How Can I Find Duplicate Values in an Oracle Table Using SQL?

Efficiently Identifying Duplicate Data in Oracle Tables with SQL

Maintaining data integrity in Oracle databases requires effective methods for detecting and managing duplicate values. This article demonstrates a straightforward SQL approach to pinpoint duplicate entries within a specific table column.

The core technique involves using SQL's aggregation capabilities to count occurrences of each value in the target column, then filtering the results to show only those values appearing more than once. This is achieved using GROUP BY and HAVING clauses.

Here's the SQL statement:

<code class="language-sql">SELECT column_name, COUNT(*) AS duplicate_count
FROM table_name
GROUP BY column_name
HAVING COUNT(*) > 1;</code>

Understanding the SQL Statement:

1. *`SELECT column_name, COUNT() AS duplicate_count**: This selects the column containing potential duplicates and counts the occurrences of each unique value, aliasing the count asduplicate_count`.
2. FROM table_name: Specifies the table to be queried.
3. GROUP BY column_name: Groups the rows based on the unique values in the specified column.
4. *`HAVING COUNT() > 1`**: Filters the grouped results, displaying only those groups (unique values) with a count greater than one, thus highlighting duplicates.

Practical Example:

Let's say we have a table named EMPLOYEES with a column EMPLOYEE_ID. To find duplicate EMPLOYEE_ID values, we'd use the following query:

<code class="language-sql">SELECT EMPLOYEE_ID, COUNT(*) AS duplicate_count
FROM EMPLOYEES
GROUP BY EMPLOYEE_ID
HAVING COUNT(*) > 1;</code>

This query will return a list of duplicate EMPLOYEE_ID values and the number of times each appears, facilitating efficient identification and resolution of duplicate records.

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