mysql - 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
    ...

第二个表有一个课程列表，其名称包含部门的缩写。

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

我想编写一个查询，对于departments表中的每一行，我都会计算表中有多少行courses就像我拥有的​​缩写一样。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)

尽管我可以在 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。这是我正在考虑的伪代码...

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

Answer 2

我快速的解决方案，但不是最好的，可能是：

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

Answer 3

试一试：

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

尝试这个

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