How to Add a Unique Constraint to an Existing MySQL Field?

Creating Unique Constraints on Existing MySQL Fields

An existing table may have a field that should be unique but is not. This can lead to data inconsistencies and errors. This guide demonstrates how to make an existing field unique to ensure data integrity.

Solution

To make a field unique in MySQL:

For MySQL Versions Prior to 5.7.4:

1. Use the ALTER IGNORE TABLE statement to add the UNIQUE constraint:

   ALTER IGNORE TABLE mytbl ADD UNIQUE (columnName);

For MySQL Versions 5.7.4 and Later:

1. Use the ALTER TABLE statement to add the UNIQUE constraint:

   ALTER TABLE mytbl ADD UNIQUE (columnName);

Note: Prior to MySQL 5.7.4, the IGNORE clause in ALTER TABLE ignored any duplicate values during constraint creation. However, as of MySQL 5.7.4, this clause is removed, and duplicate values must be removed before creating the UNIQUE constraint.

Reference

• [ALTER TABLE documentation](https://dev.mysql.com/doc/refman/8.0/en/alter-table.html)

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