Question

我正在寻找以下查询的MySQL等效项：

(select course_id
from section
where semester = 'Fall' and year= 2009)
except
(select course_id
from section
where semester = 'Spring' and year= 2010);

节表在哪里：

+-----------+--------+----------+------+----------+-------------+--------------+
| course_id | sec_id | semester | year | building | room_number | time_slot_id |
+-----------+--------+----------+------+----------+-------------+--------------+
| BIO-101   | 1      | Summer   | 2009 | Painter  | 514         | B            |
| BIO-301   | 1      | Summer   | 2010 | Painter  | 514         | A            |
| CS-101    | 1      | Fall     | 2009 | Packard  | 101         | H            |
| CS-101    | 1      | Spring   | 2010 | Packard  | 101         | F            |
| CS-190    | 1      | Spring   | 2009 | Taylor   | 3128        | E            |
| CS-190    | 2      | Spring   | 2009 | Taylor   | 3128        | A            |
| CS-315    | 1      | Spring   | 2010 | Watson   | 120         | D            |
| CS-319    | 1      | Spring   | 2010 | Watson   | 100         | B            |
| CS-319    | 2      | Spring   | 2010 | Taylor   | 3128        | C            |
| CS-347    | 1      | Fall     | 2009 | Taylor   | 3128        | A            |
| EE-181    | 1      | Spring   | 2009 | Taylor   | 3128        | C            |
| EE-302    | 1      | Summer   | 2010 | Watson   | 327         | C            |
| FIN-201   | 1      | Spring   | 2010 | Packard  | 101         | B            |
| HIS-351   | 1      | Spring   | 2010 | Painter  | 514         | C            |
| MU-199    | 1      | Spring   | 2010 | Packard  | 101         | D            |
| PHY-101   | 1      | Fall     | 2009 | Watson   | 100         | A            |
+-----------+--------+----------+------+----------+-------------+--------------+

换句话说，我想找到2009年秋季学期而不是2010年春季学期教授的所有课程。

Answer 1

MySQL不支持except，因此只需使用not exists或not in：

select courseid
from section sf
where semester = 'Fall' and year = 2009 and not exists
      (select 1
       from section ss
       where sf.courseid = ss.courseid and ss.semester = 'Spring' and ss.year = 2010
      );

（我更喜欢not exists，因为它对NULL值具有更直观的支持。）

这是不正确的，因为except会删除重复项。您可以使用select distinct，但我确实确实需要这样做。

Answer 2

使用not in，因为except在mysql中不可用

select *
from section 
where courseid not in 
(
select courseid
from section
where semester = 'Spring' and year= 2010
) and semester = 'Fall' and year = 2009

Answer 3

我会使用not exists：

select s.*
from section s
where semester = 'Fall' and year = 2009 and
      not exists (select 1 
                  from section s1
                  where s.course_id = s1.course_id and 
                        s1.semester = 'Spring' and s1.year = 2010
                 );

Answer 4

我喜欢在可能的情况下坚持使用JOIN，在这里当然有可能。

SELECT sFa.course_id
FROM section AS sFa
LEFT JOIN section AS sSpr 
   ON sFa.course_id = sSpr.course_id
   AND sSpr.semester = 'Spring' AND sSpr.year= 2010
WHERE sFa.semester = 'Fall' 
   AND sFa.year= 2009
   AND sSpr.course_id IS NULL
;

相当于MySQL EXCEPT

4 个答案: