没有记录时显示列值为 0 的记录

Question

我有以下表格：

架构

tbl_semester_empid {id，semester_name varchar(50)，start_date}

tbl_batch_empid {id、semester_id、batch_name、session_room}

tbl_associate_empid {id, associate_name, batch_id, contact, join_date,duction_result, stream_result int}

问题查询是：显示学期明智的员工人数。

我写了 3 个查询：

--basic query
SELECT s.ID "Semester Id", COUNT(*) "Associate Count" FROM TBL_SEMESTER_593932 s, TBL_ASSOCIATE_593932 a, TBL_BATCH_593932 b 
        WHERE s.ID=b.SEMESTER_ID AND a.BATCH_ID=b.ID  
                GROUP BY s.ID;

--used NVL function
SELECT s.ID "Semester Id", NVL(COUNT(*),0) "Associate Count" FROM TBL_SEMESTER_593932 s, TBL_ASSOCIATE_593932 a, TBL_BATCH_593932 b 
        WHERE s.ID=b.SEMESTER_ID AND a.BATCH_ID=b.ID  
                GROUP BY s.ID;

-- used LEFT OUTER JOIN and  JOIN
SELECT s.ID "Semester Id", NVL(COUNT(*),0) "Associate Count" 
        FROM TBL_SEMESTER_593932 s LEFT OUTER JOIN TBL_BATCH_593932 b ON s.ID=b.SEMESTER_ID 
             JOIN TBL_ASSOCIATE_593932 a ON a.BATCH_ID=b.ID  
                   GROUP BY s.ID;

全部显示如下输出（只显示至少有1个学生的学期对应的记录，如果该学期没有学生，则不显示该学期的信息）：

我希望在 Associates Count 列中没有员工的学期为 0。所以我用两个 LEFT OUTE JOIN 尝试了同样的方法：

SELECT s.ID "Semester Id", NVL(COUNT(*),0) "Associate Count" 
    FROM TBL_SEMESTER_593932 s LEFT OUTER JOIN TBL_BATCH_593932 b ON s.ID=b.SEMESTER_ID 
         LEFT OUTER JOIN TBL_ASSOCIATE_593932 a ON a.BATCH_ID=b.ID  
                GROUP BY s.ID;

但是结果很奇怪：

Answer 1

基本上，您要做的就是使用所有可用学期的列表，然后计算每个学期的金额：

select s.ID, 
       s.Semester_Name,
       NVL ((select count(*) 
          from TBL_ASSOCIATE_593932 a
               join TBL_BATCH_593932 b 
                 on a.BATCH_ID=b.ID
        where  s.ID=b.SEMESTER_ID
       ),0) as Amount
 from TBL_SEMESTER_593932 s

另一种方法是加入您的所有学期列表，然后计算

select s.ID, 
       s.Semester_Name,
       count(*) as Amount 
 from TBL_SEMESTER_593932 s
        left join (
          TBL_BATCH_593932 b
          join TBL_ASSOCIATE_593932 a  
            on a.BATCH_ID=b.ID)
           on s.ID=b.SEMESTER_ID

group by s.ID, s.Semester_Name
order by s.ID

[编辑]：根据@Mahesha999 更正了最后一个查询。

Answer 2

您尝试过使用CASE when 吗？

根据您的关联表中的数据，所有批次/学期都有记录。

所以试试这个，你的 NVL 和 case 很相似……

select s.id, s.name, CASE WHEN count(a.id) IS NULL THEN 0 ELSE count(a.id) END as ACount
from tblsem s
left join tblbatch b
on b.sid = s.id
left join tblaso a
on b.id = a.batchid
group by s.id, s.name
;

结果：

ID  NAME    ACOUNT
1   sem1    4
2   sem2    3
3   sem3    0

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