title: Update null values by merging another table in mssql
body: <p>I have two tables like :</p>
  
  <p>Table 1:</p>
  
  <pre><code>Customer    employee
  ASD_1234    WF001
  ASD_1235    WF002
  ASD_1236    WF003
  ASD_1237    NULL
  ASD_1238    NULL
  ASD_1239    NULL
  ASD_1240    WF004
  ASD_1234    WF001
  ASD_1236    WF003
  ASD_1240    WF004
  </code></pre>
  
  <p>Table2:</p>
  
  <pre><code>Customer    com_employee
  ASD_1234    WF001
  ASD_1235    WF002
  ASD_1236    WF003
  ASD_1237    WF005
  ASD_1238    WF006
  ASD_1239    WF007
  ASD_1240    WF004
  </code></pre>
  
  <p>The table 2 is the metadata which consists of unique <strong>Customer</strong>.Now I need to update <strong>only the null values</strong> in table1 by joining table2 on Customer. How can I achieve this?
  Thanks </p>