我在回答以下问题时遇到了一些麻烦。
对于Student表中显示的每个年龄值，找到该值的级别值 最常出现。例如，如果年龄超过18岁的FR级学生比较多， JR，或18岁的SO学生，你应该打印这对（18，FR）。
来自此数据集。
create database university;
use university;
create table student(
snum decimal(9) primary key,
sname varchar(30),
major varchar(25),
level varchar(2),
age int
);
create table faculty(
fid decimal(9) primary key,
fname varchar(30),
deptid decimal(2)
);
create table class(
cname varchar(40) primary key,
meets_at varchar(20),
room varchar(10),
fid decimal(9),
foreign key(fid) references faculty(fid)
);
create table enrolled(
snum decimal(9),
cname varchar(40),
primary key(snum,cname),
foreign key(snum) references student(snum),
foreign key(cname) references class(cname)
);
insert into student values(051135593,'Maria White','English','SR',21);
insert into student values(060839453,'Charles Harris','Architecture','SR',22);
insert into student values(099354543,'Susan Martin','Law','JR',20);
insert into student values(112348546,'Joseph Thompson','Computer Science','SO',19);
insert into student values(115987938,'Christopher Garcia','Computer Science','JR',20);
insert into student values(132977562,'Angela Martinez','History','SR',20);
insert into student values(269734834,'Thomas Robinson','Psychology','SO',18);
insert into student values(280158572,'Margaret Clark','Animal Science','FR',18);
insert into student values(301221823,'Juan Rodriguez','Psychology','JR',20);
insert into student values(318548912,'Dorthy Lewis','Finance','FR',18);
insert into student values(320874981,'Daniel Lee','Electrical Engineering','FR',17);
insert into student values(322654189,'Lisa Walker','Computer Science','SO',17);
insert into student values(348121549,'Paul Hall','Computer Science','JR',18);
insert into student values(351565322,'Nancy Allen','Accounting','JR',19);
insert into student values(451519864,'Mark Young','Finance','FR',18);
insert into student values(455798411,'Luis Hernandez','Electrical Engineering','FR',17);
insert into student values(462156489,'Donald King','Mechanical Engineering','SO',19);
insert into student values(550156548,'George Wright','Education','SR',21);
insert into student values(552455318,'Ana Lopez','Computer Engineering','SR',19);
insert into student values(556784565,'Kenneth Hill','Civil Engineering','SR',21);
insert into student values(567354612,'Karen Scott','Computer Engineering','FR',18);
insert into student values(573284895,'Steven Green','Kinesiology','SO',19);
insert into student values(574489456,'Betty Adams','Economics','JR',20);
insert into student values(578875478,'Edward Baker','Veterinary Medicine','SR',21);
insert into faculty values(142519864,'Ivana Teach',20);
insert into faculty values(242518965,'James Smith',68);
insert into faculty values(141582651,'Mary Johnson',20);
insert into faculty values(011564812,'John Williams',68);
insert into faculty values(254099823,'Patricia Jones',68);
insert into faculty values(356187925,'Robert Brown',12);
insert into faculty values(489456522,'Linda Davis',20);
insert into faculty values(287321212,'Michael Miller',12);
insert into faculty values(248965255,'Barbara Wilson',12);
insert into faculty values(159542516,'William Moore',33);
insert into faculty values(090873519,'Elizabeth Taylor',11);
insert into faculty values(486512566,'David Anderson',20);
insert into faculty values(619023588,'Jennifer Thomas',11);
insert into faculty values(489221823,'Richard Jackson',33);
insert into faculty values(548977562,'Ulysses Teach',20);
insert into class values('Data Structures','MWF 10','R128',489456522);
insert into class values('Database Systems','MWF 12:30-1:45','1320 DCL',142519864);
insert into class values('Operating System Design','TuTh 12-1:20','20 AVW',489456522);
insert into class values('Archaeology of the Incas','MWF 3-4:15','R128',248965255);
insert into class values('Aviation Accident Investigation','TuTh 1-2:50','Q3',011564812);
insert into class values('Air Quality Engineering','TuTh 10:30-11:45','R15',011564812);
insert into class values('Introductory Latin','MWF 3-4:15','R12',248965255);
insert into class values('American Political Parties','TuTh 2-3:15','20 AVW',619023588);
insert into class values('Social Cognition','Tu 6:30-8:40','R15',159542516);
insert into class values('Perception','MTuWTh 3','Q3',489221823);
insert into class values('Multivariate Analysis','TuTh 2-3:15','R15',090873519);
insert into class values('Patent Law','F 1-2:50','R128',090873519);
insert into class values('Urban Economics','MWF 11','20 AVW',489221823);
insert into class values('Organic Chemistry','TuTh 12:30-1:45','R12',489221823);
insert into class values('Marketing Research','MW 10-11:15','1320 DCL',489221823);
insert into class values('Seminar in American Art','M 4','R15',489221823);
insert into class values('Orbital Mechanics','MWF 8','1320 DCL',011564812);
insert into class values('Dairy Herd Management','TuTh 12:30-1:45','R128',356187925);
insert into class values('Communication Networks','MW 9:30-10:45','20 AVW',141582651);
insert into class values('Optical Electronics','TuTh 12:30-1:45','R15',254099823);
insert into class values('Intoduction to Math','TuTh 8-9:30','R128',489221823);
insert into enrolled values(112348546,'Database Systems');
insert into enrolled values(115987938,'Database Systems');
insert into enrolled values(348121549,'Database Systems');
insert into enrolled values(322654189,'Database Systems');
insert into enrolled values(552455318,'Database Systems');
insert into enrolled values(455798411,'Operating System Design');
insert into enrolled values(552455318,'Operating System Design');
insert into enrolled values(567354612,'Operating System Design');
insert into enrolled values(112348546,'Operating System Design');
insert into enrolled values(115987938,'Operating System Design');
insert into enrolled values(322654189,'Operating System Design');
insert into enrolled values(567354612,'Data Structures');
insert into enrolled values(552455318,'Communication Networks');
insert into enrolled values(455798411,'Optical Electronics');
insert into enrolled values(301221823,'Perception');
insert into enrolled values(301221823,'Social Cognition');
insert into enrolled values(301221823,'American Political Parties');
insert into enrolled values(556784565,'Air Quality Engineering');
insert into enrolled values(099354543,'Patent Law');
insert into enrolled values(574489456,'Urban Economics');
我最好的尝试是正确的，即：
select age, level from (select age, level, count(level) as levelCount from student group by age, level order by age, levelCount desc) as counts group by age;
但是它打破了标准sql的规则，其中每个选定的值必须是分组的，我不会用最外面的select语句，level。我正在利用MySQl的功能来返回非聚合数据。
问题是如何根据标准的sql最佳实践返回从组创建的每个组的最大计数。
select age, level, count(level) as levelCount from student group by age, level order by age, levelCount desc;
请，谢谢
答案 0 :(得分：0)
这是MySQL的痛苦。一种方法是使用变量。但是，最简单的方法是使用group_concat() / substring_index()技巧：
select age,
substring_index(group_concat(level order by levelCount desc), ',', 1) as mode_level
from (select age, level, count(level) as levelCount
from student
group by age, level
order by age, levelCount desc
) as counts
group by age;
（统计上，你要找的是mode。）
注意：这是一个技巧。 group_concat()的中间空间 - 默认情况下 - 1,024个字符，因此可能导致出现空间/溢出错误。这个限制很容易增加。