技術標籤：SQL

LeetCode 182：Duplicate Emails

【Description】

Write a SQL query to find all duplicate emails in a table named Person.

For example, your query should return the following for the above table:

Note: All emails are in lowercase.

【Solution】

Personal solution:

select distinct p1.Email
from Person p1,
 
 Person p2
where p1.Email=p2.Email
    and p1.Id!=p2.Id

Reference solution:
1.Duplicated emails existed more than one time. We can build a temp table and count the times each email exists, and then get the duplicated emails.

select Email from
(
  select Email, count(Email) as num
  from Person
  group
 
 by Email
) as statistic
where num > 1
;

2.A more common used way to add a condition to a GROUP BY is to use the HAVING clause, which is much simpler and more efficient.

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