Update Duplicate Data Based on the First One

Introduction

In this blog post, we’ll explore a common database problem: updating duplicate data based on the first occurrence. The problem presented in the question involves updating VLI_OMDF_ID values in the VL_Liegenschaften table if there are duplicates with the same B.OTO_ID, but one of them has a NULL value.

The solution involves using a self-join to compare duplicate data and update the VLI_OMDF_ID values accordingly. We’ll break down this process into smaller steps, explaining each part in detail.

Problem Statement

Suppose we have two tables: VL_Liegenschaften and VL_Wohneinheiten. The VL_Liegenschaften table contains data with duplicate entries for the same B.OTO_ID, but one of them has a NULL value for VLI_OMDF_ID.

For example:

We want to update the VLI_OMDF_ID values in the VL_Liegenschaften table if there are duplicates with the same B.OTO_ID, but one of them has a NULL value.

SQL Solution

To solve this problem, we can use a self-join to compare duplicate data and update the VLI_OMDF_ID values accordingly.

Here’s an example query that achieves this:

MERGE INTO VL_Liegenschaften t
USING (
  SELECT VLW_OTOID, VLI_OMDF_ID
  FROM VL_Wohneinheiten
  GROUP BY VLW_OTOID
) t2
ON (t.VLW_OTOID = t2.VLW_OTOID AND t.VLI_OMDF_ID IS NULL)
WHEN MATCHED THEN 
    UPDATE SET t.VLI_OMDF_ID = t2.VLI_OMDF_ID;

This query uses a MERGE statement to compare duplicate data in the VL_Liegenschaften table with the same data in the VL_Wohneinheiten table, but grouped by VLW_OTOID. The ON clause specifies the join condition, which is t.VLW_OTOID = t2.VLW_OTOID AND t.VLI_OMDF_ID IS NULL. This means that we only want to update rows in VL_Liegenschaften where the corresponding row in VL_Wohneinheiten has a non-NULL value for VLI_OMDF_ID.

How it Works

Let’s break down the query step by step:

1. The subquery selects all rows from VL_Wohneinheiten, groups them by VLW_OTOID, and returns the corresponding VLI_OMDF_ID values.
2. The MERGE statement joins this result with the VL_Liegenschaften table on the condition specified in the ON clause.
3. When a match is found, the WHEN MATCHED clause updates the VLI_OMDF_ID value in the corresponding row in VL_Liegenschaften.

Error Messages

The question mentions an error message related to the MERGE statement: “The MERGE statement tried to update or delete the same row more than once. This happens when a target line matches more than one source line.”

This is because the original query had a problem with optimizing the join condition, which caused the MERGE statement to fail.

Conclusion

In this blog post, we’ve explored how to update duplicate data based on the first occurrence using a self-join. We provided an example query that achieves this and explained each part of the query in detail. By following these steps, you should be able to solve similar problems involving duplicate data updates.

Additional Considerations

Optimizing the Query

While the query we provided works correctly, it may not be the most efficient way to solve this problem. To optimize the query, consider using indexing on the columns used in the join condition and update clause.

Additionally, if you need to update a large number of rows, consider using a transaction to ensure that either all or no rows are updated, depending on the outcome of the query.

Alternative Solutions

While self-joins can be an effective solution for this problem, they may not always be the best approach. Consider alternative solutions, such as:

• Using window functions (e.g., ROW_NUMBER()) to identify duplicate rows and update them accordingly.
• Creating a temporary table to store the updated data and then updating the original table.

By exploring different approaches, you can find the most efficient solution for your specific use case.

Last modified on 2023-08-13