如何根据mysql中的另一个表计算一个表中的行数

Question

我在MySQL数据库中有两个表.第一个有一个部门名称列表.

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

第二个表有一个名称包含部门缩写的课程列表.

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

我想写一个查询,对于departments表中的每一行,我会计算一下courses表中有多少行就像我的缩写.像这样的东西:

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

我可以通过查询为一个单独的部门执行此操作

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

虽然我可以在PHP中循环并多次执行上述查询,但我宁愿在单个查询中执行此操作.这是我想到的伪代码......

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

有任何想法吗？

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小提琴