title: Postgres array of json, get all the unique values of a specific key
body: <p>I have a table that has data like:</p>
  
  <pre><code>id, array_json
  123, [{"Name": "John", "Age": "12"}, {"Name": "Jane", "Age": "18"}, {"Name": "Jake", "Age": "34"}]
  124, [{"Name": "Dolly", "Age": "12"}, {"Name": "Molly", "Age": "18"}, {"Name": "Rosa", "Age": "34"}]
  123, []
  125, [{"Name": "Dolly", "Age": "12"}, {"Name": "Dolly", "Age": "18"}, {"Name": "Holt", "Age": "34"}]
  </code></pre>
  
  <p>Its basically an id mapped to an array of jsons, which is a jsonb.
  I want the output to be all the unique names (Key is "Name") &amp; their count as follows:</p>
  
  <pre><code>id, Unique_Names, Count
  123, ["John", "Jane", "Jake"], 3
  124, ["Dolly", "Molly", "Rosa"], 3
  125, [], 0
  126, ["Dolly", "Holt"], 2
  </code></pre>
  
  <p>How do I do this in postgres?</p>