select u_i.uid as uid, count(distinct act_month) as act_month_total, count(distinct case when year(act_time) = 2021 then act_day end) as act_days_2021, count(distinct case when year(act_time) = 2021 and tag = 'exam' then act_day end) as act_days_2021_exam, count(distinct case when year(act_time) = 2021 and tag = 'question' then act_day end) as act_days_2021_question from user_info u_i left join (select uid, start_time as act_time, date_format(start_time, '%Y%m') as act_month, date_format(start_time, '%Y%m%d') as act_day, 'exam' as tag from exam_record union all select uid, submit_time as act_time, date_format(submit_time, '%Y%m') as act_month, date_format(submit_time, '%Y%m%d') as act_day, 'question' as tag from practice_record ) exam_and_practice on exam_and_practice.uid = u_i.uid where u_i.level >= 6 group by uid order by act_month_total desc, act_days_2021 desc; 请用oracle语法来改写这段sql

以下是使用 Oracle 语法改写后的 SQL 查询语句：

SELECT u_i.uid, 
       COUNT(DISTINCT act_month) AS act_month_total, 
       COUNT(DISTINCT CASE 
                        WHEN EXTRACT(YEAR FROM act_time) = 2021 THEN act_day
                      END) AS act_days_2021, 
       COUNT(DISTINCT CASE 
                        WHEN EXTRACT(YEAR FROM act_time) = 2021 AND tag = 'exam' THEN act_day
                      END) AS act_days_2021_exam, 
       COUNT(DISTINCT CASE 
                        WHEN EXTRACT(YEAR FROM act_time) = 2021 AND tag = 'question' THEN act_day
                      END) AS act_days_2021_question
FROM user_info u_i 
LEFT JOIN (SELECT uid, 
                  start_time AS act_time, 
                  TO_CHAR(start_time, 'YYYYMM') AS act_month, 
                  TO_CHAR(start_time, 'YYYYMMDD') AS act_day, 
                  'exam' AS tag 
           FROM exam_record 
           UNION ALL 
           SELECT uid, 
                  submit_time AS act_time, 
                  TO_CHAR(submit_time, 'YYYYMM') AS act_month, 
                  TO_CHAR(submit_time, 'YYYYMMDD') AS act_day, 
                  'question' AS tag 
           FROM practice_record) exam_and_practice 
ON exam_and_practice.uid = u_i.uid 
WHERE u_i.level >= 6 
GROUP BY u_i.uid 
ORDER BY act_month_total DESC, act_days_2021 DESC;

在 Oracle 中，日期时间的函数使用 `EXTRACT`，日期格式化使用 `TO_CHAR`。同时，Oracle 中使用分号作为语句结束符。