sql 聚合函数和group by 联合使用
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很多时候单独使用聚合函数的时候觉得很容易,求个平均值,求和,求个数等,但是和分组一起用就有点混淆了,好记性不如烂笔头,所以就记下来以后看看。
常用聚合函数罗列
(01)AVG() - 返回平均值COUNT() - 返回行数FIRST() - 返回第一个记录的值LAST() - 返回最后一个记录的值max() - 返回最大值MIN() - 返回最小值SUM() - 返回总和
创建一张表
(01)CREATE TABLE [dbo].[stuscore]([name] [varchar](50) ,--学生名字[subject] [varchar](50) ,--课程名字[score] [int] ,--分数[stuid] [int] ,--学号 [d_date] datetime,--登记时间)
(02)添加多条数据insert into [stuscore]select '张三','数学','78','1',GETDATE()union allselect '张三','语文','98','1',GETDATE()union allselect '张三','英语','88','1',GETDATE()union allselect '李四','数学','81','2',GETDATE()union allselect '李四','语文','83','2',GETDATE()union allselect '李四','英语','60','2',GETDATE()
(03)问题罗列:1. 计算每个人的总成绩并排名(要求显示字段:姓名,总成绩)2. 计算每个人的总成绩并排名(要求显示字段: 学号,姓名,总成绩,时间)3. 计算每个人单科的最高成绩(要求显示字段: 学号,姓名,课程,最高成绩)4. 计算每个人的平均成绩(要求显示字段: 学号,姓名,平均成绩)5. 列出各门课程成绩最好的学生(要求显示字段: 学号,姓名,科目,成绩)6. 列出各门课程成绩最好的两位学生(要求显示字段: 学号,姓名,科目,成绩)7. 统计如下:学号 姓名 语文 数学 英语 总分 平均分8.列出各门课程的平均成绩(要求显示字段:课程,平均成绩)9.列出数学成绩的排名(要求显示字段:学号,姓名,成绩,排名)10.列出数学成绩在2-3名的学生(要求显示字段:学号,姓名,科目,成绩)11.求出李四的数学成绩的排名12.统计如下:课程 不及格(0-59)个 良(60-80)个 优(81-100)个
问题解决
(01)计算每个人的总成绩并排名(要求显示字段:姓名,总成绩)select name, SUM(score) as totalscore from stuscore group by name order by totalscore desc(求和用sum,计算每个人按name 分组,排序order by)
(02)计算每个人的总成绩并排名(要求显示字段: 学号,姓名,总成绩)select stuid,SUM(score) as totalscore,name,d_date from stuscore group by name,stuid ,d_date order by totalscore desc(如果一个表有很多字段呢?难道也要一个一个字段罗列出来group by?)select distinct ,d,core,t1.d_date from stuscore t1,(select stuid,sum(score) as allscore from stuscore group by stuid)t2where d=dorder by core desc
(03)计算每个人单科的最高成绩(要求显示字段: 学号,姓名,课程,最高成绩) select d,,ect,e from stuscore t1,(select stuid,max(score) as maxscore from stuscore group by stuid) t2where d=d and e=core
(04)计算每个人的平均成绩(要求显示字段: 学号,姓名,平均成绩)select distinct d,,core from stuscore t1,(select stuid,avg(score) as avgscore from stuscore group by stuid) t2where d=d
(05)列出各门课程成绩最好的学生(要求显示字段: 学号,姓名,科目,成绩)select distinct d,,e from stuscore t1,(select subject ,max(score) as maxscore from stuscore group by subject) t2where e=core and ect=ect
(06)统计如下:学号 姓名 语文 数学 英语 总分 平均分select stuid as 学号,name as 姓名, sum(case when subject='语文' then score else 0 end) as 语文, sum(case when subject='数学' then score else 0 end) as 数学, sum(case when subject='英语' then score else 0 end) as 英语, sum(score) as 总分,(sum(score)/count(*)) as 平均分 from stuscore group by stuid,name order by 总分 desc
(07)列出各门课程成绩最好的两位学生(要求显示字段: 学号,姓名,科目,成绩)select * from stuscore t1 where d in (select top 2 d from stuscore where subject=ect order by score desc)order by ect
(08)列出各门课程的平均成绩(要求显示字段:课程,平均成绩)select subject,AVG(score) from stuscore group by subject
(09)列出数学成绩的排名(要求显示字段:学号,姓名,成绩,排名)declare @temp table(pm int identity(1,1),stuid int,name varchar(50),score int)insert into @temp select stuid,name,score from stuscore where subject='数学' order by score descselect * from @temp
(10)列出数学成绩在2-3名的学生(要求显示字段:学号,姓名,科目,成绩)select t3.* from(select top 2 t2.* from (select top 3 name,subject,score,stuid from stuscore where subject='数学'order by score desc) t2 order by e) t3 order by e desc
(11)求出李四的数学成绩的排名declare @tmp table(pm int,name varchar(50),score int,stuid int)insert into @tmp select null,name,score,stuid from stuscore where subject='数学' order by score descdeclare @id intset @id=0;update @tmp set @id=@id+1,pm=@idselect * from @tmp where name='李四'
(12)统计如下:课程 不及格(0-59)个 良(60-80)个 优(81-100)个select subject, (select count(*) from stuscore where score<60 and subject=ect) as 不及格,(select count(*) from stuscore where score between 60 and 80 and subject=ect) as 良,(select count(*) from stuscore where score >80 and subject=ect) as 优from stuscore t1 group by subject
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