MySQL Basic operation

Database Connection diagram

Student table and class table take left outer connection

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 |+-------+---------------+----------------+ Student table and class table take left outer join, and subtract student table from curriculum intersection section select S.stuid,s.name,c.class from S Tudents as S left join classes as C in S.classid=c.classid where C.classid is null;+-------+-------------+-------+| Stuid | name |    Class |+-------+-------------+-------+| 24 | Xu Xian |    NULL | | 25 | Sun Dasheng | NULL |+-------+-------------+-------+ calculates the average of girls ' scores select AVG (age) from students group by gender have gender= ' F ';

MySQL basic operations