Mysql 5.5.38 ERROR 1062 (23000): Duplicate entry '' for key 'anykey'
If you want to hunt down what key combinations are preventing the unique key from working, run this

SELECT add1, city, state,COUNT(1) rowcount
FROM mytable GROUP BY add1, city, state
HAVING COUNT(1) &gt; 1;


To see the actual rows, run this

SELECT A.* FROM mytable A INNER JOIN
(SELECT add1, city, state,COUNT(1) rowcount
FROM mytable GROUP BY add1, city, state
HAVING COUNT(1) &gt; 1) B USING (add1, city, state);


This will give you the opportunity to clean up the data

Alternatively, you may decide to make it a regular nonunique index.