title: Specifying NOT NULL when writing pandas to SQL
body: <p>I'm creating a table using SQLAlchemy like this:</p>
  
  <pre><code>import sqlalchemy
  
  df.to_sql(table_name, engine, schema='dbo', 
            index=True, index_label='IdColumn', 
            if_exists='replace',
            dtype={'IdColumn': sqlalchemy.types.VARCHAR(length=255)})
  </code></pre>
  
  <p>Is it possible to specify in <code>dtypes</code> that <code>IdColumn</code> should be <code>not null</code>? The above writes <code>df</code> to the SQL-server, but gives <code>IdColumn</code> the type <code>(varchar(255), null)</code> by default.</p>