#查所有年龄在20岁以下的学生姓名及年龄
#SELECT sname,sage from s where sage<20
#查考试成绩有不及格的学生的学号
#SELECT sno from sc where grade < 90
#查所年龄在20至23岁之间的学生姓名、系别及年龄。
#SELECT sname,sdept,sage from s where sage BETWEEN 20 and 23
#查计算机系、数学系、信息系的学生姓名、性别。
#SELECT sname,ssex from s where sdept in (1409,1408)
#查既不是计算机系、数学系、又不是信息系的学生姓名、性别
#SELECT sname,ssex from s where sdept not in (1409,1408)
#查所有姓“刘”的学生的姓名、学号和性别。
#SELECT sname,ssex,sno from s where sname like '刘%'
#查姓“上官”且全名为3个汉字的学生姓名。
#SELECT sname from s where sname like '上官_'
#查所有不姓“张”的学生的姓名。
#SELECT sname,ssex,sno from s where sname not like '刘%'
#9：查DB_Design相关课程的课程号。
#SELECT cno,cname from c where cname like '%DBc_design%' ESCAPE 'c'
#10：查缺考的学生的学号和课程号。
#SELECT sno,cno from sc where grade is null
#11：查年龄为空值的学生的学号和姓名。
#SELECT sno,sname from s where sage is null
#12：查计算机系20岁以下的学生的学号和姓名。
#SELECT sno,sname from s where sage<20 and sdept = 1408
#14：查询选修了C3课程的学生的学号和成绩，其结果按分数的降序排列。
#SELECT sno,grade from sc,c where sc.cno=c.cno and cname='php' ORDER BY grade desc
#15：查询全体学生的情况，查询结果按所在系升序排列，对同一系中的学生按年龄降序排列。
#SELECT * from s ORDER BY sdept asc ,sage desc
#16：查询学生总人数。
#SELECT count(sno) as studentnum from s
#17：查询选修了课程的学生人数。
#SELECT COUNT(DISTINCT(sno)) from sc
#18：计算选修了C1课程的学生平均成绩。
#SELECT avg(grade) from sc INNER JOIN c on sc.cno = c.cno where cname = 'php'
#19：查询学习C3课程的学生最高分数。
#SELECT max(grade) from sc where cno = (SELECT cno from c where cname = 'php');
#20：查询各个课程号与相应的选课人数。
#SELECT cno,count(sno) from sc GROUP BY cno
#21：查询计算机系选修了3门以上课程的学生的学号。
#SELECT sc.sno,count(cno) as num from s,sc where s.sno=sc.sno and sdept = '1505' GROUP BY sno HAVING num>3
#22：求基本表S中男同学的每一年龄组（超过50人）有多少人？要求查询结果按人数升序排列，人数相同按年龄降序排列。
#SELECT sage,COUNT(sage) as num from s where ssex = '男' GROUP BY sage HAVING num>50 ORDER BY num asc,sage desc;
#23：查询每个学生及其选修课程的情况.
#SELECT * from s,sc,c where s.sno = sc.sno and sc.cno = c.cno
#24：查询选修了C2课程且成绩在90分以上的所有学生。
#SELECT * from s,sc,c where s.sno = sc.sno and sc.cno = c.cno and grade>90 and cname= 'php'
#SELECT * from s INNER JOIN sc on s.sno = sc.sno INNER JOIN c on sc.cno = c.cno where grade >90 and cname = 'php'
#25：查询每个学生选修的课程名及其成绩。
#SELECT s.sno,sname,cname,grade from s,sc,c where s.sno = sc.sno and sc.cno = c.cno
#26：统计每一年龄选修课程的学生人数。
#SELECT sage,COUNT(sc.sno) from s,sc where s.sno=sc.sno GROUP BY sage ;
#27：查询选修了C2课程的学生姓名。
#SELECT sname from s,sc,c where s.sno = sc.sno and sc.cno = c.cno and cname='php'
#28：查询与“张三”在同一个系学习的学生学号、姓名和系别。
#SELECT sno,sname,sdept from s where sdept=(SELECT sdept from s where sname = '张三')
#29：查询选修课程名为“数据库”的学生学号和姓名。
#SELECT sno,sname from s,sc,c where s.sno = sc.sno and sc.cno = c.cno and cname = '数据库'
#30：查询所有未选修C2课程的学生姓名。
#SELECT sname from s where sname not in (SELECT sname from s,sc,c where s.sno = sc.sno and sc.cno = c.cno and cname='php')
#31：查询选修了全部课程的学生姓名。
#SELECT sname FROM s WHERE NOT EXISTS (SELECT * FROM c WHERE NOT EXISTS (SELECT * FROM sc WHERE sc.sno = s.sno AND sc.cno = c.cno))
#SELECT sname FROM s INNER JOIN sc ON s.sno = sc.sno INNER JOIN c ON sc.cno = c.cno GROUP BY sname having COUNT(DISTINCT(sc.cno)) = (select count(cno) from c)
#32：查询所学课程包含学生S3所学课程的学生学号
#SELECT DISTINCT(sno) from sc,c where sc.cno = c.cno and cname in (SELECT cname from s,sc,c where s.sno = sc.sno and sc.cno = c.cno and sname = '梁斌斌')
# DISTINCT 去重 max 最大值 avg 平均值 min 最小值 is null 空的 escape 转译