sql - Selecting only one duplicate record

Sql - Selecting only one duplicate record

If you want to select only one duplicate record from a table in SQL, you can use a combination of the ROW_NUMBER() window function and a common table expression (CTE). This allows you to assign a row number to each row based on certain criteria and then filter for rows with a row number greater than 1 (indicating duplicates).

Assuming you have a table named your_table with a unique identifier column named id and you want to find one duplicate based on a specific column, you can do something like this:

WITH RankedDuplicates AS ( SELECT id, your_column, ROW_NUMBER() OVER (PARTITION BY your_column ORDER BY id) AS row_num FROM your_table ) SELECT id, your_column FROM RankedDuplicates WHERE row_num > 1;

Explanation:

  • The ROW_NUMBER() function assigns a unique number to each row within a partition.
  • The PARTITION BY your_column ensures that the numbering restarts for each unique value in your_column.
  • The ORDER BY id determines the order of rows within each partition.
  • The common table expression (CTE) RankedDuplicates is used to calculate the row numbers.
  • The final SELECT statement filters for rows where row_num is greater than 1, indicating duplicates.

Examples

  1. SQL select only one duplicate record using GROUP BY and HAVING

    • SQL Code:
      SELECT column1, column2, ... FROM your_table GROUP BY column1, column2, ... HAVING COUNT(*) > 1 LIMIT 1;
    • Description: Groups by columns and selects only one duplicate record using GROUP BY and HAVING COUNT(*) > 1.

  2. SQL select only one duplicate record using ROW_NUMBER()

    • SQL Code:
      SELECT column1, column2, ... FROM ( SELECT column1, column2, ..., ROW_NUMBER() OVER (PARTITION BY column1, column2 ORDER BY column1, column2) AS row_num FROM your_table ) AS ranked WHERE row_num = 2;
    • Description: Uses ROW_NUMBER() to assign row numbers within partitions and selects only one duplicate record with row_num = 2.

  3. SQL select only one duplicate record using INNER JOIN

    • SQL Code:
      SELECT t1.column1, t1.column2, ... FROM your_table t1 INNER JOIN ( SELECT column1, column2 FROM your_table GROUP BY column1, column2 HAVING COUNT(*) > 1 ) t2 ON t1.column1 = t2.column1 AND t1.column2 = t2.column2 LIMIT 1;
    • Description: Performs an inner join with a subquery to select only one duplicate record.

  4. SQL select only one duplicate record using EXISTS

    • SQL Code:
      SELECT column1, column2, ... FROM your_table t1 WHERE EXISTS ( SELECT 1 FROM your_table t2 WHERE t1.column1 = t2.column1 AND t1.column2 = t2.column2 AND t1.primary_key_column <> t2.primary_key_column ) LIMIT 1;
    • Description: Uses EXISTS to check for duplicates and selects only one duplicate record.

  5. SQL select only one duplicate record using DISTINCT and LIMIT

    • SQL Code:
      SELECT DISTINCT column1, column2, ... FROM your_table LIMIT 1;
    • Description: Uses DISTINCT and LIMIT to select only one duplicate record.

  6. SQL select only one duplicate record based on a condition

    • SQL Code:
      SELECT column1, column2, ... FROM your_table WHERE column1 = 'specific_value' AND column2 = 'specific_value' LIMIT 1;
    • Description: Filters rows based on specific values and selects only one duplicate record.

  7. SQL select only one duplicate record using OFFSET

    • SQL Code:
      SELECT column1, column2, ... FROM your_table GROUP BY column1, column2, ... HAVING COUNT(*) > 1 LIMIT 1 OFFSET 1;
    • Description: Groups by columns, selects only one duplicate record using GROUP BY, and skips the first with OFFSET 1.

  8. SQL select only one duplicate record with ORDER BY and LIMIT

    • SQL Code:
      SELECT column1, column2, ... FROM your_table ORDER BY column1, column2 LIMIT 1;
    • Description: Orders by columns and selects only one duplicate record using ORDER BY and LIMIT.

  9. SQL select only one duplicate record using RANK()

    • SQL Code:
      SELECT column1, column2, ... FROM ( SELECT column1, column2, ..., RANK() OVER (PARTITION BY column1, column2 ORDER BY column1, column2) AS rank_num FROM your_table ) AS ranked WHERE rank_num = 2;
    • Description: Uses RANK() to assign ranks within partitions and selects only one duplicate record with rank_num = 2.

  10. SQL select only one duplicate record using a self-join

    • SQL Code:
      SELECT t1.column1, t1.column2, ... FROM your_table t1 INNER JOIN your_table t2 ON t1.column1 = t2.column1 AND t1.column2 = t2.column2 WHERE t1.primary_key_column < t2.primary_key_column LIMIT 1;
    • Description: Performs a self-join to select only one duplicate record based on primary key comparison.

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