private void checkAttribute()
	{
		ExpressionDecomposer ed=new ExpressionDecomposer();
		//check select
		Iterator<Expression> s=select.iterator();
		while(s.hasNext())
		{
			ArrayList<String> selectAttrs=ed.getIdentifiers(s.next());
			Iterator<String> selectIterator=selectAttrs.iterator();
			while(selectIterator.hasNext())
			{				
				attributesInTable(selectIterator.next(),"select clause");
			}
		}

		//check where
		if(where!=null)
		{
			ArrayList<String> whereAttrs=ed.getIdentifiers(where);

			Iterator<String> whereIterator=whereAttrs.iterator();
			while(whereIterator.hasNext())
			{			
				attributesInTable(whereIterator.next(),"where clause");
			}
		}

		//check group
		if(group!=null)
		{
			attributesInTable(group,"aggregation");
		}
	}