👀 Views: 4 💬 Answers: 1 📅 Created: 2025-06-11

mysql database unique-constraint sql

I'm dealing with I'm upgrading from an older version and I'm sure I'm missing something obvious here, but I've searched everywhere and can't find a clear answer....... I'm working with an unexpected scenario while using the `INSERT ... ON DUPLICATE KEY UPDATE` syntax with MySQL 8.0. I have a table called `user_data` with a unique index on the `email` column, which is intended to prevent duplicate email addresses. The relevant part of the table structure is as follows: ```sql CREATE TABLE user_data ( id INT AUTO_INCREMENT PRIMARY KEY, email VARCHAR(255) NOT NULL, name VARCHAR(100), UNIQUE INDEX (email) ); ``` I am attempting to insert a new user using the following query: ```sql INSERT INTO user_data (email, name) VALUES ('test@example.com', 'Test User') ON DUPLICATE KEY UPDATE name = VALUES(name); ``` The first execution of this query works as expected, and the user is inserted. However, when I try inserting the same email again, I see the following behavior: 1. The existing entry is updated with the new name as expected, but also, I noticed that an additional duplicate entry is being created from elsewhere in the application. 2. The duplicate entry does not trigger the unique constraint, which seems odd, since I would expect the operation to unexpected result or at least not to create a new row. I've tried to ensure that no other part of my application is inadvertently inserting duplicate records, and I have run a check using: ```sql SELECT * FROM user_data WHERE email = 'test@example.com'; ``` This confirms that there is only one entry after the insert/update but the unique index should have prevented any duplicate inserts in the first place. Is there any reason why the unique constraint on the `email` column isn't behaving as intended? I've also checked if there's any trigger defined on this table that might be conflicting, but there are none. Any insights into this behavior would be greatly appreciated! This is part of a larger service I'm building. Thanks for taking the time to read this!