Question

我有一个看起来像这样的表：

Teacher   | Class | Number of students
Ms. Smith | 101   | 101
Mr. Frank | 300   | 20
Mr. Frank | 401   | 0
Mr. Smith | 102   | 20
Ms. Date  | 300   | 0
Mr. First | 100   | 10

我想查询并找回一张已删除任何教师（他们所有的行）的表，这些教师的班级为零，所以我要找的结果是：

Teacher   | Class | Number of students
Ms. Smith | 101   | 101
Mr. Smith | 102   | 20
Mr. First | 100   | 10

知道我怎么能这样做吗？我很难过。

Answer 1

使用not exists：

select *
from teachers t
where not exists (
    select 1 
    from teachers
    where number_of_students = 0 and teacher = t.teacher)

  teacher  | class | number_of_students 
-----------+-------+--------------------
 Ms. Smith |   101 |                101
 Mr. Smith |   102 |                 20
 Mr. First |   100 |                 10
(3 rows)

Answer 2

如果您不能使用子查询：

SELECT t1.Teacher, t1.Class, t1.number_of_students 
FROM yourTable t1
LEFT JOIN yourTable t2
  ON t1.teacher = t2.teacher
 AND t2.number_of_students = 0
WHERE t2.teacher IS NULL

Answer 3

您的表格未正常化。您别无选择，只能使用教师姓名。如果有同名教师，这将是一个问题，但这是你如何做到的：

SELECT *
FROM teachers t1
WHERE t1.teacher NOT IN (SELECT t2.teacher
                    FROM teachers t2
                    WHERE t2.teacher = t1.teacher
                    AND number_of_students = 0)

或者可能更简单的方法：

SELECT *
FROM teachers t1
WHERE (SELECT COUNT(*)
       FROM teachers t2
       WHERE t2.teacher = t1.teacher
       AND number_of_students = 0) > 0

或者你可以使用Klin稍微好一点的答案。

如何从postgres表中删除列中具有相同值的行，如果这些行中的任何行在另一列中包含特定值？

3 个答案: