Скачать или смотреть Resolving the ORA-00001: unique constraint (constraint_name) violated Error in Oracle SQL

Are you facing the `ORA-00001: unique constraint (constraint_name) violated` error in Oracle SQL? This guide explores the common reasons behind this error and provides practical solutions to avoid it.
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Understanding the ORA-00001 Error

The ORA-00001: unique constraint (constraint_name) violated error is a common issue when working with databases, particularly in Oracle SQL. This error occurs when there is an attempt to insert a duplicate value into a column that has a unique constraint. In simpler terms, it signifies that you are trying to add an entry that already exists in the database, particularly when it comes to primary keys or unique keys.

The Scenario

You might find yourself encountering this error even when you have a NOT EXISTS check in place before executing the insert statement. Such was the case for a user who attempted to insert records into an EMPLOYEE table, protected by a primary key on the EMP_ID column. Despite their precautions, the error still occurred.

The Table Structure

Here’s a look at the SQL structure of the table involved:

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The Problematic Insert Statement

The user executed the following insert statement:

[[See Video to Reveal this Text or Code Snippet]]

Despite including a NOT EXISTS check, they still faced the unique constraint violated issue.

Analyzing the Cause

Concurrent Transactions

The key factor here is parallel transactions. When multiple sessions attempt to insert the same EMP_ID almost simultaneously, they may both pass the NOT EXISTS check before either one successfully inserts the record into the database. This results in one session failing due to a unique constraint violation.

Example Reproduction of the Error

Here’s how the issue can be reproduced with two sessions attempting to insert the same employee ID:

Session A attempts to insert an employee with EMP_ID = 123.

[[See Video to Reveal this Text or Code Snippet]]

Session B runs the same insert with EMP_ID = 123.

[[See Video to Reveal this Text or Code Snippet]]

Session A commits the transaction.

[[See Video to Reveal this Text or Code Snippet]]

As a result, Session B fails with the constraint violation as the insert attempt sees that the EMP_ID is now occupied.

Solutions to Avoid the Error

To prevent the ORA-00001 error, consider the following strategies:

1. Use Application-Level Locking

Implement application-level locking mechanisms to ensure that if one session is inserting a record, other sessions are temporarily blocked from doing so with the same key.

2. Retry Logic

Consider adding retry logic around your insert statements. If an insertion fails due to a unique constraint violation, wait for a brief period before attempting the insert again.

3. Adjust Isolation Levels

Review and adjust the transaction isolation level if necessary. A higher isolation level may reduce concurrency-related issues but at the cost of performance.

4. Use MERGE Statement

Instead of relying solely on INSERT, consider using an INSERT ... ON DUPLICATE KEY UPDATE or a MERGE statement which can handle duplicates more gracefully.



By understanding the root cause of the ORA-00001 error and implementing these strategies, you can ensure smoother operations with your Oracle SQL databases, minimizing the potential for frustrating unique constraint violations.