mysql - embed count query within another query

Question

In postgres I am fairly sure you can do something like this

SELECT
  authors.stage_name, 
  count(select id from books where books.author_id  = authors.id) 
FROM
  authors,
  books;

Essentially, in this example I would like to return a list of authors and how many books each has written.... in the same query.

Is this possible? I suspect this approach is rather naive..

Thanks :)

Answer 1

How about using a join:

SELECT authors.stage_name, count(*) 
FROM authors INNER JOIN books on books.author_id = authors.id
GROUP BY authors.stage_name

Answer 2

Well, for one thing, it returns a Cartesian product of all authors to all books, regardless of whether that author wrote that book.

Here's how I'd write a query to get the result you say you want:

SELECT a.stage_name, COUNT(b.id)
FROM authors a
  LEFT OUTER JOIN books b ON (a.id = b.author_id)
GROUP BY a.id;

You need to learn how to write join queries if you use SQL. The join is to SQL what the loop is to application programming languages. I.e. it's a fundamental programming construct that you need to know.