title: How to create column in SQL query with custom text
body: <p>Is it possible to create custom column with sql query, and in that query to add some custom text, to say e.g what type of error is in that query, or i need to create table, where i will insert types of errors, and after with joins to add that column ?</p>
  
  <p>E.g query would look like this (without adding that extra column that will say what is type of error, because, i don't know is it possible to do that)</p>
  
  <pre><code>SELECT id,aa,bb,NULL AS cc, NULL AS dd
  FROM test
  WHERE aa=SomeRequirement AND 
        bb=SomeRequirement
  
  UNION ALL
  
  SELECT id,NULL,NULL,cc,dd
  FROM test2
  WHERE cc=SomeRequirement AND
        dd=SomeRequirement
  </code></pre>
  
  <p>and wanted result is</p>
  
  <pre><code>id| TextForError               |  aa      |   bb    |    cc    |    dd
  1 | thisRowIsNotGoodBecause..  |  someData| someData| someData | someData
  </code></pre>
  
  <p>And one more question, if i need to create table with errors that i wanted to display, how is possible to add that error , if i don't have keys that will reference typical error for that row?</p>