查询未显示正确的结果

Question

我已经建立了一个这样的查询：

        SELECT 
           c.*,
           (SELECT COUNT(cursus_id) FROM cursusdienst_bestellingen where cursus_id = c.id AND datum_afhaling IS  NULL AND datum_verwijderd IS NULL AND studentid = '$studentid') as besteld,
           (SELECT COUNT(cursus_id) FROM cursusdienst_bestellingen where cursus_id = c.id AND datum_afhaling IS NOT NULL AND datum_verwijderd IS NULL AND datum_afgehaald IS NULL AND studentid = '$studentid') as afhalen,
           (SELECT datum_afhaling + INTERVAL 14 DAY FROM cursusdienst_bestellingen where cursus_id = c.id AND datum_afhaling IS NOT NULL AND datum_verwijderd IS NULL AND datum_afgehaald IS NULL AND studentid = '$studentid') as datum_afhaling,
           (SELECT COUNT(cursus_id) FROM cursusdienst_bestellingen where cursus_id = c.id AND datum_afgehaald IS NOT NULL AND datum_verwijderd IS NULL AND cb.studentid = '$studentid') as afgehaald
        FROM 
           cursusdienst c
        LEFT JOIN
           cursusdienst_bestellingen cb
        ON
           cb.cursus_id = c.id    
        WHERE 
           c.studierichting = '1ste Bachelor'
        ORDER BY 
           c.artikel asc, cb.studentid

这是针对学生和课程的注册/订购系统。有很多学生使用这个系统。学生必须先注册，然后才能订购课程。 我在这里的查询有问题，因为我在同一个列表中看到每个学生的课程。这里有一个例子：

Checkbox         Course              Price     Status
Checkbox         Course 1            12.00     Ordered
Checkbox         Course 1            12.00     Ordered
Checkbox         Course 2            25.50     Ready to deliver
Checkbox         Course 3            15.00     

正如您在此处看到的，查询向我显示了 2 名学生的所有课程信息（这只是一个示例），他们已登录并订购了课程。 第一个学生订购了 Course 1 en 2，第二个学生只订购了 Course 1。 如果我以学生身份登录，我只想查看我订购的课程的状态，但我也会看到其余课程（没有像课程 3 这样的任何状态），因此我可以以相同的形式订购其他课程。 不想看到的是别人点的！

因此，如果我作为学生订购了课程 1 和 2，我只想看到这个。课程 3 也在列表中，因为我现在可以订购但我之前没有订购（所以它没有状态） ：

Checkbox         Course              Price     Status
Checkbox         Course 1            12.00     Ordered
Checkbox         Course 2            25.50     Ready to deliver
Checkbox         Course 3            15.00     

如果我将“AND cb.studentid = '$studentid'”添加到 WHERE，我只会看到我订购的课程，而不是我未订购的其他课程。

编辑：（感谢 SunKnight0）在 ON 部分而不是 WHERE 部分设置“AND cb.studentid = '$studentid'”！所以工作代码如下所示：

        SELECT 
           c.*,
           (SELECT COUNT(cursus_id) FROM cursusdienst_bestellingen where cursus_id = c.id AND datum_afhaling IS  NULL AND datum_verwijderd IS NULL AND studentid = '$studentid') as besteld,
           (SELECT COUNT(cursus_id) FROM cursusdienst_bestellingen where cursus_id = c.id AND datum_afhaling IS NOT NULL AND datum_verwijderd IS NULL AND datum_afgehaald IS NULL AND studentid = '$studentid') as afhalen,
           (SELECT datum_afhaling + INTERVAL 14 DAY FROM cursusdienst_bestellingen where cursus_id = c.id AND datum_afhaling IS NOT NULL AND datum_verwijderd IS NULL AND datum_afgehaald IS NULL AND studentid = '$studentid') as datum_afhaling,
           (SELECT COUNT(cursus_id) FROM cursusdienst_bestellingen where cursus_id = c.id AND datum_afgehaald IS NOT NULL AND datum_verwijderd IS NULL AND cb.studentid = '$studentid') as afgehaald
        FROM 
           cursusdienst c
        LEFT JOIN
           cursusdienst_bestellingen cb
        ON
           cb.cursus_id = c.id AND cb.student_id = '$studentid'  
        WHERE 
           c.studierichting = '1ste Bachelor'
        ORDER BY 
           c.artikel asc, cb.studentid

Answer 1

SELECT 
   c.*,
   (SELECT COUNT(cursus_id) FROM cursusdienst_bestellingen where cursus_id = c.id AND datum_afhaling IS  NULL AND datum_verwijderd IS NULL AND studentid = '$studentid') as besteld,
   (SELECT COUNT(cursus_id) FROM cursusdienst_bestellingen where cursus_id = c.id AND datum_afhaling IS NOT NULL AND datum_verwijderd IS NULL AND datum_afgehaald IS NULL AND studentid = '$studentid') as afhalen,
   (SELECT datum_afhaling + INTERVAL 14 DAY FROM cursusdienst_bestellingen where cursus_id = c.id AND datum_afhaling IS NOT NULL AND datum_verwijderd IS NULL AND datum_afgehaald IS NULL AND studentid = '$studentid') as datum_afhaling,
   (SELECT COUNT(cursus_id) FROM cursusdienst_bestellingen where cursus_id = c.id AND datum_afgehaald IS NOT NULL AND datum_verwijderd IS NULL AND cb.studentid = '$studentid') as afgehaald
FROM 
   cursusdienst c
LEFT JOIN
   cursusdienst_bestellingen cb
ON
   cb.cursus_id = c.id AND cb.studentid = '$studentid'
WHERE 
   c.studierichting = '1ste Bachelor'
ORDER BY 
   c.artikel asc, cb.studentid