查找重复的电子邮箱
select email as Email from Person group by email having count(email) > 1
select email as Email from ( select email ,count(email) as c from Person group by email ) r where r.c > 1
select distinct p1.email as Email from Person p1 left join Person p2 on p1.email = p2.email where p1.id != p2.id
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