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

Question

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!

Answer 1

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.