如何根據mysql中的另一個表計算一個表中的行數

Question

我在MySQL數據庫中有兩個表。 第一個有一個部門名稱列表。

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

第二個表有一個名稱包含部門縮寫的課程列表。

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

我想編寫一個查詢，對於departments表中的每一行，我會計算一下我在courses表中有多少行的縮寫。 像這樣的東西：

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

我可以通過查詢為一個單獨的部門執行此操作

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

雖然我可以在PHP中循環並多次執行上述查詢，但我寧願在單個查詢中執行此操作。 這是我想到的偽代碼......

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

有任何想法嗎？

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

我快速解決方案，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

試一試：

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

試試這個

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