MySQL - Make an existing Field Unique

Just write this query in your db phpmyadmin.

ALTER TABLE TableName ADD UNIQUE (FieldName)

Eg: ALTER TABLE user ADD UNIQUE (email)

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

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

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

ALTER TABLE myTable
  ADD CONSTRAINT constraintName 
    UNIQUE (columnName);