Leetcode: find all duplicate

Leetcode : Duplicate Emails

Write a SQL query to find all duplicate emails in a table named Person.
+—-+———+
| Id | Email |
+—-+———+
| 1 | [email protected] |
| 2 | [email protected] |
| 3 | [email protected] |
+—-+———+
For example, your query should return the following for the above table:
+———+
| Email |
+———+
| [email protected] |
+———+

Method:

Use count() group by Email.

select Email from 
(
select count(*) as c, Email from Person group by Email 
) as a
where c > 1;

Add ORDER in subquery. That will accelerate the parent query.

select Email from 
(
select count(*) as c, Email from Person group by Email  order by c
) as a
where c > 1;