```
SELECT stu_name,score_sum,
@curRank := @curRank + 1 AS rank
FROM student,(SELECT @curRank := 0) r
ORDE
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SELECT student_name, course_name,score FROM student_course WHERE score>70;
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SELECT stu_no FROM student_course WHERE course_no IN (SELECT course_no FROM student_course WHERE stu_no=01
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```
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
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mysql>showglobalstatuslikeuptime;
+---------------+--------+
|Variable_name|Value|
+---------------+--------
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1. 查看该数据库实例下所有库大小，得到的结果是以MB为单位mysql>selecttable_schema,sum(data_length)/1024/1024asdata_length,sum(index_len
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Student(S#,Sname,Sage,Ssex) 学生表Course(C#,Cname,T#) 课程表SC(S#,C#,score) 成绩表Teacher(T#,Tname) 教师表问题：1、查询“001”课程
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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
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