How to Make an Existing Field Unique in MySQL?

Making Existing Field Unique in MySQL

Existing tables in MySQL can face the issue of having non-unique fields that can lead to data integrity problems. To rectify this, it becomes necessary to enforce uniqueness on such fields.

Solution

To make an existing field unique, the following syntax can be used:

ALTER TABLE <table_name> ADD UNIQUE (<field_name>);

MySQL Version Considerations

Note that for MySQL 5.7.4 or later, the IGNORE clause for ALTER TABLE is removed, so the syntax becomes:

ALTER TABLE <table_name> ADD UNIQUE (<field_name>);

Important: For versions 5.7.4 and above, it's essential to remove duplicate entries before adding the unique constraint, as the IGNORE keyword is no longer supported.

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