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 | +---------+
Note: All emails are in lowercase.
GROUP BY and a temporary table [Accepted]Algorithm
\nDuplicated emails existed more than one time. To count the times each email exists, we can use the following code.
\nselect Email, count(Email) as num\nfrom Person\ngroup by Email;\n
| Email | num |\n|---------|-----|\n| a@b.com | 2 |\n| c@d.com | 1 |\n
Taking this as a temporary table, we can get a solution as below.
\nselect Email from\n(\n select Email, count(Email) as num\n from Person\n group by Email\n) as statistic\nwhere num > 1\n;\n
GROUP BY and HAVING condition [Accepted]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.
So we can rewrite the above solution to this one.
\nMySQL
\nselect Email\nfrom Person\ngroup by Email\nhaving count(Email) > 1;\n