LeetCode Database: Delete Duplicate Emails

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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  |
+----+------------------+

# Write your MySQL query statement below

delete from Person where Id not in
(
select Id from 
(
    select Id,
    if(STRCMP(@prev, Email)=0, false, true) as Diff,
    @prev := Email

    from 
    (select * from Person p2 order by Email, Id asc) x,
    (select @prev := ‘‘) y
) t where t.Diff=true
)

LeetCode Database: Delete Duplicate Emails

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原文地址：http://www.cnblogs.com/ydlme/p/4899417.html