在sql情況下when語句為NULL

Question

我將按年齡將人們按年齡分類，並帶有case when語句，因此

select 
        case when age >= 11 and age <= 20 then name end column1 as "11-20",
        case when age >= 21 and age <= 30 then name end column2 as "21-30",
        case when age >= 31 and age <= 40 then name end column3 as "31-40"
from passenger

結果是這樣的：

11-20    21-30    31-40
John     NULL     NULL
NULL     Kevin    NULL
Michael  NULL     NULL
NULL     NULL     Beckey

我希望輸出看起來像：

11-20    21-30    31-40
John     Kevin    Beckey
Michael 

所以我該如何刪除NULL或將非NULL值向上移動？

Answer 1

根據戈登·林多夫（G​​ordon Lindoff）的Oracle（？）答案，您可以在MySQL中執行以下操作：

select
  max(case when agegroup = '11-20' then name end) as "11-20",
  max(case when agegroup = '21-30' then name end) as "21-30",
  max(case when agegroup = '31-40' then name end) as "31-40"
from 
  (select @rownum1 := 0, @rownum2 := 0, @rownum3 := 0) v,
  (select
     case
       when Age > 10 and Age <= 20 then '11-20' 
       when Age > 20 and Age <= 30 then '21-30' 
       when Age > 30 and Age <= 40 then '31-40'
     end as agegroup,
     case
       when Age > 10 and Age <= 20 then @rownum1 := @rownum1 + 1 
       when Age > 20 and Age <= 30 then @rownum2 := @rownum2 + 1 
       when Age > 30 and Age <= 40 then @rownum3 := @rownum3 + 1
     end as seqnum,
     name
   from
     Passengers) p
group by
  seqnum

演示： http : //sqlfiddle.com/#!2/5ea1a/9

擺弄給定的數據。

Answer 2

通過對group by進行group by可以刪除NULL 。 但是，您沒有group by的密鑰。 大多數數據庫都支持row_number()函數。 使用該功能，您可以執行以下操作：

select max(case when agegroup = '11-20' then name end) as "11-20",
       max(case when agegroup = '21-30' then name end) as "21-30",
       max(case when agegroup = '31-40' then name end) as "31-40"
from (select p.*, row_number() over (partition by agegroup order by name) as seqnum
      from (select p.*,
                   (case when age >= 11 and age <= 20 then '11-20'
                         when age >= 21 and age <= 30 then '21-30'
                         when age >= 31 and age <= 40 then '31-40'
                    end) as agegroup
            from passenger p
           ) p
group by seqnum;

編輯：

MySQL中的等效項使用變量：

select max(case when agegroup = '11-20' then name end) as "11-20",
       max(case when agegroup = '21-30' then name end) as "21-30",
       max(case when agegroup = '31-40' then name end) as "31-40"
from (select p.*,
             @rn := if(agegroup = @agegroup, @rn + 1, 1) as seqnum,
             @agegroup := agegroup
      from (select p.*,
                   (case when age >= 11 and age <= 20 then '11-20'
                         when age >= 21 and age <= 30 then '21-30'
                         when age >= 31 and age <= 40 then '31-40'
                    end) as agegroup
            from passenger p
           ) p cross join
           (select @rn := 0, @agegroup := '') var
       order by agegroup
      ) p
group by seqnum;