Question

I have an already existing table with a field that should be unique but is not. I only know this because an entry was made into the table that had the same value as another, already existing, entry and this caused problems.

How do I make this field only accept unique values?

Answer 1

ALTER IGNORE TABLE mytbl ADD UNIQUE (columnName);

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For MySQL 5.7.4 or later:

ALTER TABLE mytbl ADD UNIQUE (columnName);

As of MySQL 5.7.4, the IGNORE clause for ALTER TABLE is removed and
its use produces an error.

So, make sure to remove duplicate entries first as IGNORE keyword is no longer supported.

Reference