没有记录时显示列值为0的记录

Question

我有以下表格：

模式

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，joined_date，induction_result，stream_result int}

问题查询是：显示学期明智的关联计数。

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;

所有这些都显示以下输出（它显示的记录仅对应于其中至少有一名学生的学期，如果该学期不是学生，则不会显示该学期的信息）：

我希望没有关联者的学期在“关联计数”列中为0。所以我尝试了两个LEFT OUTE 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 
         LEFT OUTER JOIN TBL_ASSOCIATE_593932 a ON a.BATCH_ID=b.ID  
                GROUP BY s.ID;

但结果很奇怪：

Answer 1

您是否尝试过使用CASE when？

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与案例类似......

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

• SQLFIDDLE DEMO

Answer 2

基本上，你要做的就是使用所有可用学期的清单，然后计算每个学期的数量：

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

另一种方法是加入你所有学期的名单，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。