描述
现有试卷作答记录表exam_record（uid用户ID, exam_id试卷ID, start_time开始作答时间, submit_time交卷时间, score得分）：
| id | uid | exam_id | start_time | submit_time | score |
| 1 | 1001 | 9001 | 2021-09-01 09:01:01 | 2021-09-01 09:41:01 | 81 |
| 2 | 1002 | 9002 | 2021-09-01 12:01:01 | 2021-09-01 12:31:01 | 70 |
| 3 | 1002 | 9001 | 2021-09-01 19:01:01 | 2021-09-01 19:40:01 | 80 |
| 4 | 1002 | 9002 | 2021-09-01 12:01:01 | 2021-09-01 12:31:01 | 70 |
| 5 | 1004 | 9001 | 2021-09-01 19:01:01 | 2021-09-01 19:40:01 | 85 |
| 6 | 1002 | 9002 | 2021-09-01 12:01:01 | (NULL) | (NULL) |
题目练习表practice_record（uid用户ID, question_id题目ID, submit_time提交时间, score得分）：
| id | uid | question_id | submit_time | score |
| 1 | 1001 | 8001 | 2021-08-02 11:41:01 | 60 |
| 2 | 1002 | 8001 | 2021-09-02 19:30:01 | 50 |
| 3 | 1002 | 8001 | 2021-09-02 19:20:01 | 70 |
| 4 | 1002 | 8002 | 2021-09-02 19:38:01 | 70 |
| 5 | 1003 | 8001 | 2021-08-02 19:38:01 | 70 |
| 6 | 1003 | 8001 | 2021-08-02 19:48:01 | 90 |
| 7 | 1003 | 8002 | 2021-08-01 19:38:01 | 80 |
请统计每个题目和每份试卷被作答的人数和次数，分别按照"试卷"和"题目"的uv & pv降序显示，示例数据结果输出如下：
| tid | uv | pv |
| 9001 | 3 | 3 |
| 9002 | 1 | 3 |
| 8001 | 3 | 5 |
| 8002 | 2 | 2 |
解释：“试卷”有3人共练习3次试卷9001，1人作答3次9002；“刷题”有3人刷5次8001，有2人刷2次8002
SELECT
exam_id AS tid,
COUNT(DISTINCT exam_record.uid) uv,
COUNT(*) pv
FROM
exam_record
GROUP BY
exam_id
UNION
SELECT
question_id AS tid,
COUNT(DISTINCT practice_record.uid) uv,
COUNT(*) pv
FROM
practice_record
GROUP BY
question_id
ORDER BY
LEFT(tid, 1) DESC,
uv DESC,
pv DESC;
🔍 分步拆解与核心知识点
🧱 第一部分：考试行为统计（Exam UV/PV）
SELECT
exam_id AS tid,
COUNT(DISTINCT uid) AS uv,
COUNT(*) AS pv
FROM exam_record
GROUP BY exam_id| 字段 | 说明 |
|---|---|
exam_id AS tid |
统一别名为 tid（目标 ID），便于合并 |
COUNT(DISTINCT uid) |
计算该试卷的独立用户数（UV） |
COUNT(*) |
总作答次数（PV） |
GROUP BY exam_id |
按试卷分组统计 |
🧱 第二部分：练习行为统计（Question UV/PV）
SELECT
question_id AS tid,
COUNT(DISTINCT uid) AS uv,
COUNT(*) AS pv
FROM practice_record
GROUP BY question_id结构与第一部分完全一致，只是数据源不同。
🧱 合并：UNION
-- 第一个查询
...
UNION
-- 第二个查询- ✅
UNION将两个结构相同的查询结果纵向拼接 - 要求：各列类型和顺序必须一致（这里都是
tid,uv,pv） - 自动去重（如用
UNION ALL则不去重）
🧱 排序：ORDER BY LEFT(tid, 1) DESC, uv DESC, pv DESC
ORDER BY
LEFT(tid, 1) DESC, -- 按 tid 的第一个字符降序
uv DESC,
pv DESC