數據庫連接關係圖
學生表與班級表取左外連接
select s.stuid,s.name,c.class from students as s left join classes as c on s.classid=c.classid;
+-------+---------------+----------------+
| stuid | name | class |
+-------+---------------+----------------+
| 1 | Shi Zhongyu | Emei Pai |
| 2 | Shi Potian | Shaolin Pai |
| 3 | Xie Yanke | Emei Pai |
| 4 | Ding Dian | Wudang Pai |
| 5 | Yu Yutong | QingCheng Pai |
| 6 | Shi Qing | Riyue Shenjiao |
| 7 | Xi Ren | QingCheng Pai |
| 8 | Lin Daiyu | Ming Jiao |
| 9 | Ren Yingying | Lianshan Pai |
| 10 | Yue Lingshan | QingCheng Pai |
| 11 | Yuan Chengzhi | Lianshan Pai |
| 12 | Wen Qingqing | Shaolin Pai |
| 13 | Tian Boguang | Emei Pai |
| 14 | Lu Wushuang | QingCheng Pai |
| 15 | Duan Yu | Wudang Pai |
| 16 | Xu Zhu | Shaolin Pai |
| 17 | Lin Chong | Wudang Pai |
| 18 | Hua Rong | Ming Jiao |
| 19 | Xue Baochai | Lianshan Pai |
| 20 | Diao Chan | Ming Jiao |
| 21 | Huang Yueying | Lianshan Pai |
| 22 | Xiao Qiao | Shaolin Pai |
| 23 | Ma Chao | Wudang Pai |
# | 24 | Xu Xian | NULL |
# | 25 | Sun Dasheng | NULL |
+-------+---------------+----------------+
學生表與班級表取左外連接,並且減去學生表與課程表交集部分
select s.stuid,s.name,c.class from students as s left join classes as c on s.classid=c.classid where c.classid is NULL;
+-------+-------------+-------+
| stuid | name | class |
+-------+-------------+-------+
| 24 | Xu Xian | NULL |
| 25 | Sun Dasheng | NULL |
+-------+-------------+-------+
計算女生成績的平均值
select avg(age) from students group by gender having gender='F';