Understanding the Task

Understanding the Task:

• Identify Duplicates: The goal is to pinpoint rows in a table that have identical values in specific columns, often referred to as "duplicate rows."
• Preserve the First: Among these duplicates, the task is to retain the first occurrence while removing the rest. This ensures that unique data is maintained in the table.

SQL Approach: To achieve this in SQL, we typically employ a combination of techniques involving a common table expression (CTE) and the DELETE statement. Here's a general outline:

1. Create a CTE:

2. Filter for Duplicates:

3. Delete Duplicates:

Example:

WITH DuplicateRows AS (
  SELECT *,
         ROW_NUMBER() OVER (PARTITION BY Column1, Column2 ORDER BY Column3) AS RowNumber
  FROM YourTable
)
DELETE FROM DuplicateRows
WHERE RowNumber > 1;

In this example:

• YourTable is the name of the table you want to process.
• Column1 and Column2 are the columns used to identify duplicates.
• Column3 is used to determine the order of rows within each duplicate group, ensuring the first row is preserved.

Key Points:

• The ordering criteria in the ROW_NUMBER() function is essential for determining which row is considered the "first."
• Ensure that the columns used for partitioning and ordering are appropriate for your specific use case.
• Test the query carefully before executing it, as deleting rows is irreversible.
• Consider creating a backup of the table before performing the deletion.

Code Example: Using a Common Table Expression (CTE)

This is a common approach in SQL Server:

WITH DuplicateRows AS (
  SELECT *,
         ROW_NUMBER() OVER (PARTITION BY Column1, Column2 ORDER BY Column3) AS RowNumber
  FROM YourTable
)
DELETE FROM DuplicateRows
WHERE RowNumber > 1;

Breakdown:

1. CTE Definition:

   • DuplicateRows: A temporary result set created to identify duplicates.
   • SELECT *: Selects all columns from the YourTable.
   • ROW_NUMBER(): Assigns a sequential number to each row within a partition.
     • PARTITION BY Column1, Column2: Groups rows based on the values in these columns.
     • ORDER BY Column3: Determines the order within each partition.

2. DELETE Statement:

Explanation:

• The CTE assigns a unique number to each row based on its values in Column1 and Column2.
• If multiple rows have the same values in Column1 and Column2, the ROW_NUMBER() function will assign consecutive numbers to them.
• The DELETE statement then targets rows with a RowNumber greater than 1, which are the duplicates.

Code Example: Using a Temporary Table

Another approach involves creating a temporary table to store unique rows:

CREATE TABLE #UniqueRows (
  -- List of columns from YourTable
);

INSERT INTO #UniqueRows
SELECT DISTINCT *
FROM YourTable;

DELETE FROM YourTable
WHERE NOT EXISTS (
  SELECT 1
  FROM #UniqueRows
  WHERE YourTable.Column1 = #UniqueRows.Column1
    AND YourTable.Column2 = #UniqueRows.Column2
);

DROP TABLE #UniqueRows;

1. Create Temporary Table:
2. Insert Unique Rows:
3. Delete Duplicates:
4. Drop Temporary Table:
   • Removes the temporary table.

Alternative Methods for Deleting Duplicate Rows in SQL Server

While the CTE and temporary table methods are commonly used, here are some additional approaches:

Using a Window Function and Common Table Expression (CTE):

WITH DuplicateRows AS (
  SELECT *,
         ROW_NUMBER() OVER (PARTITION BY Column1, Column2 ORDER BY Column3) AS RowNumber
  FROM YourTable
)
DELETE FROM YourTable
WHERE EXISTS (
  SELECT 1
  FROM DuplicateRows
  WHERE YourTable.PrimaryKey = DuplicateRows.PrimaryKey
    AND DuplicateRows.RowNumber > 1
);

This method directly deletes rows from the original table using a DELETE statement that references the CTE.

Using a Subquery:

DELETE FROM YourTable
WHERE EXISTS (
  SELECT 1
  FROM YourTable AS T2
  WHERE YourTable.PrimaryKey <> T2.PrimaryKey
    AND YourTable.Column1 = T2.Column1
    AND YourTable.Column2 = T2.Column2
);

This method uses a subquery to identify duplicate rows based on matching values in specific columns.

Using a Common Table Expression and DELETE with OUTPUT:

WITH DuplicateRows AS (
  SELECT *,
         ROW_NUMBER() OVER (PARTITION BY Column1, Column2 ORDER BY Column3) AS RowNumber
  FROM YourTable
)
DELETE FROM DuplicateRows
OUTPUT DELETED.*
WHERE RowNumber > 1;

This method not only deletes the duplicate rows but also outputs the deleted rows, which can be useful for logging or further analysis.

Using a WHILE Loop and Cursors:

DECLARE @PrimaryKey INT;
DECLARE @RowNumber INT;

DECLARE CursorToDeleteDuplicates CURSOR FOR
SELECT PrimaryKey, ROW_NUMBER() OVER (PARTITION BY Column1, Column2 ORDER BY Column3) AS RowNumber
FROM YourTable;

OPEN CursorToDeleteDuplicates;

FETCH NEXT FROM CursorToDeleteDuplicates INTO @PrimaryKey, @RowNumber;

WHILE @@FETCH_STATUS = 0
BEGIN
  IF @RowNumber > 1
  BEGIN
    DELETE FROM YourTable
    WHERE PrimaryKey = @PrimaryKey;
  END

  FETCH NEXT FROM CursorToDeleteDuplicates INTO @PrimaryKey, @RowNumber;
END

CLOSE CursorToDeleteDuplicates;
DEALLOCATE CURSOR CursorToDeleteDuplicates;

This method uses a cursor to iterate over the rows and delete duplicates based on a specific condition. While this approach is less efficient than the other methods, it can be useful in certain scenarios.

Choosing the Best Method:

The most suitable method depends on factors such as:

• Performance: For large datasets, CTE-based methods are often more efficient.
• Functionality: If you need to log or analyze deleted rows, the OUTPUT clause can be helpful.
• Complexity: The subquery method is generally simpler to understand.
• Specific requirements: If you have specific constraints or preferences, you might choose a different method.