Question

让我们说我有一个带有类ID的表“ trips”，这是另一个表“ classes”的外键，还有一个列“ trip”，它只是文本。

62 'Zoo'
62 'Park'
62 'Observatory'
64 'Park'
64 'Zoo'
81 'Park'

我想算一下没有上天文台的班级数。我怎么知道的？

Answer 1

一种方法是聚合：

select class
from trips t
group by class
having sum(case when trip = 'Observatory' then 1 else 0 end) = 0;

如果有一个类表（您可能这样做），请使用not exists：

select c.*
from classes c
where not exists (select 1
                  from trips t
                  where t.classid = c.classid and
                        t.trip = 'Observatory'
                 );

Answer 2

计算班级数量，计算去天文台的班级数量，然后减去：

WITH cteTotal_class_count AS (SELECT COUNT(*) AS TOTAL_CLASS_COUNT
                                FROM CLASSES),
     cteObservatory_class_count AS (SELECT COUNT(DISTINCT CLASS_ID) AS OBSERVATORY_CLASS_COUNT
                                      FROM TRIPS
                                      WHERE UPPER(TRIP) = 'OBSERVATORY')
SELECT TOTAL_CLASS_COUNT - OBSERVATORY_CLASS_COUNT AS COUNT_OF_CLASSES_WHO_DIDNT_GO
  FROM cteTotal_class_count
  CROSS JOIN cteObservatory_class_count

dbfiddle here

Answer 3

与上一个答案的想法相同，但有所简化：

SELECT (SELECT count(distinct class_id) FROM trips) - 
       (SELECT count(distinct class_id) FROM trips WHERE trip = 'Observatory');

计算有多少ID（fk），其中ID（fk）相同，但是列值！=给定值

3 个答案: