[hobs]

Most of your queries shouldn't be so verbose as to confuse which aliases you are talking about, but I agree - here's the best format :) (because why put ON on another line anyway?)

  SELECT
    bo.title,
    au.last_name,
    au.first_name
  FROM books AS bo
  LEFT JOIN authors AS au ON 
    bo.author_id = au.id

[dragonwriter]

actually, I'd say the best is “column aliases are like full identifiers that name what the entity is in the context of the query”.

In simple cases that may be just the singular of the table name, e.g.:

  SELECT
    book.title,
    author.last_name,
    author.first_name
  FROM books AS book
  LEFT JOIN authors AS author ON 
    book.author_id = author.id

But in other cases, it will be different, e.g.:

  SELECT    
    manager.last_name || ', ' || manager.first_name AS manager_name,
    coalesce(employee.title, 'All Titles') AS staff_title,
    count(employee.id) AS count
  FROM employees AS manager
  LEFT JOIN employees AS employee ON
    manager.id = employee.manager_id
  GROUP BY manager.id, ROLLUP(employee.title)
  HAVING employee.title IS NOT NULL
      OR GROUPING(employee.title)=1

[mulmen]

ON should be on another line because it gives context to bo.author_id = au.id.

[hobs]

You never write NOT ON, so no, I'd disagree that its required for all the inequalities you express.

[mulmen]

Huh?

ON is required by the syntax. Why start every line with a keyword that describes the line except the ON portion of a JOIN? It's inconsistent and has no clear benefit.