如何根据mysql中的另一个表计算一个表中的行数

Question

I have two tables in a MySQL database. The first one has a list of department names.

departments
    abbreviation | name
    -------------|-------------
    ACC          | accounting
    BUS          | business
    ...

The second table has a list of courses with names that contain the department's abbreviation.

courses
    section      | name
    -------------|-------------
    ACC-101-01   | Intro to Accounting
    ACC-110-01   | More accounting
    BUS-200-02   | Business etc.
    ...

I'd like to write a query that will, for each row in the departments table, give me a count of how many rows in the courses table are like the abbreviation I have. Something such as this:

    abbreviation | num
    -------------|--------------
    ACC          | 2
    BUS          | 1
    ...

I can do this for one individual department with the query

SELECT COUNT(*) FROM courses WHERE section LIKE '%ACC%'
    (gives me 2)

Although I could loop through in PHP and do the above query many times, I'd rather do it in a single query. This is the pseudocode I'm thinking of...

SELECT department.abbreviation, num FROM
    for each row in departments
        SELECT COUNT(*) AS num FROM classes WHERE section LIKE CONCAT('%',departments.abbreviation,'%)

Any ideas?

Answer 1

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

Sql Fiddle

Answer 2

I quick solution, but not the best, could be:

SELECT abbreviation, 
(SELECT COUNT(*) FROM courses C WHERE D.abbreviation = SUBSTRING(C.abbreviation, 0, 3)) AS c 
FROM departments D;

Answer 3

Give this a shot:

select
    d.abbreviation,
    count(d.abbreviation) count
from
    departments d
inner join
    courses c
on (LOCATE(d.abbreviation,c.section) <> 0)
group by d.abbreviation;

Answer 4

Try this

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