#1062 - Duplicate entry '' for key 'unique_id' When Trying to add UNIQUE KEY (MySQL)

I've got an error on MySQL while trying to add a UNIQUE KEY. Here's what I'm trying to do. I've got a column called 'unique_id' which is VARCHAR(100). There are no indexes defined on the table. I'm getting this error:

#1062 - Duplicate entry '' for key 'unique_id' 

When I try to add a UNIQUE key. Here is a screenshot of how I'm setting it up in phpMyAdmin:

Here is the MySQL query that's generate by phpMyAdmin:

ALTER TABLE  `wind_archive` ADD  `unique_id` VARCHAR( 100 ) NOT NULL FIRST ,
ADD UNIQUE (
`unique_id`
)

I've had this problem in the past and never resolved it so I just rebuilt the table from scratch. Unfortunately in this case I cannot do that as there are many entries in the table already. Thanks for your help!

The error says it all:

Duplicate entry '' 

So run the following query:

SELECT unique_id,COUNT(unique_id)
FROM yourtblname
GROUP BY unique_id
HAVING COUNT(unique_id) >1

This query will also show you the problem

SELECT *
FROM yourtblname
WHERE unique_id=''

This will show you where there are values that have duplicates. You are trying to create a unique index on a field with duplicates. You will need to resolve the duplicate data first then add the index.

This is 3rd time i am looking for solution to this problem so for the reference I am posting the answer here.

Depending on the data we may use IGNORE keyword with Alter command. If IGNORE is specified, only the first row is used of rows with duplicates on a unique key, The other conflicting rows are deleted. Incorrect values are truncated to the closest matching acceptable value.

The IGNORE keyword extension to MySQL seems to have a bug in the InnoDB version on some version of MySQL.

You could always, convert to MyISAM, IGNORE-ADD the index and then convert back to InnoDB

ALTER TABLE table ENGINE MyISAM;
ALTER IGNORE TABLE table ADD UNIQUE INDEX dupidx (field);
ALTER TABLE table ENGINE InnoDB;

Note, if you have Foreign Key constraints this will not work, you will have to remove those first, and add them back later.