e-mail]

select email from Person group by email having count(email)>1;

问题2：交换男(m)女(f)

salary表

id   name  salary  sex

1     A       1000      m

2     B       2000       f

3     C       3000      m

4     D       4000      f

update salary set sex=

case sex when 'm' then 'f'

              when 'f' then 'm'                 

end; 

问题3：返回第二高的薪水

id      salary

1        100

2        200

3        300   

4        400

select salary from tb_salary order by salary desc limit 1 offset 1;