SQL計算一個表相對於另一表的行

Question

我有3個不同的表：

客戶

+----+-----------+----------+
| id | firstName | lastName |
+----+-----------+----------+
|  1 | John      | Doe      |
|  2 | Jane      | Doe      |
+----+-----------+----------+

貸款

+----+--------+-----------+----------------+
| id | amount | client_id | institution_id |
+----+--------+-----------+----------------+
|  1 |    200 |         2 |              3 |
|  2 |    400 |         1 |              1 |
+----+--------+-----------+----------------+

機構

+----+---------------+
| id |     name      |
+----+---------------+
|  1 | Institution A |
|  2 | Institution B |
|  3 | Institution C |
+----+---------------+

我正在尋找一個清單，列出每個機構向客戶提供的貸款數量（機構表中的每一行）。 包括客戶在某機構獲得0筆貸款時。

看起來像：

+-----------+-----------+----------+--------------------------+-----------+
| client_id | firstName | lastName | financialInstitutionName | loanCount |
+-----------+-----------+----------+--------------------------+-----------+
|         1 | John      | Doe      | Institution A            |         1 |
|         1 | John      | Doe      | Institution B            |         0 |
|         1 | John      | Doe      | Institution C            |         0 |
|         2 | Jane      | Doe      | Institution A            |         0 |
|         2 | Jane      | Doe      | Institution B            |         0 |
|         2 | Jane      | Doe      | Institution C            |         1 |
+-----------+-----------+----------+--------------------------+-----------+

我嘗試了各種方式的連接，子查詢和where子句，但均未成功。 我不理解的概念是如何在每個機構，每個客戶（總機構x客戶）中獲得一行。 我很想如果沒有子查詢或聯合聯接就可以進行該查詢。

感謝您的時間！

Answer 1

SELECT 
    loan.client_id, 
    client.firstName, 
    client.lastName, 
    institution.name as financialInstitutionName, 
    COUNT(loan.id) as loanCount 
FROM client 
INNER JOIN loan ON client.id = loan.client_id 
INNER JOIN institution ON loan.institution_id = institution.id 
GROUP BY client.id;

Answer 2

FROM中的第一個子查詢會設置該數據，以便每個客戶都有每個機構的記錄。 然后將其連接到一個子查詢，該子查詢計算貸款的數量。

SELECT 
    d.client_id,
    d.firstName,
    d.lastName,
    d.name AS financialInstitutionName,
    CASE WHEN l IS NULL
        THEN 0
        ELSE l.loanCount
    END AS loanCount

FROM
(
    SELECT
        Client.id AS client_id,
        Client.firstName,
        Client.lastName,
        Institution.id AS institution_id,
        Institution.name
    FROM Client, Institution
) AS d

LEFT JOIN (
    SELECT client_id, institution_id, COUNT(id) AS loanCount 
    FROM Loan
    GROUP BY client_id, institution_id
) AS l ON d.client_id = l.client_id AND d.institution_id = l.institution_id

編輯：包括每個機構的記錄

編輯：拼寫