查询选修了课程的学生姓名 hive> select distinct Sname from student inner join sc on student.Sno=Sc.Sno;
----hive的group by 和集合函数
查询学生的总人数 hive> select count(distinct Sno)count from student;
计算1号课程的学生平均成绩 hive> select avg(distinct Grade) from sc where Cno=1; 查询各科成绩平均分 hive> select Cno,avg(Grade) from sc group by Cno; 查询选修1号课程的学生最高分数 select Grade from sc where Cno=1 sort by Grade desc limit 1; (注意比较:select * from sc where Cno=1 sort by Grade select Grade from sc where Cno=1 order by Grade)
求各个课程号及相应的选课人数 hive> select Cno,count(1) from sc group by Cno;
查询选修了3门以上的课程的学生学号 hive> select Sno from (select Sno,count(Cno) CountCno from sc group by Sno)a where a.CountCno>3; 或 hive> select Sno from sc group by Sno having count(Cno)>3;
----hive的Order By/Sort By/Distribute By Order By ,在strict 模式下(hive.mapred.mode=strict),order by 语句必须跟着limit语句,但是在nonstrict下就不是必须的,这样做的理由是必须有一个reduce对最终的结果进行排序,如果最后输出的行数过多,一个reduce需要花费很长的时间。
查询学生信息,结果按学号全局有序 hive> set hive.mapred.mode=strict; <默认nonstrict> hive> select Sno from student order by Sno; FAILED: Error in semantic analysis: 1:33 In strict mode, if ORDER BY is specified, LIMIT must also be specified. Error encountered near token 'Sno' Sort By,它通常发生在每一个redcue里,“order by” 和“sort by"的区别在于,前者能给保证输出都是有顺序的,而后者如果有多个reduce的时候只是保证了输出的部分有序。set mapred.reduce.tasks=<number>在sort by可以指定,在用sort by的时候,如果没有指定列,它会随机的分配到不同的reduce里去。distribute by 按照指定的字段对数据进行划分到不同的输出reduce中 此方法会根据性别划分到不同的reduce中 ,然后按年龄排序并输出到不同的文件中。
查询学生信息,按性别分区,在分区内按年龄有序 hive> set mapred.reduce.tasks=2; hive> insert overwrite local directory '/home/hadoop/out' select * from student distribute by Sex sort by Sage;
----Join查询,join只支持等值连接 查询每个学生及其选修课程的情况 hive> select student.*,sc.* from student join sc on (student.Sno =sc.Sno); 查询学生的得分情况。 hive>select student.Sname,course.Cname,sc.Grade from student join sc on student.Sno=sc.Sno join course on sc.cno=course.cno;
查询选修2号课程且成绩在90分以上的所有学生。 hive> select student.Sname,sc.Grade from student join sc on student.Sno=sc.Sno where sc.Cno=2 and sc.Grade>90;
----LEFT,RIGHT 和 FULL OUTER JOIN ,inner join, left semi join 查询所有学生的信息,如果在成绩表中有成绩,则输出成绩表中的课程号 hive> select student.Sname,sc.Cno from student left outer join sc on student.Sno=sc.Sno; 如果student的sno值对应的sc在中没有值,则会输出student.Sname null.如果用right out join会保留右边的值,左边的为null。 Join 发生在WHERE 子句之前。如果你想限制 join 的输出,应该在 WHERE 子句中写过滤条件——或是在join 子句中写。
重写以下子查询为LEFT SEMI JOIN SELECT a.key, a.value FROM a WHERE a.key exist in (SELECT b.key FROM B); 可以被重写为: SELECT a.key, a.val FROM a LEFT SEMI JOIN b on (a.key = b.key)
查询与“刘晨”在同一个系学习的学生 hive> select s1.Sname from student s1 left semi join student s2 on s1.Sdept=s2.Sdept and s2.Sname='刘晨';
注意比较: select * from student s1 left join student s2 on s1.Sdept=s2.Sdept and s2.Sname='刘晨'; select * from student s1 right join student s2 on s1.Sdept=s2.Sdept and s2.Sname='刘晨'; select * from student s1 inner join student s2 on s1.Sdept=s2.Sdept and s2.Sname='刘晨'; select * from student s1 left semi join student s2 on s1.Sdept=s2.Sdept and s2.Sname='刘晨'; select s1.Sname from student s1 right semi join student s2 on s1.Sdept=s2.Sdept and s2.Sname='刘晨';