title: Cannot delete primary key constraint in SQL Server
body: <p>When I open designer for a table in SSMS, first column Id has a primary key assigned to it (<code>IDENTITY</code>), and key icon is shown next to it. In the Indexes / Keys window I can see a <code>PK_dbo.Lines</code> entry with Name <code>[PK_dbo.Lines]</code> and type <code>Primary Key</code>.</p>
  
  <pre><code>ALTER TABLE dbo.Lines DROP CONSTRAINT [PK_dbo.Lines]
  </code></pre>
  
  <p>returns</p>
  
  <blockquote>
    <p>Msg 3728, Level 16, State 1, Line 33<br>
    'PK_dbo.Lines' is not a constraint.</p>
  </blockquote>
  
  <p>If I execute</p>
  
  <pre><code>SELECT * 
  FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE
  </code></pre>
  
  <p>I can see primary keys for all tables, except for table <code>Lines</code>...</p>
  
  <p>This query also does not return any results:</p>
  
  <pre><code>select object_name (parent_obj) ObjectName, name
  from sysobjects
  where xtype = 'PK'
    and parent_obj = (object_id('[dbo].[Lines]'))
  </code></pre>
  
  <p>Any idea what is going on here?</p>