MySQL - Make an existing Field Unique

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?

Solution 1:

ALTER IGNORE TABLE mytbl ADD UNIQUE (columnName);

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

Solution 2:

Just write this query in your db phpmyadmin.

ALTER TABLE TableName ADD UNIQUE (FieldName)

Eg: ALTER TABLE user ADD UNIQUE (email)

Solution 3:

If you also want to name the constraint, use this:

ALTER TABLE myTable
  ADD CONSTRAINT constraintName 
    UNIQUE (columnName);

Solution 4:

CREATE UNIQUE INDEX foo ON table_name (field_name)

You have to remove duplicate values on that column before executes that sql. Any existing duplicate value on that column will lead you to mysql error 1062