Скачать или смотреть How to Avoid Cannot add or update a child row: a foreign key constraint fails in MySQL Queries

If you’re encountering the `Cannot add or update a child row: a foreign key constraint fails` error in MySQL when using REPLACE or INSERT queries, this guide explains how to effectively navigate around these constraints.
---
This video is based on the question https://stackoverflow.com/q/74300673/ asked by the user 'zosmex' ( https://stackoverflow.com/u/11130036/ ) and on the answer https://stackoverflow.com/a/74300727/ provided by the user 'Being_shawn' ( https://stackoverflow.com/u/20399922/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: How to avoid "Cannot add or update a child row: a foreign key constraint fails" while running "REPLACE INTO" query in MySQL?

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Avoiding Foreign Key Constraint Errors in MySQL

Introduction

If you’ve ever worked with MySQL, you know that foreign key constraints are important for maintaining data integrity. However, these constraints can sometimes create hurdles when you're trying to run queries like REPLACE INTO or INSERT ... ON DUPLICATE KEY UPDATE. One common error that arises in this situation is the "Cannot add or update a child row: a foreign key constraint fails" error. In this guide, we'll delve into why this error happens and, more importantly, how to resolve it.

Understanding the Error

When you see an error like this, it indicates that the operation you’re trying to perform violates referential integrity. In simpler terms, the query is trying to add or update a row in a target table that references a non-existing row in a parent table. Here’s what that looks like in your case:

Query Example:

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

Error Message:

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

The error is saying that your target_table has a foreign key constraint that must point to an existing project_id in the project table before you can proceed with your operation.

Solutions to Bypass the Constraint Error

Temporary Disabling of Foreign Key Checks

The most straightforward solution to this problem is to temporarily disable foreign key checks in MySQL. By doing this, you can run your INSERT or REPLACE INTO queries without being blocked by the constraints. Here are the steps you need to follow:

Disable Foreign Key Checks: Start by executing the following command:

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

Perform Your Database Operation: Run your REPLACE INTO or desired INSERT statement:

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

Re-enable Foreign Key Checks: Make sure to turn the foreign key checks back on once you're finished by executing:

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

Notes and Precautions

Data Integrity: Disabling foreign key checks can lead to data integrity issues. Ensure that the data you're inserting or replacing does not violate the referential integrity when foreign checks are re-enabled.

Testing: It’s recommended to perform this operation in a safe testing environment first, especially with sensitive or production data.

Backup: Always have a backup of your database before making changes that could affect data integrity.

Alternative Queries

If you wish to avoid disabling foreign key checks altogether, you might consider restructuring your queries or database schema. Here are a few alternatives:

Double-check Foreign Key Values: Ensure that all the values for foreign keys exist in the referenced tables before performing your query.

Break Down Queries: Instead of attempting a large bulk insert, try inserting records individually after confirming that they meet all constraints.

Conclusion

Encountering the "Cannot add or update a child row: a foreign key constraint fails" error can be frustrating, especially when you’re trying to manage and populate your database efficiently. By temporarily disabling foreign key checks, you can bypass the error and complete your REPLACE INTO or INSERT operations. However, remember to be vigilant about data integrity and always restore constraints once you're done.

By understanding the underlying cause and solutions for this error, you can streamline your database management tasks and keep your data clean and reliable.