How to delete duplicate data in oracle

During data processing, data duplication problems are often encountered. In order to ensure the accuracy of the data and the clarity of the data warehouse, we often need to delete duplicate data in the database. This article will introduce how to delete duplicate data in Oracle database.

1. Judge data duplication

Before deleting duplicate data, you first need to determine which data is duplicated. We can use the SELECT statement and GROUP BY statement to query for duplicate data. For example, we have a table named "tablename" which contains a field named "columnname". You can use the following statement to find all duplicate data:

SELECT columnname, COUNT(*) as count
FROM tablename
GROUP BY columnname
HAVING COUNT(*) > 1;

This statement will return the number of all duplicate data and the value of the duplicate field. If we want to delete all duplicate data, we can use the following statement:

DELETE FROM tablename
WHERE columnname IN (SELECT columnname
FROM tablename
GROUP BY columnname
HAVING COUNT( *) > 1);

The "IN" keyword in this statement means to select duplicates in the "columnname" field and then delete these rows.

2. Use ROWID to remove duplicate data

The ROWID in Oracle database is a unique identifier in the database. We can use ROWID to remove duplicate data. The following is an example of using ROWID to delete duplicate data:

DELETE FROM tablename
WHERE ROWID NOT IN

(SELECT MIN(ROWID) 
 FROM tablename 
 GROUP BY columnname);

This statement will delete all duplicate rows except the first duplicate row. In the "SELECT" statement, we query for the smallest ROWID of the "columnname" field and compare it with other duplicate values ​​in the row ID. Only the first duplicate row's ROWID will not match any other ROWID and will remain in the database.

3. Use CTE to remove duplicate data

Using common table expressions (CTE) is also a way to remove duplicate data. The following is an example of using CTE to remove duplicate data:

WITH cte AS

(SELECT columnname, 
        ROW_NUMBER() OVER 
            (PARTITION BY columnname ORDER BY columnname) AS rn 
 FROM tablename)

DELETE FROM cte
WHERE rn > 1;

In this example, we Use the "SELECT" statement to name the CTE "cte". This statement uses the ROW_NUMBER() function to partition and sort the data based on the "columnname" field in ascending order (ORDER BY). After that, we delete all rows with "rn"rn" greater than 1.

4. Use the DISTINCT keyword to delete duplicate data

The last method deletes duplicate data in the Oracle database The method is to use the DISTINCT keyword. The following is an example of using the DISTINCT keyword to remove duplicate data:

DELETE FROM tablename
WHERE columnname NOT IN

(SELECT DISTINCT columnname 
 FROM tablename);

In this example, we use Nested SELECT statement. In the subquery, we use the DISTINCT keyword to find the unique "columnname" value and compare it with the "columnname" field. This statement will remove all duplicate data contained in the subquery results.

Summary

Deleting duplicate data is an important step when processing data. Oracle database provides a variety of methods to delete duplicate rows. We can use the SELECT statement and the GROUP BY statement to Determine which data is duplicated, and then use the DELETE statement to delete these data. In addition, we can also use the ROWID, CTE and DISTINCT keywords to delete duplicate data in the database. Using these techniques, we can ensure the accuracy of the data and data warehouse clarity.

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