title: Postgresql invalid input syntax for type boolean: "null"
body: <p>I' learning postgresql, and I typed the following commands.</p>
  
  <pre><code> CREATE TABLE t(co1 boolean, col2 text);
   INSERT INTO t VALUES(NULL, 'NULL');
   SELECT * FROM t where col2::boolean;
  </code></pre>
  
  <p>And the result came out like this:</p>
  
  <pre><code>ERROR:  invalid input syntax for type boolean: "NULL".
  </code></pre>
  
  <p>Why is that happened since things like 'True'::boolean is correct?</p>