Flagging First Duplicate Entries in Oracle SQL using Row Numbers or CTEs

Using Row Numbers to Flag First Duplicate Entries in Oracle SQL

As a beginner in SQL Oracle, working with large datasets can be overwhelming. In this article, we’ll explore how to use the row_number function to flag first duplicate entries in an Oracle SQL query.

Understanding the Problem

We have a table named CATS with four columns: country, hair, color, and firstItemFound. The task is to update the firstItemFound column to 'true' for each new tuple that doesn’t already have a corresponding entry in the firstItemFound column.

To achieve this, we’ll consider two approaches:

Using a temporary view to partition by columns and assign row numbers.
Creating Common Table Expressions (CTEs) to process rows incrementally.

Both methods will help us update the firstItemFound column while handling duplicate entries efficiently.

Approach 1: Using Row Numbers

We can use the row_number function to assign a unique number to each row within partitions of the same group. In this case, we’ll partition by country, hair, and color.

Here’s an example query:

SELECT *, 
       CASE WHEN rn = 1 THEN 'true' ELSE 'false' END AS firstItemFound 
FROM (
  SELECT country,
         hair,
         color,
         row_number() OVER (PARTITION BY country, hair, color ORDER BY country) AS rn
  FROM CATS
)

This query works as follows:

row_number() assigns a unique number to each row within partitions of the same group (country, hair, and color).
The outer query uses a CASE statement to check if the row number is equal to 1 (rn = 1). If it is, we assign 'true' to the firstItemFound column. Otherwise, we assign 'false'.

Approach 2: Using Common Table Expressions (CTEs)

Alternatively, we can create two CTEs: one for the initial data and another for processing rows incrementally.

Here’s an example query:

WITH 
  -- Initial data
  t AS (
    SELECT country,
           hair,
           color,
           row_number() OVER (PARTITION BY country, hair, color ORDER BY country) AS rn
    FROM CATS
  ),
  
  -- Process rows incrementally
  t2 AS (
    SELECT country,
           hair,
           color,
           rn,
           CASE WHEN rn = 1 THEN 'true' ELSE 'false' END AS firstItemFound
    FROM t
  )
SELECT * FROM t2;

This query works as follows:

The t CTE contains the initial data, with row numbers assigned using the same logic as before.
The t2 CTE processes rows incrementally by adding a new column (firstItemFound) that assigns 'true' for rows with an rn = 1 and 'false' otherwise.

Example Use Cases

Let’s consider the following example data:

NAME	COUNTRY	HAIR	COLOR
France	Shorthair	Red	Red
Brazil	Longhair	Yellow	Yellow
France	Shorthair	Red	Red
France	Longhair	Brown	Brown
France	Longhair	Black	Black
Brazil	Longhair	Yellow	Yellow
Brazil	Longhair	Black	Black
Brazil	Longhair	Brown	Brown
Brazil	Longhair	Yellow	Yellow

The desired output would be:

COUNTRY	HAIR	COLOR	FIRSTITEMFOUND
Brazil	Longhair	Yellow	true
France	Shorthair	Red	true
France	Shorthair	Red	false
France	Longhair	Brown	true
France	Longhair	Black	true
Brazil	Longhair	Yellow	false
Brazil	Longhair	Black	true
Brazil	Longhair	Brown	true
Brazil	Longhair	Yellow	false

By using row_number() or CTEs, we can efficiently update the firstItemFound column while handling duplicate entries.

Conclusion

In this article, we’ve explored two approaches to flag first duplicate entries in an Oracle SQL query:

Using row numbers to partition by columns and assign unique numbers.
Creating Common Table Expressions (CTEs) to process rows incrementally.

Both methods can be used depending on the specific requirements of your data.

Last modified on 2025-03-28