我想知道如何根据配置单元sql中的条件运行不同的select语句。
以下查询不起作用，但引发错误。
编译语句时出错：失败：parseexception行4:2无法识别表达式规范中“（”选择“1”）附近的输入

SELECT 
    CASE WHEN '${UN}'!= '' THEN 
        (
        SELECT * 
            from table1 t 
            WHERE t.yymmddval BETWEEN '${D1}' AND '${D2}'
            AND t.un in ('${UN}')
        )
    ELSE
        (
        SELECT * 
            from table1 t 
            WHERE t.yymmddval BETWEEN '${D1}' AND '${D2}'
            AND t.un in (
               (SELECT
                o.unq_num as un
                FROM table2 as o
                WHERE o.date >= '2017-01-01'
                    AND upper(o.srl_num) in ('${R}')
                LIMIT 1)            
            )       
        )
    END