Leetcode: find all duplicate
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Leetcode : Duplicate Emails
Write a SQL query to find all duplicate emails in a table named Person.
+—-+———+
| Id | Email |
+—-+———+
| 1 | a@b.com |
| 2 | c@d.com |
| 3 | a@b.com |
+—-+———+
For example, your query should return the following for the above table:
+———+
| Email |
+———+
| a@b.com |
+———+
Method:
Use count() group by Email.
select Email from (select count(*) as c, Email from Person group by Email ) as awhere 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 awhere c > 1;
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