SQL Command for Delete only duplicate records and Select only duplicate records in a Table?

Certainly! Handling duplicate records is a common task in SQL. Here are the SQL commands to select and delete only duplicate records in a table.

Selecting Duplicate Records

To select only the duplicate records from a table, you can use the GROUP BY clause along with HAVING to find records that occur more than once:

SELECT column1, column2, COUNT(*)

FROM tablw_name

GROUP BY column1, column2

HAVING COUNT(*) > 1;

Deleting Duplicate Records

To delete duplicate records, you can use a CTE (Common Table Expression) along with a ROW_NUMBER() window function to identify and remove duplicates. Here's an example of how to do this:

WITH duplicates AS (

  SELECT 

    column1,

    column2,

    ROW_NUMBER() OVER (PARTITION BY column1, column2 ORDER BY column1) AS row_num

  FROM your_table

)

DELETE FROM duplicates

WHERE row_num > 1;

Example Breakdown

• Selecting Duplicates:

  • SELECT column1, column2, COUNT(*)

  • FROM your_table

  • GROUP BY column1, column2

  • HAVING COUNT(*) > 1

This selects records where the combination of column1 and column2 appears more than once.

• Deleting Duplicates:

  • WITH duplicates AS (SELECT ..., ROW_NUMBER() OVER (PARTITION BY column1, column2 ORDER BY column1) AS row_num FROM your_table)

  • DELETE FROM duplicates WHERE row_num > 1

This creates a temporary table (duplicates) where each duplicate row is assigned a unique row number, and then deletes all rows where the row number is greater than 1, effectively removing duplicates while keeping one instance.

If you want to handle duplicates without using partitions or window functions like ROW_NUMBER(), you can achieve it through a different approach. Here's how you can select and delete duplicate records using a temporary table approach.

Selecting Duplicate Records

To select duplicate records, you can use the GROUP BY clause along with HAVING:

SELECT column1, column2, COUNT(*)

FROM your_table

GROUP BY column1, column2

HAVING COUNT(*) > 1;

Deleting Duplicate Records Without Partition

To delete duplicates without using partition, follow these steps:

1. Create a Temporary Table:

CREATE TEMPORARY TABLE temp_table AS

SELECT MIN(rowid) as min_rowid

FROM your_table

GROUP BY column1, column2

HAVING COUNT(*) > 1;

2.Delete Duplicate Records:

DELETE FROM your_table

WHERE rowid NOT IN (SELECT min_rowid FROM temp_table);

Explanation

• Step 1: Create a temporary table to store the minimum row IDs of duplicate records. This helps identify the records to keep.

• Step 2: Delete the records from the original table where the row ID is not in the list of minimum row IDs from the temporary table. This removes the duplicates, keeping only one instance.

This approach ensures that duplicates are handled without using window functions or partitions.

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