如何在单个 SQL CASE 语句中获取多个列？

Question

我正在尝试从单个 CASE 语句中获取多个列（在本例中为被保险人代码、被保险人姓名）。

已尝试以下查询，但它将被保险人代码和被保险人名称连接为一列。

从此类CASE 语句中准确返回两列的正确语法是什么？

select
    case
        when a.policyno[2] in ('E', 'W') then c.insuredcode || c.insuredname
        else b.insuredcode || b.insuredname
    end
from prpcmain a
left join prpcinsured_1 b on b.proposalno=a.proposalno 
left join prpcinsured_2 c on c.proposalno=a.proposalno
where a.policyno in (select policyno from policyno_t);

Answer 1

CASE 语句只能返回单列而不是多列

您需要两个不同的CASE 语句来执行此操作

select
    case
        when a.policyno[2] in ('E', 'W') then c.insuredcode 
        else b.insuredcode 
    end as insuredcode ,
    case
        when a.policyno[2] in ('E', 'W') then  c.insuredname
        else b.insuredname
    end as insuredname
from prpcmain a
left join prpcinsured_1 b on b.proposalno=a.proposalno 
left join prpcinsured_2 c on c.proposalno=a.proposalno
where a.policyno in (select policyno from policyno_t);

Answer 2

我可以建议其他一些可能会稍微快一点的方法：

SELECT s.insuredcode,s.insuredname FROM (
    SELECT a.policyno,b.insuredcode,b.insuredname
    FROM prpcmain a
    left join prpcinsured_1 b on b.proposalno=a.proposalno 
    WHERE a.policyno[2] not in ('E', 'W')
    UNION ALL
    SELECT a.policyno,c.insuredcode,c.insuredname
    FROM prpcmain a
    left join prpcinsured_2 c on c.proposalno=a.proposalno 
    WHERE a.policyno[2]  in ('E', 'W')
) s
where s.policyno in (select policyno from policyno_t);

至于你的问题，@Prdp 显示了你需要做什么。

Answer 3

就像任何语言的if/else条件一样，你可以在When语句中定义你的条件，如果为真，SQL执行Then语句，否则执行 Else 部分，如下所述：

     Select 
                 CASE 
                    WHEN (cs.ResultSubmitToHOD = 1) THEN 'HOD'
                    WHEN (cs.ResultSubmitToExamDep = 1) THEN 'Exam'
                    ELSE 'Teacher'
                END AS ResultSubmitStatus
     From dbo.CourseSection as cs