如何根据mysql中的另一个表计算一个表中的行数

How to count rows in one table based on another table in mysql(如何根据mysql中的另一个表计算一个表中的行数)

问题描述

我在 MySQL 数据库中有两个表.第一个是部门名称列表.

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

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

第二个表列出了课程名称，其中包含系的缩写.

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.
    ...

我想写一个查询，对于 departments 表中的每一行，给我一个 courses 表中的行数的计数我有的缩写.像这样的东西:

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
    ...

我可以通过查询为一个单独的部门执行此操作

I can do this for one individual department with the query

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，但我更愿意在单个查询中执行.这是我想到的伪代码...

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,'%)

有什么想法吗?

推荐答案

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

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