title: Replace null value by latest value
body: <p>Say I have below table. How do I pick up the latest previous value in case my joined table to not match the date and currency? On the null DKK value I want it to pick up 3. Note that dates do not exist every day since I do not load the tables on weekends. </p>
  
  <pre><code>Select
      PositionDate,
      Currency,
      T2.Value,
      isnull(t2.value, ? )
  From t1
  left join t2
  on t1.currency = t2.Currency
  and t1.PositionDate = t2.PositionDate
  </code></pre>
  
  <p>.</p>
  
  <pre><code>PositionDate    Currency        Value
  2017-04-11      SEK               1
  2017-04-11      DKK               NULL
  2017-04-11      EUR               7
  2017-04-10      SEK               4 
  2017-04-10      DKK               3
  2017-04-10      EUR               5
  2017-04-07      SEK               4 
  2017-04-07      DKK               3
  2017-04-07      EUR               5
  </code></pre>
  
  <p>.</p>