title: SQL: create new field, populate with query results
body: <p>I have two tables: tbl_listings, with the column: prop_id;
  and another table: tbl_bookings, with the columns: prop_id, booking_date.</p>
  
  <p>I want to write a query that counts all the times prop_id appears in tbl_bookings, and then populates a new column in tbl_listings with the results of that query.</p>
  
  <p>my query looks like:</p>
  
  <pre><code>ALTER TABLE tbl_listings
  ADD COLUMN prop_count INT
  
  UPDATE tbl_listings
      SET prop_count =
      (SELECT COUNT(*)
      FROM tbl_bookings
      GROUP BY prop_id)
  </code></pre>
  
  <p>but for some reason I get an error saying: Subquery returns more than 1 row.
  How do I fix this?</p>