棘手的 SQL 连接查询

Posted 2023-05-09

【中文标题】棘手的 SQL 连接查询

【英文标题】：Tricky SQL Join Query

【发布时间】：2012-12-07 07:24:38

【问题描述】：

我有两张表，一张名为 StudentCerts，包含电子邮件（主键）和证书，另一张名为 CertReqs，包含证书和课程。表格的一部分可能如下所示：

     StudentCerts:                                 CertReqs:    
Email           Certificate             Certificate          Course
[e-mail]      Programmer            Programmer           CS 101
[e-mail]      English               Programmer           CS 202
[e-mail]            Econ                  Programmer           CS 303
[e-mail]     Programmer            English              ENG 101
                                         English              ENG 102
                                         Econ                 ECON 102
                                         Econ                 ECON 304
                                         Art                  Art 101
                                         Art                  Art 102
                                         Journalism           J 101
                                         Journalism           J 202

我想要做的是获得特定学生不属于的所有证书。例如，[e-mail] 注册了程序员和英语证书，我想获得一条 SQL 语句，该语句将返回 CertReqs 中特定学生未注册的所有证书。所以对于这个例子它应该返回 Econ、Art 和 Journalism。 我一直在努力得到这个，所以任何帮助将不胜感激！

【参考方案1】：

SELECT cr.Certificate FROM CertReqs cr
WHERE cr.Certificate NOT IN (
    SELECT sc.Certificate FROM StudentCerts sc
    WHERE sc.Email = '[e-mail]'
);

查询几乎读作要求：“我想要所有 CertReqs，学生不在其中”。

【讨论】：

非常感谢！这正是我想做的！

【参考方案2】：

游戏有点晚了，但一个简单的LEFT JOIN 很好地解决了这个问题：

SELECT DISTINCT cr.Certificate 
FROM CertReqs cr
LEFT JOIN StudentCerts sc ON cr.Certificate = sc.Certificate AND sc.Email = '[e-mail]'
WHERE sc.Email IS NULL

【参考方案3】：

这将列出所有缺少某个证书的学生

SELECT  DISTINCT x.*
FROM
    (
        SELECT  a.Email, b.Certificate
        FROM    (SELECT DISTINCT Email FROM StudentCerts) a
                CROSS JOIN
                (SELECT DISTINCT Certificate FROM CertReqs) b
    ) x LEFT JOIN studentCerts y
        ON x.Email = y.Email AND
            x.Certificate = y.Certificate
WHERE  y.Email IS NULL
ORDER BY x.EMAIL

SQLFiddle Demo

如果你想要特定的电子邮件，那么你只需要添加额外的条件

SELECT  DISTINCT x.*
FROM
    (
        SELECT  a.Email, b.Certificate
        FROM    (SELECT DISTINCT Email FROM StudentCerts) a
                CROSS JOIN
                (SELECT DISTINCT Certificate FROM CertReqs) b
    ) x LEFT JOIN studentCerts y
        ON x.Email = y.Email AND
            x.Certificate = y.Certificate
WHERE  y.Email IS NULL AND
       x.Email = '[e-mail]'
ORDER BY x.EMAIL

SQLFiddle Demo

【参考方案4】：

SELECT DISTINCT A.* fROM 
(
SELECT DISTINCT EMAIL, CERTIFICATE fROM #StudentCerts A
CROSS JOIN #CertReqs B)  A
LEFT JOIN #StudentCerts B ON A.EmaiL = B.EMAIL AND B.CERTIFICATES = A.Certificate
WHERE B.EMAIL IS NULL
ORDER BY A.EMAIL 

【讨论】：

欢迎来到 Stack Overflow！虽然这段代码 sn-p 可以解决问题，但including an explanation 确实有助于提高帖子的质量。请记住，您正在为将来的读者回答问题，而这些人可能不知道您的代码建议的原因。也请尽量不要用解释性的 cmets 挤满你的代码，这会降低代码和解释的可读性！

【参考方案5】：

SELECT Certificate FROM `CertReqs` 

WHERE Certificate NOT in(
    SELECT Certificate 
    FROM `StudentCerts` 
    WHERE Email  = '[e-mail]' 
)