title: Select distinct from usercolumn and minimum value of dates for each field in usercolumn
body: <p>I have some data like this but more than 1500000 records and more than 700 users:</p>
  
  <pre><code>usercolumn   ,       datecolumn\     
  a1            ,      1998/2/11\
  a2            ,      1998/3/11\
  a1            ,      1998/2/15\
  a4            ,      1998/4/14\
  a3            ,      1999/1/15\
  a2            ,      1998/11/12\
  a2            ,      1999/2/11\
  a3            ,      2000/2/9\
  a1            ,      1998/6/5\
  a3            ,      1998/7/7\
  a1            ,      1998/3/11\
  a5            ,      1998/3/18\
  a2            ,      1998/2/8\
  a1            ,      1998/12/11\
  a4            ,      1998/12/1\
  a5            ,      1998/2/11\
  ....
  </code></pre>
  
  <p>I would like to have distinct data from usercolumn and minimum value of date for each user like this:</p>
  
  <pre><code>usercolumn      ,    datecolumn \        
  a1              ,    1998/2/11\
  a2              ,    1998/2/8\
  a3              ,    1998/7/7\
  a4              ,    1998/4/14\
  a5              ,    1998/2/11\
  ....
  </code></pre>
  
  <p>please help me to write an SQL command to do this for oledb adapter in c#, thanks.</p>