Redshift 嵌套案例 when/ If then else

Question

当一个语句有多个条件时，是否有嵌套 case when 语句的方法。例如下面，其中num_students = 2，有很多差异类。因此 num_of students =2 的 else 语句是 'Stu_2，但对于整体数据来说是 'unk'

select id, test_a, test_b,
case when num_students = 5 then 'Stu_5'
when num_students = 4 then 'Stu_4'
when num_students = 3 then 'Stu_3'
  when num_students = 2 and class = 'Eng' then 'Stu_Eng_2'
  when num_students =2 and class = 'Fre' then 'Stu_Fre_2'
  when num_students = 2 and class = 'His' then 'Stu_His_2'
  when num_students =2 and class = 'Geo' then 'Stu_Geo_2'
  else 'Stu_2'
else 'unk'
from table

Answer 1

您可以按不同的顺序来表述：

(case when num_students = 5 then 'Stu_5'
      when num_students = 4 then 'Stu_4'
      when num_students = 3 then 'Stu_3'
      when num_students <> 2 or num_students is null then 'unk'
      -- num_students must be 2
      when class = 'Eng' then 'Stu_Eng_2'
      when class = 'Fre' then 'Stu_Fre_2'
      when class = 'His' then 'Stu_His_2'
      when class = 'Geo' then 'Stu_Geo_2'
      else 'Stu_2'
 end)

Answer 2

是的，这是嵌套 case 语句的目的之一。嵌套时，需要再次调用CASE语句如下：

select 
  id, 
  test_a, 
  test_b,
  case 
    when num_students = 5 then 'Stu_5'
    when num_students = 4 then 'Stu_4'
    when num_students = 3 then 'Stu_3'
    when num_students = 2 then
      case when class = 'Eng' then 'Stu_Eng_2'
           when class = 'Fre' then 'Stu_Fre_2'
           when class = 'His' then 'Stu_His_2'
           when class = 'Geo' then 'Stu_Geo_2'
      else 'Stu_2' end
  else 'unk' end as <column_name>
from <table_name>

不要忘记每个嵌套的 CASE WHEN 后的 END 子句 :)

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