如何在Proc SQL的CASE表达式中调用存储为变量值的条件表达式？

我使用CASE表达式验证var1的值并为新变量var1_val赋值，相关规则依赖两个数据集：

• 数据集response_options_scan的valid_values列存储var1的所有有效值；
• 数据集response_options_select的requirement_status列定义var1的必填时机，值包括"All Records"、"Required"或"var2 contains xyz"这类动态条件。

由于代码要用于宏中批量验证数十个变量，我希望在CASE的第三个WHEN条件中直接调用requirement_status的存储值，而非硬编码条件。但硬编码写法能得到正确结果，直接SELECT调用requirement_status的写法却不符合预期。

硬编码方式（结果正确） #

proc sql noprint;
    create table validate_select_test
    as select *
        ,case when var1 in (select valid_values from response_options_scan where variable="var1") then "var1 valid value"
        when var1="" and (select requirement_status from response_options_select where variable="var1") in ("All Records", "Required") then "var1 invalid value: missing" 
        when var1="" and var2 contains 'XYZ' then "var1 invalid value: missing"
        when var1="" then "var1 valid value: missing"
        else "var1 invalid value"
        end as var1_val format=$100.
    from validate_select;
quit;

SELECT调用方式（结果不符合预期） #

注：response_options_select中variable="var1"对应的requirement_status值为"var2 contains 'XYZ'"，预期var2不含XYZ且var1为空时，var1_val应为"var1 valid value: missing"。

proc sql noprint;
    create table validate_select_test
    as select *
        ,case when var1 in (select valid_values from response_options_scan where variable="var1") then "var1 valid value"
        when var1="" and (select requirement_status from response_options_select where variable="var1") in ("All Records", "Required") then "var1 invalid value: missing" 
        when var1="" and (select requirement_status from response_options_select where variable="var1") then "var1 invalid value: missing"
        when var1="" then "var1 valid value: missing"
        else "var1 invalid value"
        end as var1_val format=$100.
    from validate_select;
quit;

内容的提问来源于stack exchange，提问作者mvanduym