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Learn how to effectively troubleshoot and resolve the `ORA-01427` error in Oracle SQL when using subqueries with case statements. This guide provides solutions for common pitfalls and best practices.
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Resolving the ORA-01427 Error: How to Fix Single-Row Subquery Returns More Than One Row in Oracle

Encountering the ORA-01427 error in Oracle SQL can be frustrating, especially when it arises during complex queries involving case statements and subqueries. This error indicates that a single-row subquery is attempting to return more than one row, which violates the expectations of the SQL query.

In this guide, we will break down the issues surrounding this error and provide clear solutions to help you resolve it effectively.

Understanding the Problem

The error message you receive, ORA-01427: single-row subquery returns more than one row, typically occurs in a context like this:

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Why This Error Happens:

Multiple Rows from RANK: The RANK analytic function can return multiple rows ranked 1st within each partition. If there are multiple identical values, the query returns more than one row.

Inadequate Filtering: The CASE statement expects that the inner subquery will return a single row. If the subquery is not adequately filtered based on unique identifiers, it may return multiple rows.

Solutions to Fix the Error

Now that we have identified the problem areas, here are a few effective strategies to resolve the ORA-01427 error.

1. Use ROW_NUMBER() instead of RANK()

Switching from RANK() to ROW_NUMBER() ensures that only one row is returned for each partition, as ROW_NUMBER() assigns a unique sequential integer to each row within a partition.

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2. Aggregate Results with LISTAGG()

If multiple entries need to be consolidated into a single output, consider using the LISTAGG() function to aggregate results into a single row.

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3. Ensure Proper Correlation with the Outer Query

In some cases, the subquery may need to reference values from the outer query to ensure unique results. This can be done by correlating the subquery to the outer query:

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Conclusion

The ORA-01427: single-row subquery returns more than one row error in Oracle SQL can stem from the use of analytic functions, improper filtering, or a lack of correlation in subqueries. By implementing strategies like switching to ROW_NUMBER(), using LISTAGG() for aggregation, and ensuring correlation with the outer query, you can eliminate this frustrating error and enhance your SQL's accuracy.

Next time you encounter this issue, refer back to these tips to resolve it swiftly and efficiently!