棘手的SQL加入查询

Question

我有两个表，一个名为StudentCerts，包含一个电子邮件（主键）和一个证书，另一个名为CertReqs，包含证书和课程。 表的一部分可能如下：

     StudentCerts:                                 CertReqs:    
Email           Certificate             Certificate          Course
This@that.com      Programmer            Programmer           CS 101
This@that.com      English               Programmer           CS 202
A@B.com            Econ                  Programmer           CS 303
john@smith.com     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，This@that.com注册了程序员和英语证书。我想得到一个SQL语句，它将返回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归还经济，艺术和新闻。 我一直在努力争取这个，所以任何帮助将不胜感激！

Answer 1

SELECT cr.Certificate FROM CertReqs cr
WHERE cr.Certificate NOT IN (
    SELECT sc.Certificate FROM StudentCerts sc
    WHERE sc.Email = 'This@that.com'
);

该查询几乎可以作为要求：“我想要所有CertReqs，学生不在其中”。

Answer 2

游戏稍晚，但简单的LEFT JOIN可以很好地解决这个问题：

SELECT DISTINCT cr.Certificate 
FROM CertReqs cr
LEFT JOIN StudentCerts sc ON cr.Certificate = sc.Certificate AND sc.Email = 'This@that.com'
WHERE sc.Email IS NULL

Answer 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演示

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 = 'This@that.com'
ORDER BY x.EMAIL

• SQLFiddle演示

Answer 4

SELECT Certificate FROM `CertReqs` 

WHERE Certificate NOT in(
    SELECT Certificate 
    FROM `StudentCerts` 
    WHERE Email  = 'This@that.com' 
)

Answer 5

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