title: Postgres - Subtraction of two queries
body: <p>I want to make a subtraction like this:</p>
  
  <pre><code>SELECT 
      (SELECT ST_Y(ST_Centroid(ST_Collect(column1))) AS distance_km 
       FROM table1) - 
      (SELECT ST_Y(ST_Transform(column1,4326)) AS distance_km 
       FROM table1) AS Difference
  </code></pre>
  
  <p>But the <code>ST_Y(ST_Centroid(ST_Collect(column1)))</code> has only one row and <code>ST_Y(ST_Transform(column1,4326))</code> has multiple rows. I want to subtract the multiple rows by the column and see the results for each multiple row. With this query I get: </p>
  
  <blockquote>
    <p>ERROR:  more than one row returned by a subquery used as an expression</p>
  </blockquote>
  
  <p>Any help?</p>