title: error occured : "Cannot call methods on nvarchar."
body: <p>I wrote a query to find answers related to a specific question, but I received this error when runing this code. </p>
  
  <blockquote>
    <p>Cannot call methods on nvarchar</p>
  </blockquote>
  
  <pre><code> select Posts.Id as Answer, ParentId as question, User.DisplayName as Answerer 
   FROM  Posts
  INNER JOIN Users    ON Posts.OwnerUserId = Users.Id
  WHERE 
  ParentId in (SELECT Posts.Id
  FROM  Posts
  INNER JOIN PostTags ON Posts.Id          = PostTags.PostId
  INNER JOIN Tags     ON PostTags.TagId    = Tags.id
  INNER JOIN Users    ON Posts.OwnerUserId = Users.id
  INNER JOIN PostTypes ON Posts.PostTypeId = PostTypes.Id
  WHERE Tags.TagName   in 
  (select 
     num.TagName as Tag
  
   from
  
  (select TagName
  from
  Tags, PostTags, Posts
  where Tags.Id = PostTags.TagId and Posts.Id = PostId
  and Posts.CreationDate &lt; '2015-04-20'
  and Posts.CreationDate &gt; '2015-01-01'
  group by TagName) as rate
  
  INNER JOIN
  (select TagName 
  from Tags, PostTags, Posts
  where Tags.Id = PostTags.TagId and Posts.Id = PostId
  group by TagName
  having count(PostId) &gt; 8000)as num 
  ON rate.TagName = num.TagName
  )
    AND Posts.AnswerCount &gt; 1
    AND Posts.AcceptedAnswerId is not null
    AND Posts.Score &gt; 0
    AND Posts.PostTypeId  = 1                -- PostTypeId = 1 -&gt; Question
    AND Posts.CommunityOwnedDate is null
    )
  AND Posts.PostTypeId  = 2                -- -&gt; Answers
  </code></pre>
  
  <p>I think the problem is <code>ParentId</code> , but I do not understand what is wrong. </p>