Unique Constraint Handling Empty Values in MySQL

As a common database requirement, it's crucial to guarantee data integrity by enforcing unique constraints on specific fields. However, a unique constraint typically prohibits empty values, which pose a challenge when working with fields that may legitimately remain empty, such as product codes that rely on external providers.

In MySQL, it is possible to create a unique constraint that allows empty values. This feature, as described in the MySQL reference, can be implemented as follows:

CREATE UNIQUE INDEX `my_unique_index` ON `my_table` (`my_field`)

This index will ensure that all non-empty values in the my_field column remain unique. However, it will allow rows with empty values to be inserted.

It's important to note that the my_field column should not be defined as NOT NULL for this approach to work. If the column is set as NOT NULL, it will not allow empty values, thus defeating the purpose of the unique constraint that allows emptiness.