How MySQL queries duplicate records

SELECT * FROM Hengtu_demandpush a
where (A.did,a.mid) in (select Did,mid from Hengtu_demandpush GROUP by Did,mid have count (*) > 1)

Or

SELECT * FROM Hengtu_demandpush GROUP by Did,mid have Count (*) >1

1, look for redundant records in the table, duplicate records are based on a single field (Peopleid) to determine

1. SELECT * from People
2. where Peopleid in (select Peopleid from People GROUP by Peopleid have Count (Peopleid) > 1)

2, delete redundant records in the table, duplicate records are based on a single field (Peopleid) to judge, leaving only the smallest ROWID records

1. Delete from people
2. where Peopleid in (select Peopleid from People GROUP by Peopleid have Count (Peopleid) > 1)
3. and rowID not in (select min (rowid) from people GROUP by Peopleid have Count (Peopleid)>1)

3. Find redundant duplicate records (multiple fields) in the table

1. SELECT * FROM Vitae a
2. where (A.PEOPLEID,A.SEQ) in (select Peopleid,seq from Vitae GROUP by PEOPLEID,SEQ have Count (*) > 1)

4. Delete extra duplicate records (multiple fields) in the table, leaving only the record with ROWID minimum

1. Delete from Vitae a
2. where (A.PEOPLEID,A.SEQ) in (select Peopleid,seq from Vitae GROUP by PEOPLEID,SEQ have Count (*) > 1)
3. and rowID not in (select min (rowid) from Vitae GROUP by PEOPLEID,SEQ have Count (*)>1)

5. Find redundant duplicate records (multiple fields) in the table, not including the smallest ROWID records

1. SELECT * FROM Vitae a
2. where (A.PEOPLEID,A.SEQ) in (select Peopleid,seq from Vitae GROUP by PEOPLEID,SEQ have Count (*) > 1)
3. and rowID not in (select min (rowid) from Vitae GROUP by PEOPLEID,SEQ have Count (*)>1)

How MySQL queries duplicate records