mysql查询学生平均成绩及其名次
``` SELECT student_name,AVG(grade) as avg_grade,(SELECT COUNT(DISTINCT(AVG(grade))) FROM student_grade WHERE
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``` SELECT student_name,AVG(grade) as avg_grade,(SELECT COUNT(DISTINCT(AVG(grade))) FROM student_grade WHERE
``` SELECT * FROM SCORE ORDER BY SUBJECT, SCORE DESC LIMIT 3; ```
： SELECT course_name, COUNT(*) AS students_number FROM course_registration GROUP BY course_name;
``` SELECT s.name AS 姓名, c.name AS 课程名称, s.score AS 成绩, AVG(s.score) AS 平均成绩 FROM student s, course c WHERE
``` select stu_id,stu_name,AVG(score) from student inner join score on student.stu_id=score.stu_id where sco
SELECT teacher,AVG(score) AS avg_score FROM course GROUP BY teacher ORDER BY avg_score DESC;
centos定时备份mysql数据库并删除过期的（自己设定）备份 新建 mysql_autobackup.sh脚本，内容如下 ``` #!/bin/bash /usr/bin/mysqldump -uus
SELECT stu_no FROM student_course WHERE course_no IN (SELECT course_no FROM student_course WHERE stu_no=01
SELECT stu_no,stu_name FROM student WHERE stu_no IN (SELECT stu_no FROM course_stu WHERE course_no = 01 I
``` SELECT a.name,a.chinese,a.math,a.english, (SELECT COUNT(*) FROM Table b WHERE b.chinese > a.ch
1、减少不必要的数据库操作：尽量将多次查询合并成一次查询，使用存储过程，将多次查询合并成一次查询，减少数据库的操作次数。 2、优化SQL语句：可以在SQL语句中使用表的别名，加快查询速度；使用EXPLAIN关键
1、为已有表添加索引： ALTER TABLE `table_name` ADD INDEX `index_name` (`column_1`, `column_2`, ...); 2、为已有表添加唯一索引
可以使用--ignore-table参数来排除某些表，比如： mysqldump -u root -p db_name --ignore-table=db_name.table_name > db_name.s
1、清除主库上的复制信息： ``` mysql> RESET MASTER; ``` 2、清除从库的复制信息： ``` mysql> RESET SLAVE; ``` 3、在主库上清除从库的连接信息： ```
这是一个 mySQL 错误，表明 INFORMATION_SCHEMA.GLOBAL_STATUS 功能被禁用了。这意味着用户无法查询全局状态信息，如果你想使用该功能，你需要给 INFORMATION_SCHEM