title: Update existing row in database from pandas df
body: <p>I have a PostgreSQL db. Pandas has a 'to_sql' function to write the records of a dataframe into a database. But I haven't found any documentation on how to update an existing database row using pandas when im finished with the dataframe.</p>
  
  <p>Currently I am able to read a database table into a dataframe using pandas read_sql_table. I then work with the data as necessary. However I haven't been able to figure out how to write that dataframe back into the database to update the original rows.</p>
  
  <p>I dont want to have to overwrite the whole table. I just need to update the rows that were originally selected.</p>