mysql - 给出所有不教数学的老师的名字

Question

我有两张 table 。第一个是Course 表，第二个是Teacher 表。我想要找到所有不教'数学'的老师。我怎样才能做到这一点？

类(class)表

course_id   course  teacher_id  marks
 1          Physics     1        60
 2          Math        1        60
 3          Chemestry   1        60
 4          English     2        60
 5          Hindi       2        60
 6          Physics     2        60
 7          Chemestry   3        60
 8          English     4        60
 9          Math        5        60
10          Math        6        60

教师表

teacher_id  name      salary    gender
   1       Teacher1    20         1
   2       Teacher2    30         1
   3       Teacher3    40         2
   4       Teacher4    50         2
   5       Teacher5    60         1
   6       Teacher6    70         2

Answer 1

I want to get all teacher who does not teachs math.

您需要连接 teacher_id 上的两个表，然后根据类(class)过滤掉行。

SQL> SELECT DISTINCT t.name
  2  FROM course c,
  3    teacher t
  4  WHERE c.teacher_id = t.teacher_id
  5  AND c.course     <> 'Math';

NAME
--------
Teacher2
Teacher1
Teacher4
Teacher3

SQL>

编辑由于您有教授多门类(class)的老师，因此您需要进一步过滤:

SQL> WITH DATA AS
  2    (SELECT c.*,
  3      t.name
  4    FROM course c,
  5      teacher t
  6    WHERE c.teacher_id = t.teacher_id
  7    AND c.course      <> 'Math'
  8    )
  9  SELECT DISTINCT name
 10  FROM data
 11  WHERE teacher_id NOT IN
 12    (SELECT teacher_id FROM course WHERE course = 'Math'
 13    )
 14  /

NAME
--------
Teacher2
Teacher4
Teacher3

SQL>

注意请记住，使用NOT EXISTS子句的另一种解决方案在性能方面更好，因为表扫描更少，甚至索引扫描也更少。有了适当的索引，不存在查询将是最佳方法。