SQL查询，是否需要GROUP BY？如何避免不必要的复杂语法

Question

我正在构建一个查询，该查询将基于 4 tables 构建一个非常大的数据集。到目前为止，这是我的查询：

SELECT  
CLIENT_CA."Requisition_ID",  
CLIENT_REQS."Requisition Title",  
CLIENT_REQS."Country",  
CLIENT_CD."Application_Status",  
CLIENT_CA."Candidate",  
CLIENT_CD."Gender",  
max(case when 
CLIENT_CA."Process_of_Activity" = 'Application Entry' then 
CLIENT_CA."Completed_on" end) as "ENTRY",  
max(case when 
CLIENT_CA."Process_of_Activity" = 'Candidate Selection' then 
CLIENT_CA."Completed_on" end) as "Selection",  
max(case when 
CLIENT_CA."Process_of_Activity" = 'Job Offer' then 
CLIENT_CA."Completed_on" end) as "Offer",  
CLIENT_RR."Status_Reason(E-Rec)"   
FROM CLIENT_CA  
LEFT JOIN CLIENT_CD ON (CLIENT_CA."Candidate_ID"=CLIENT_CD."Candidate_ID"
and CLIENT_CA."Requisition_ID"=CLIENT_CD."Requisition_ID" )  
LEFT JOIN CLIENT_REQS ON CLIENT_REQS."Requisition_ID"=CLIENT_CA."Requisition_ID"  
LEFT JOIN CLIENT_RR
ON (CLIENT_CA."Candidate_ID"=CLIENT_RR."Candidate_ID" and CLIENT_CA."Requisition_ID"=CLIENT_RR."Requisition_ID" )  
GROUP BY CLIENT_CA."Candidate"
   ,CLIENT_CA."Requisition_ID" 
  ,CLIENT_CD."Gender"
  ,CLIENT_REQS."Requisition Title"
  ,CLIENT_REQS."Country" 
  ,CLIENT_RR."Status_Reason(E-Rec)" 
  ,CLIENT_CD."Application_Status";

从技术上讲，这个查询工作得很好，准确无误地提供了我需要的信息。问题是我需要在 SELECT 子句中添加大约 80 列，而系统或更确切地说是逻辑迫使我将我添加的每一列都放在 GROUP BY 子句中......我猜这是因为 "max" 函数。基本上 CLIENT_CA 是我的 "base" table，我正在尝试实现在 MS Excel 中由其他 3 table 上的 VLOOKUP 函数完成的功能。我只是有点担心我让这件事变得有点不必要的复杂。请毫不犹豫地告诉我，我只是个愚蠢的人 :) 但前提是您有建议如何做得更好 :) 谢谢。哦....我在 dashDB 上运行这个。

Answer 1

使用子查询从主查询中提取 table，连接列和计算列（条目、选择、报价），这样您就可以将分组列限制为固定数量

SELECT  
CA."Requisition_ID",  
REQS."Requisition Title",  
REQS."Country",  
CD."Application_Status",  
CA."Candidate",  
CD."Gender",  
"ENTRY",  
"Selection",  
"Offer",  
RR."Status_Reason(E-Rec)"   
FROM ( 
    SELECT 
        "Requisition_ID", "Candidate_ID", "Candidate",
        max(case when "Process_of_Activity" = 'Application Entry' then "Completed_on" end) as "ENTRY",  
        max(case when "Process_of_Activity" = 'Candidate Selection' then "Completed_on" end) as "Selection",  
        max(case when "Process_of_Activity" = 'Job Offer' then "Completed_on" end) as "Offer"
    FROM CLIENT_CA
    GROUP BY "Candidate", "Requisition_ID", "Candidate_ID" 
    ) AS CA 
LEFT JOIN CLIENT_CD AS CD ON (CA."Candidate_ID"=CD."Candidate_ID" and CA."Requisition_ID"=CD."Requisition_ID" )  
LEFT JOIN CLIENT_REQS AS REQS ON REQS."Requisition_ID"=CA."Requisition_ID" 
LEFT JOIN CLIENT_RR AS RR ON (CA."Candidate_ID"=RR."Candidate_ID" and CA."Requisition_ID"=RR."Requisition_ID" )

SQL查询，是否需要GROUP BY？如何避免不必要的复杂语法

SQL query, is GROUP BY needed? how to avoid unnecessarily complicated syntax

sql

dashdb