Question

我正在尝试在包含多个值的基于一列的表中添加标志。例如。

Student_id |       Exam_type   
   123             Practice  
   123          Recertification
   123            Certification
   456             Practice
   456            Certification  
   789          Recertification
   135             Practice
   246             Practice
   246            Certification

我希望能够标记出哪些学生参加了实践考试。输出必须是：

Student_id | Practice_taken
123              Y
456              Y
789              N
135              Y
246              Y

Answer 1

您可以使用条件聚合和case表达式：

select student_id,
       (case when sum(exam_type = 'Practice' then 1 else 0 end) > 0
             then 'Y' else 'N'
        end) as practice_taken
from t
group by student_id;

Answer 2

您可以尝试使用row_number（）

select student_id, case when max(rn)=1 then 'Y' else 'N' end as Practice_taken from
(select student_id,row_number() over(partition by student_id order by case when exam_type = 'Practice' then 1 else 0 end desc) as rn
from t) where rn=1
group by student_id

Answer 3

其他选择是使用exists：

select student_id, (case when exists (select 1 
                                      from table t1 
                                      where t1.student_id = t.student_id and
                                            t1.Exam_type = 'Practice'
                                     ) 
                         then 'Y' else 'N' 
                    end) as practice_taken
from table t
group by student_id;

SQL根据具有多个值的一列添加标志

3 个答案: