在配置單元中的指定條件下從單行創建多行

Question

我試圖執行空檢查。對於e.g：

Col_A | Col_B | Col_C | Col_D 
null | boy | null | dust 

然後我想作爲輸出：

Col_A | Col_B | Col_C | Col_D | New_Col 
null | boy | null | dust | Col_A failed null check 
null | boy | null | dust | Col_D failed null check 

什麼是做到這一點的正確方法？

Answer 1

select t.* 
     ,concat(elt(e.pos+1,'Col_A','Col_B','Col_C','Col_D'),' failed null check') as New_Col 
from mytable t lateral view posexplode (array(Col_A,Col_B,Col_C,Col_D)) e 
where e.val is null 

Answer 2

一種方法是使用union all：

select Col_A, Col_B, Col_C, Col_D, 'Col_A failed NULL check' as new_col 
from t 
where Col_A is null 
union all 
select Col_A, Col_B, Col_C, Col_D, 'Col_B failed NULL check' as new_col 
from t 
where Col_B is null 
union all 
select Col_A, Col_B, Col_C, Col_D, 'Col_C failed NULL check' as new_col 
from t 
where Col_C is null 
union all 
select Col_A, Col_B, Col_C, Col_D, 'Col_D failed NULL check' as new_col 
from t 
where Col_D is null; 

這是相當強力。如果您有很多列，則可以使用電子表格生成SQL。這也需要爲每個子查詢單獨掃描。