Querying database Duplicate logging method __ Database

SQL statement Query method for database duplicate records SQL statement query database duplicate record method SQL statement query database record SQL statement query database duplicate record

Suppose you have an existing list of people (table name: person), and if you want to find out the exact same records in the three fields of the name, ID number and address, use

SELECT p1.*

From persons p1,persons P2

WHERE p1.id<>p2.id

and P1.cardid =p2.cardid

and p1.pname = P2.pname

and P1.address =p2.address

You can implement this feature.

To delete a duplicate record's SQL statement

1. Using the rowID method

2. Using the group by method

3. Using the Distinct method

1. Using rowID method

According to the Oracle band ROWID attribute, to determine whether there are duplicates, the statement is as follows:

Check the data:

SELECT * FROM table1 a where rowID!= (select Max (ROWID) from table1 b where a.name1=b.name1 and a.name2=b.name2 ...)

Delete data:

Delete from table1 a where rowID!= (select Max (ROWID) from table1 b where a.name1=b.name1 and a.name2=b.name2 ...)

2.group by method

Check the data:

Select COUNT (num), max (name) from student--Lists the number of duplicate records and lists his Name property

GROUP BY Num

Having count (num) >1--After grouping by NUM, find out that the NUM column in the table repeats, which occurs more than once

Delete data:

Delete from student

GROUP BY Num

Having count (num) >1

In this case, all the duplicates are deleted.

3. Using the distinct method-useful for small tables

CREATE TABLE Table_new as SELECT DISTINCT * FROM table1 Minux

TRUNCATE TABLE table1;

INSERT INTO table1 select * from Table_new;

How to query and delete duplicate records

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

SELECT * from people where Peopleid to (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 rowid minimal records

Delete from people where Peopleid on (select Peopleid from People GROUP by Peopleid have count (Peopleid) > 1)

and rowID not in (select min (rowid) from people GROUP by Peopleid have Count (Peopleid) >1)

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

SELECT * from Vitae a WHERE (A.PEOPLEID,A.SEQ) in (select Peopleid,seq to Vitae GROUP by PEOPLEID,SEQ have Count (*) &G T 1)

4, delete redundant records in the table (multiple fields), leaving only the smallest ROWID records

Delete from Vitae a where (A.PEOPLEID,A.SEQ) in (select Peopleid,seq to Vitae GROUP by PEOPLEID,SEQ have count (*) > 1)

and rowID not in (select min (rowid) from Vitae GROUP by PEOPLEID,SEQ have Count (*) >1)

5, look for redundant records in the table (multiple fields), does not contain the smallest ROWID records

SELECT * from Vitae a WHERE (A.PEOPLEID,A.SEQ) in (select Peopleid,seq to Vitae GROUP by PEOPLEID,SEQ have Count (*) &G T 1)

and rowID not in (select min (rowid) from Vitae GROUP by PEOPLEID,SEQ have Count (*) >1)

(b) For example, there is a field "name" in Table A and

and the "name" value may be the same between different records, and now you need to query between the records in the table, the "name" value duplicates the item;

Select Name,count (*) from-A Group by Name has Count (*) > 1

If the same gender is also the same large as the following:

Select Name,sex,count (*) from-A Group by Name,sex have Count (*) > 1

Method One

Declare @max integer, @id integer

Declare cur_rows cursor Local for select main field, COUNT (*) from table name Group by main field having count (*) >; 1

Open Cur_rows

Fetch cur_rows into @id, @max

While @ @fetch_status =0

Begin

Select @max = @max-1

SET ROWCOUNT @max

Delete from table name where main field = @id

Fetch cur_rows into @id, @max

End

Close Cur_rows

SET ROWCOUNT 0

Method Two

"Duplicate record" has two duplicate records, one is a completely duplicate record, that is, all the fields are duplicate records, the second is a part of key fields duplicate records, such as the Name field repeat, and other fields do not necessarily repeat or repeat can be ignored.

1, for the first repetition, easier to solve, the use of

SELECT DISTINCT * FROM tablename

You can get a result set without duplicate records. If the table needs to delete duplicate records (1 of duplicate records are retained), you can delete them in the following ways

SELECT DISTINCT * into #Tmp tablename

DROP TABLE TableName

SELECT * INTO TableName from #Tmp

drop table #Tmp

This duplication occurs because the table is poorly designed and can be resolved by adding a unique index column.

2. This type of repetition usually requires the first record in the duplicate record to be retained, as follows, assuming that there are duplicate fields name,address, requiring the unique result set of the two field

Select Identity (int,1,1) as Autoid, * into #Tmp from TableName

Select min (autoid) as autoid into #Tmp2 from #Tmp Group by name,autoid

SELECT * from #Tmp where autoid in (select Autoid from #tmp2)

The last select gets the name,address result set (but one more autoid field, which can be written in the SELECT clause to omit this column)

Query duplication

SELECT * FROM tablename where ID in (

Select ID from tablename

GROUP BY ID

Having count (ID) > 1)

Excerpted from Baidu Library