LeetCode force buckle brush title database (182): find duplicate e-mail

topic

Write a SQL query to find all duplicate Person table e-mail.

Example:

Based on the above inputs, your query should return the following results:

Note: All e-mail all lowercase letters.

analysis

Method One: Use GROUP BY and temporary tables

Algorithms
exist repeated e-mail. To calculate the number of the existence of each email, we can use the following code.

MySQL

select Email, count(Email) as num
from Person
group by Email;

Email	on one
[email protected]	2
[email protected]	1

As a temporary table, we get the following solutions.

MySQL

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

Method Two: Use GROUP BY and HAVING conditions

GROUP BY to add one kind of the condition of the more common method is to use a HAVING clause, which is more simple and efficient.

So we can rewrite the above solutions are:

MySQL

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

Method three: Since the connection

1. View the table

SELECT
	* 
FROM
	person;

2. Since the two connection tables

SELECT
	* 
FROM
	person AS a,
	person AS b;

The final result 3

SELECT DISTINCT
	a.Email AS Email 
FROM
	person AS a,
	person AS b 
WHERE
	a.Id < b.Id 
	AND a.Email = b.Email;

answer

The first

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

The second

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

The third

SELECT DISTINCT
	a.Email AS Email 
FROM
	person AS a,
	person AS b 
WHERE
	a.Id < b.Id 
	AND a.Email = b.Email;

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