title: redshift: update a Null string into NULL
body: <p>There is a redshift table with a &quot;column_a&quot;</p>
  <pre><code>  column_a                       varchar(1000) null,
  </code></pre>
  <p>the following query will return a lot of results in 'Null' strings</p>
  <pre><code>select column_a from table where lower(column_a)='Null' limit 100;
  </code></pre>
  <p>but if i do</p>
  <pre><code>update table set column_a = NULL where lower(column_a)='';
  </code></pre>
  <p>i got an error message</p>
  <pre><code>Cannot insert a NULL value into column binhash
  </code></pre>
  <p>is there anyway i can replace the 'Null' string into NULL? Thanks</p>