【sql】刪除重複的郵件 Delete Duplicate Emails

問題：

Write a SQL query to delete all duplicate email entries in a table named Person, keeping only unique emails based on its smallest Id.

+----+------------------+
| Id | Email            |
+----+------------------+
| 1  | john@example.com |
| 2  | bob@example.com  |
| 3  | john@example.com |
+----+------------------+
Id is the primary key column for this table.

For example, after running your query, the above Person table should have the following rows:

+----+------------------+
| Id | Email            |
+----+------------------+
| 1  | john@example.com |
| 2  | bob@example.com  |
+----+------------------+

解決：

① 能夠首先找出全部不重複的郵箱，而後取個反就是重複的郵箱，都刪掉便可，那麼咱們如何找出全部不重複的郵箱呢，咱們能夠按照郵箱羣組起來，而後用Min關鍵字挑出較小的，而後取補集刪除便可。962ms

DELETE FROM Person
WHERE Id NOT IN
(SELECT Id FROM
    (SELECT MIN(Id) Id FROM Person GROUP BY Email) p
);

② 使用內交讓兩個表以郵箱關聯起來，而後把相同郵箱且Id大的刪除掉；954ms

DELETE p2 FROM Person p1 JOIN Person p2
ON p2.Email = p1.Email WHERE p2.Id > p1.Id;

③ 直接用where將兩表關聯起來；1444ms。

DELETE p2 FROM Person p1,Person p2
WHERE p1.Email = p2.Email AND p2.Id > p1.Id;