如果 count(*) 大于 0，则输出 1。否则输出 0

Question

我有一张如下所示的表格：

+--------------+----------------+
| PersonColumn | AttendedColumn |
+ ------------ + -------------- +
| person1      | attended       |
| person1      | not attended   |
| person2      | not attended   |
| person2      | not attended   |
| person2      | not attended   |
| person3      | attended       |
| person3      | attended       |
+--------------+----------------+

所以我想要一个 select 语句来输出这个：

+---------+-----+
| person1 |  1  |
| person2 |  0  |
| person3 |  1  |
+---------+-----+

也就是说，如果该人至少参加过一次，则输出 1。否则输出零。我正在使用 ORACLE

Answer 1

Select personcolumn, max(decode(attendedcolumn,'attended',1,0)) from ... group by personcolumn

Answer 2

您可以使用CASE 和GROUP BY 使用条件聚合。

例如：

select
  personcolumn,
  case when 
    sum(case when attendedcolumn = 'attended' then 1 else 0 end) > 0
    then 1
    else 0 
  end as attended
from t
group by personcolumn

结果：

 PERSONCOLUMN  ATTENDED 
 ------------- -------- 
 person1       1        
 person2       0        
 person3       1        

请参阅db<>fiddle 的运行示例。

Answer 3

你可以使用简单的max():

select personcolumn,
       max(case when attendedcolumn = 'attended' then 1 else 0 end) 
from t
group by personcolumn;