如何在mysql 2中基于一个表对一个表中的行进行计数

Question

I checked this answer but I have a problem.

SQL Fiddle (I changed INNER JOIN to LEFT JOIN )

What if there is a row in my departments (DDD for example) and no association in the courses table .

As you can see in the fiddle the count for DDD department is 1 like the BUS department has even if there is no data for DDD department. I know that LEFT JOIN does this but how can I modify this ?

How do I get count = 0 for the DDD department?

Answer 1

Try COUNT(c.section) so when section results null for no association then in count it evaluates to zero

SELECT d.abbreviation
, COUNT(c.section) num
FROM departments d
LEFT JOIN courses c ON c.section LIKE CONCAT(d.abbreviation, "%")
GROUP BY d.abbreviation

Demo