MySQL: making a column unique?

Question

I have a table that is in production. I realize that some of the columns should be unique. Is it safe to go into phpMyAdmin and change those columns to make it unique?

ALTER TABLE  `foo` ADD UNIQUE ( `bar` )

Answer 1

Follow the below steps to apply unique column value from phpmyadmin panel:

Go to the table structure. Click on the unique keyword as like below -

Click on the ok from confirmation box -

Unique value constraint for column will apply. Or you can run mysql query:

ALTER TABLE user ADD UNIQUE(email);

Answer 2

1. You do not have duplicates -> will apply the key without issues
2. You do have duplicates -> will give an error message, nothing happened to your data
3. All is unique, except several rows with NULL in them, unique constraint is still applied, as NULL is not checked when checking for unique values (you can have the entire table have a NULL value in a unique field without any error message).

One more thing, if you have a prod DB, you must also have a dev DB which you can test on without fear, right?