mysql基本操作

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資料庫連接關係圖

學生表與班級表取左外串連

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串連,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  |+-------+-------------+-------+計算女產生績的平均值select avg(age) from students group by gender having gender=‘F‘;

mysql基本操作