How to set a default value for a column in a table?

Question

SQL

I have three tables: Book(Id), Topic(Id), BookTopic(BookId, TopicId). There is a query that for each Topic returns the number of Books in this Topic:

SELECT T.Id, T.TopicName, count(BT.TopicId) AS 'BooksCount' 
FROM Topic T, BookTopic BT
WHERE T.Id = BT.TopicId
GROUP BY T.Id, T.TopicName

But there is a problem: if no book is associated with a topic, then such a topic is not returned. And I would like such a topic to be returned with the BooksCount=0 value. How can I do that?

Answer 1

The question is incorrect in the header, but what you are asking to do is like this:
SELECT T.Id, T.TopicName, count(BT.TopicId) AS 'BooksCount'
FROM Topic T
LEFT JOIN BookTopic BT on
T.Id = BT.TopicId
GROUP BY T.Id, T.TopicName