MySQL need to make column NOT UNIQUE. Error Can't DROP 'ColumnName'; check that column/key exists

Question

ColumnName is Unique (UNIQUE KEY ColumnName).

I just want to make column not unique (must be very simple, but can not understand how).

If in phpMyAdmin check at column name and at bottom click on Unique icon, get #1062 - Duplicate entry '' for key 'RegistrationNumber'. OK, see it is because, clicking on icon it ADD UNIQUE.

There is Unique icon in Structure within row. But the icon is not click-able.

As in phpMyAdmin did not found how to do it, trying with query.

Based on advices tried ALTER TABLE TableName DROP INDEX ColumnName.

Get 1091 Can't DROP 'ColumnName'; check that column/key exists

Here https://stackoverflow.com/a/4414694/2465936 found This error means that you are trying to delete a key which is being used by another table. Possibly the ColumnName is used by another table.

Please advice what need to do to make column not unique.

With SHOW CREATE TABLE get

Array
(
[0] => Array
    (
        [Table] => 18_6_TransactionPartners
        [Create Table] => CREATE TABLE `18_6_TransactionPartners` (
       `Number` int(11) NOT NULL AUTO_INCREMENT,
       `CompanyName` char(255) COLLATE utf8_unicode_ci NOT NULL,
       `RegistrationNumber` char(255) COLLATE utf8_unicode_ci NOT NULL,
        .......
        PRIMARY KEY (`Number`),
        UNIQUE KEY `Number_2` (`Number`),
        UNIQUE KEY `CompanyName` (`CompanyName`,`RegistrationNumber`),
        KEY `Number` (`Number`)
        ) ENGINE=InnoDB AUTO_INCREMENT=444 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci
    )

 )

Update

Based on @Bart Friederichs advice tried ALTER TABLE 18_6_TransactionPartners DROP INDEX Number and changed column RegistrationNumber not not unique. Do not understand why (possibly had some mess with unique keys). In any case can change to not unique.

Answer 1

Probably you have a named INDEX. By using SHOW CREATE TABLE tbl you can find out the names of the indices. Then drop them by name (e.g. some test table):

mysql> SHOW CREATE TABLE test;
CREATE TABLE `test` (
  `entry_id` int(11) DEFAULT NULL,
  `user_id` int(11) DEFAULT NULL,
  UNIQUE KEY `k` (`entry_id`)
)

To drop the index, use this:

ALTER TABLE test DROP INDEX k;

Your key name is RegistrationNumber (as is told by the error message):

ALTER TABLE TableName DROP INDEX RegistrationNumber;

Answer 2

If your column was defined unique using UNIQUE clause, then you can do something like this:

ALTER TABLE mytable DROP INDEX constraint_name

For dropping the index do this:-

ALTER TABLE mytable DROP INDEX index_name;