leetcode182 查找重複的電子郵箱 Duplicate Emails

編寫一個 SQL查詢，來查找名爲 Person 的表中的全部重複電子郵件。

 

 

 建立表和數據：

Create table If Not Exists Person (Id int,Email varchar(255));
Truncate table Person;
insert into Person (Id, Email) values ('1','a@b.com');
insert into Person (Id, Email) values ('2','c@d.com');
insert into Person (Id, Email) values ('3','a@b.com');

解法：

1.若是一個字段的值在表中重複了，那麼含有重複值的行數必定超過1。

group by 對Email分組，那麼Email重複的行個數大於1。

having 篩選出這些行。

select Email
from Person
group by Email
having count(Id) > 1

2.假設表中的字段Id是惟一的，還能夠自鏈接表，選中哪些Id不一樣，Email相同的行。注意投影后要distinct去重。

select distinct P1.Email
from Person as P1 join Person as P2 on (P1.Id != P2.Id and P1.Email = P2.Email)

3.對每行的Email，子查詢中都在表中查找一次，Email相同的行個數超過1。選中此行。效率低。

select distinct P1.Email
from Person as P1 
where 1 < (
    select count(*)
    from Person as P2
    where P1.email = P2.email
);

 

 

 

select E1.Name as Employee
from Employee as E1 join Employee as E2 on (E1.ManagerId  = E2.Id and E1.salary > E2.salary)