title: numeric overflow with "NULL" column (0,3)
body: <p>I have a table (Oracle 12.1.0.2.0) where when I sum up the values I get</p>
  
  <pre><code>ORA-01426: numeric overflow
  01426. 00000 -  "numeric overflow"
  *Cause:    Evaluation of an value expression causes an overflow/underflow.
  *Action:   Reduce the operands.
  </code></pre>
  
  <p>When I check the values in SQL-Developer I see in the column "NULL" which should be no issue for the function sum.  </p>
  
  <p>When I use dump(col1) for this table the result is </p>
  
  <pre><code>Typ=2 Len=2: 0,3
  </code></pre>
  
  <p>shouldn't this be NULL for columns with NULL? </p>
  
  <p>Also checks 'is not null' didn't filter rows. </p>
  
  <p>Exporting the data only exported a part of the first row and stopped after the first occurence of this 0,3. </p>
  
  <p>SQL Navigator showed "error" as value for this column. </p>
  
  <p>Table was created as select. Now it is truncated and refilled and NULL rows are NULL and summing up works. </p>
  
  <p>What was the content of that column? And why SQL-Developer shows NULL instead of an error?</p>