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

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.