title: Deleting rows that are duplicated in one column based on the conditions of another column
body: <p>Here is an example of my data set;</p>
  
  <pre><code>Date      Time(GMT)Depth Temp  Salinity Density Phosphate
  24/06/2002  1000    1           33.855          0.01
  24/06/2002  1000    45          33.827          0.01
  01/07/2002  1000    10  13.26   33.104  24.873  0.06
  01/07/2002  1000    30  12.01   33.787  25.646  0.13
  08/07/2002  1000    5   13.34   33.609  25.248  0.01
  08/07/2002  1000    40  12.01   34.258  26.011  1.33
  15/07/2002  1000    30  12.04   34.507  26.199  0.01
  22/07/2002  1000    5   13.93   33.792  25.269  0.01
  22/07/2002  1000    30  11.9    34.438  26.172  0.08
  29/07/2002  1000    5   13.23   34.09   25.642  0.01
  </code></pre>
  
  <p>I want to delete duplicate rows so that I only have one row per date, I want to do this based on the Depth, I would like to keep the row with the greatest (deepest) depth. Any ideas? </p>