title: Same number of columns for Union Operation
body: <p>I was going through union and union all logic and trying examples. What has puzzled me is why is it necessary to have same number of columns in both the tables to perform a union or union all operation? </p>
  
  <p>Forgive me if my question's silly, but i couldn't get the exact answer anywhere. And conceptually thinking, as long as one common column is present, merging two tables should be easy right?(like a join operation). But this is not the case, and I want to know why?</p>