Question

我正在使用下面的oracle查询来获取行数。

SELECT T.ID,T.NAME,COUNT(*) AS NO_OF_STUDENTS FROM STUDENT S RIGHT JOIN 
TEACHER T ON S.TEACHER_ID = T.ID
GROUP BY T.ID,T.NAME ORDER BY T.ID

实际结果应为：

TEACHER 1 - 10 STUDENTS   
TEACHER 2 - 5 STUDENTS   
TEACHER 3 - 0 STUDENT

我得到的结果是：

TEACHER 1 - 10 STUDENTS   
TEACHER 2 - 5 STUDENTS   
TEACHER 3 - 1 STUDENT

由于教师3没有任何学生，因此结果应为0学生。但是我得到的结果是1个学生。

Answer 1

您需要计算与外部连接的表中的特定列（不使用*，包括空值），因为该列可能没有匹配的数据。所以：

SELECT T.ID, T.NAME, COUNT(S.ID) AS NO_OF_STUDENTS
FROM STUDENT S
RIGHT JOIN TEACHER T ON S.TEACHER_ID = T.ID
GROUP BY T.ID, T.NAME
ORDER BY T.ID

唯一的区别是COUNT(S.ID)而不是COUNT(*)。

具有通过CTE提供的虚构数据的简单演示：

with teacher (id, name) as (
  select 1, 'Teacher 1' from dual
  union all select 2, 'Teacher 2' from dual
  union all select 3, 'Teacher 3' from dual
),
student (id, teacher_id) as (
  select level, 1 from dual connect by level <= 10
  union all
  select level + 10, 2 from dual connect by level <= 5
)  
SELECT T.ID, T.NAME, COUNT(S.ID) AS NO_OF_STUDENTS
FROM STUDENT S
RIGHT JOIN TEACHER T ON S.TEACHER_ID = T.ID
GROUP BY T.ID, T.NAME
ORDER BY T.ID;

        ID NAME      NO_OF_STUDENTS
---------- --------- --------------
         1 Teacher 1             10
         2 Teacher 2              5
         3 Teacher 3              0

您也可以将其作为左联接来执行，我觉得这更直观：

SELECT T.ID, T.NAME, COUNT(S.ID) AS NO_OF_STUDENTS
FROM TEACHER T
LEFT JOIN STUDENT S ON S.TEACHER_ID = T.ID
GROUP BY T.ID, T.NAME
ORDER BY T.ID

获得相同的结果。

分组依据中的Count（*）-返回1，而不是“ 0”

1 个答案: