title: MySQL need to make column NOT UNIQUE. Error Can't DROP 'ColumnName'; check that column/key exists
body: <p>ColumnName is Unique (<code>UNIQUE KEY ColumnName</code>).</p>
  
  <p>I just want to make column not unique (must be very simple, but can not understand how). </p>
  
  <p>If in phpMyAdmin check at column name and at bottom click on Unique icon, get <code>#1062 - Duplicate entry '' for key 'RegistrationNumber'</code>. OK, see it is because, clicking on icon it <code>ADD UNIQUE</code>. </p>
  
  <p>There is Unique icon in Structure within row. But the icon is not click-able.</p>
  
  <p>As in phpMyAdmin did not found how to do it, trying with query.</p>
  
  <p>Based on advices tried <code>ALTER TABLE TableName DROP INDEX ColumnName</code>. </p>
  
  <p>Get <code>1091 Can't DROP 'ColumnName'; check that column/key exists</code></p>
  
  <p>Here <a href="https://stackoverflow.com/a/4414694/2465936">https://stackoverflow.com/a/4414694/2465936</a> found <code>This error means that you are trying to delete a key which is being used by another table.</code> Possibly the ColumnName is used by another table.</p>
  
  <p>Please advice what need to do to make column not unique.</p>
  
  <p>With <code>SHOW CREATE TABLE</code> get </p>
  
  <pre><code>Array
  (
  [0] =&gt; Array
      (
          [Table] =&gt; 18_6_TransactionPartners
          [Create Table] =&gt; 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
      )
  
   )
  </code></pre>
  
  <p><strong>Update</strong></p>
  
  <p>Based on @Bart Friederichs advice tried <code>ALTER TABLE 18_6_TransactionPartners DROP INDEX Number</code> and changed column <code>RegistrationNumber</code> not not unique. Do not understand why (possibly had some mess with unique keys). In any case can change to not unique.</p>