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