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Background

A person table has two fields of ID and name. ID is the only one and cannot be duplicate. If Id is the same, it is considered as duplicate record.

• Removing duplicate rows from Oracle tables with SQL can be very tricky, and there are several techniques for identifying and removing duplicate rows from tables: Subquery to identify duplicate rows Use RANK to find and remove duplicate table rows.
• Want to write an plsql code to update a single record using rowid where all the rows are duplicated could someone help me in this case. Prodserviceid Location feature ord- - - -PP23456 Chennai Fast2 B77 PP23456 Chennai Fast2 B77 Now I need to update one of the record to madurai.
• You can use the rowid pseudocolumn like this: DELETE FROM yourtable. WHERE rowid not in (SELECT MIN(rowid) FROM yourtable. GROUP BY column1, column2, column3); Here column1, column2, and column3 make up the identifying key for each record. You can list all your columns.
• Please refer to 'ROWID Datatype' and 'UROWID Datatype' for more information. Rowid values have several important uses: They are the fastest way to access a single row. They can show you how the rows in a table are stored. They are unique identifiers for rows in a table. You should not use ROWID as the primary key of a table.

Two, solve

Duplicate records can create problems sometimes when displaying reports or performing a Multiple Insert update. Finding duplicate records in a database needs further investigation. In some cases, duplicate records are positive, but it all depends on the data and the database design as well.

select id from group by id having count(*) > 1

Group and count by ID. if the number of a group with an ID number exceeds 1, it will be considered as duplicate.

How to query duplicate data

PS: change the > to = to find no duplicate data.

Oracle delete SQL for duplicate data (delete all):

Basic structure of deleting duplicate data:

To delete the duplicate data, use the following statement to delete it

Note to the above SQL: the statement is very simple, which is to delete the queried data. However, the efficiency of this deletion is very low. For large data volume, the database may be suspended.

It is recommended to insert the duplicate data found in the query into a temporary table first, and then delete it, so that there is no need to query again when deleting. As follows:

Create table temporary table as (select field 1, field 2, count (*) from table name group by field 1, field 2 having count (*) > 1)

The above sentence is to create a temporary table and insert the queried data into it.

You can delete as follows:

Oracle delete duplicate SQL (leave a record):

In Oracle, there is a hidden automatic ROWID, which gives each record a unique ROWID. If we want to keep the latest record, we can use this field to keep the record with the largest ROWID in the duplicate data.

Use ROWID to query duplicate data:

The SQL in brackets queries the record with the largest ROWID, and the other duplicate data except for the largest ROWID is found outside.

Therefore, if we want to delete duplicate data and keep only the latest data, we can write as follows:

Delete duplicate data (leave one of the largest ROWID)

Delete duplicate data (leave one of the minimum ROWID)

Of course, the execution efficiency of the above statement is very low. You can consider creating a temporary table. You need to judge whether duplicate fields and rowids are inserted into the temporary table, and then compare them when deleting them.

For deletion of complete duplicate records

For the case where two rows of records in the table are exactly the same, you can use the following statement to obtain the records after removing duplicate data:

You can put the query records in the temporary table, delete the original table records, and finally export the data of the temporary table back to the original table. As follows:

If you want to delete the duplicate data of a table, you can first create a temporary table, import the data after removing the duplicate data into the temporary table, and then import the data into the formal table from the temporary table, as follows:

MySQL query and deletion of duplicate records
How to query and delete duplicate records (example demonstration)

1. Redundant duplicate records in the lookup table are determined by a single field (peopleid)

2. Delete redundant duplicate records in the table. Duplicate records are judged by a single field (tableid). Only the records with the lowest ROWID are left.

3. Redundant duplicate records (multiple fields) in lookup table

4. Delete redundant duplicate records (multiple fields) in the table, leaving only the records with the lowest ROWID

5. Redundant duplicate records (multiple fields) in the lookup table, excluding the record with the lowest ROWID

6. Query out the same SQL for a column in a table:

For example

There is a field “name” in table a, and the value of “name” may be the same between different records,

Now it is necessary to find out the duplicate items of “name” value among the records in the table;

If the gender is the same, it is as follows:

Oracle query whether the data in the fields in the table is duplicate

To query a single field:

Oracle queries duplicate data and deletes, keeping only one record
1. Redundant duplicate records in the lookup table are determined by a single field (ID)

2. Delete redundant duplicate records in the table. Duplicate records are judged by a single field (ID). Only the records with the lowest ROWID are left

3. Redundant duplicate records (multiple fields) in lookup table

4. Delete redundant duplicate records (multiple fields) in the table, leaving only the records with the lowest ROWID

5. Redundant duplicate records (multiple fields) in the lookup table, excluding the record with the lowest ROWID

For example, there is a personnel table (Table Name: peosons)
If you want to query the same records of name, ID number and address

Oracle Sql Find Duplicate Rows

The above effects can be achieved.
Several SQL statements for deleting duplicate records
1. Using ROWID method
2. Group by method
3. Using distinct method
1. Using ROWID method
According to the ROWID attribute of Oracle, judge whether there is duplication. The statement is as follows:
Data check:

Delete data:

2. Group by method
Data check:
select count（num）， max（name） from student
–List the number of duplicate records and his name attribute
group by num
Having count (Num) > 1 — find out the repetition of num column in the table after grouping by num, that is, the occurrence times are more than once
Delete data:

In this way, all the duplicates will be deleted.
3. Use distinct method – useful for small tables

A complete set of methods for querying and deleting duplicate records
1. Redundant duplicate records in the lookup table are determined by a single field (peopleid)

2. Delete redundant duplicate records in the table. Duplicate records are judged by a single field (peopleid). Only the records with the lowest ROWID are left

3. Redundant duplicate records (multiple fields) in lookup table

4. Delete redundant duplicate records (multiple fields) in the table, leaving only the records with the lowest ROWID

5. Redundant duplicate records (multiple fields) in the lookup table, excluding the record with the lowest ROWID

(two)
For example
There is a field “name” in table a,
And the “name” value may be the same between different records,
Now it is necessary to find out the duplicate items of “name” value among the records in the table;

If the gender is the same, it is as follows:

(three)
Method 1

Method two
“Duplicate record” has two kinds of duplicate records: one is a completely duplicate record, that is, a duplicate record of all fields; the other is a duplicate record of some key fields,
For example, the name field is duplicate, while other fields may or may not be duplicate.
1. For the first kind of repetition, it’s easier to solve, using
Select distinct * from tablename to get the result set without duplicate records.
If the table needs to delete duplicate records (keep 1 duplicate record), you can delete them as follows

The reason for this kind of duplication is due to the poor design of the table. It can be solved by adding a unique index column.
2. This kind of repetition problem usually requires to keep the first record in the duplicate record. The operation method is as follows
Assume that there are duplicate fields named name and address, and the unique result set of these two fields is required

The last select gets the result set with no duplicate name and address (but there is one more AutoID field, which can be written in the select clause when actually writing)
（4） Duplicate query

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• Selected Reading

Find Duplicates In Oracle Table

Problem Statement:
You want to find and remove duplicates from a table in Oracle.

Solution: We can use Oracle’s internal ROWID value for uniquely identifying rows in a table. The sample syntax to acheive this would like below.

To demonstrate the usage, we will begin by creating sample data.

Example

Looking at the data we just created.

Example

So, we have inserted 3 duplciates which we wanted to remove. before we go on and write a Delete statement, let us understand the inner query with ROWID.

Example

I had intentionally added the columns player_rank and player_name to this innermost subquery to make the logic understandable. Ideally, innermost subquery could be written without them to the same effect. If we execute just this innermost query offcourse with the extra columns selected for clarity, we see these results.

The SQL returns the rowid for all the rows in the table. The ROW_NUMBER() function then works over sets of id and player_name driven by the PARTITION BY instruction. This means that for every unique player_rank and player_name, ROW_NUMBER will start a running count of rows we have aliased as rnk. When a new player_rank and player_name combination is observed, the rnk counter resets to 1.

Now we can apply the DELETE operator to remove the duplicate values as below.

SQL: Remove duplicates

Oracle Find Duplicate Rows

Example

Output