#数据准备
drop table if exists class;
create table class(
class_no int(2) unsigned zerofill primary key auto_increment comment ‘班级编号‘,
class_name varchar(30) not null comment ‘班级名称‘
);
insert into class values(1, ‘培优班‘);
insert into class values(2, ‘普通班‘);
drop table if exists student;
create table student(
stu_no int(2) unsigned zerofill primary key auto_increment comment ‘学员编号‘,
stu_name varchar(30) not null comment ‘学员姓名‘,
stu_sex varchar(3) not null comment ‘学员性别‘,
stu_age tinyint(2) unsigned zerofill comment ‘学员年代‘,
grade double(5,2) zerofill comment ‘成绩‘,
class_no int(2) unsigned zerofill comment ‘所在班级编号‘,
foreign key(class_no) references class(class_no)
);
insert into student values(01, ‘李白‘, ‘男‘, 18, 60, 01);
insert into student values(02, ‘杜甫‘, ‘男‘, 20, 76, 01);
insert into student values(03, ‘张飞‘, ‘男‘, 32, 80, 02);
insert into student values(04, ‘韩信‘, ‘男‘, 26, 98, 02);
insert into student values(05, ‘了龙‘, ‘男‘, 27, 56, 02);
insert into student values(06, ‘大乔‘, ‘女‘, 17, 88, 01);
insert into student values(07, ‘小乔‘, ‘女‘, 16, 96, 01);
insert into student values(08, ‘小乔‘, ‘女‘, 16, 90, 01);
insert into student values(09, ‘关哥‘, ‘男‘, 32, 80, 02);
insert into student values(10, ‘刘备‘, ‘男‘, 36, 98, null);
alter table student drop foreign key `student_ibfk_1`;
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1: [ group by ] 分组
需求: 查询出每一个班级最高成绩是多少
例: select class_no, max(grade) from student group by class_no; --先按class_no分组,然后再拿到每一组的最高成绩
例: select class_no, sum(grade) from student group by class_no; --查询每一个班级的总成绩
例: select class_no, min(grade) from student group by class_no; --查询每一个班级的最低成绩
例: select class_no, avg(grade) from student group by class_no; --查询每一个班级的平均成绩
例: select class_no, count(*) from student group by class_no; --查询每一个班级的人数
ps: group by一般要与max,min,avg等这些函数一起使用
例: select * from student group by class_no;
--可以进多重分组
需求: 查询出每一个班级男学生和女学生的最高成绩分别是多少
例: select class_no, stu_sex ,max(grade) from student group by class_no, stu_sex;
例: select class_no, stu_name, max(grade) from student group by class_no; --不能这样做
ps: 在要查询的字段集中,这些字段要么是包含在group by语句的后面的字段, 要么就是被包含在聚合函数中, 否则会报错
--还可以对满足条件的记录进行分组
select class_no, max(grade) from student where class_no is not null group by class_no;
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2: having
例: select * from student where class_no = 1 and stu_sex = ‘男‘;
例: select * from student having class_no = 1 and stu_sex = ‘男‘;
例: select class_no, max(grade) from student group by class_no having class_no is not null; --having
例: select class_no, max(grade) from student group by class_no where class_no is not null; --报错
ps: having需要跟在group by后面，而where不能跟在group by后面
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查询它可以配合5个字句来执行,查询到相应数据(where, order by ,limit, group by, having)
这个子句有一个顺序,需要按照顺序来写
select * from student [where] [group by] [having] [order by] [limit];
例: select class_no, stu_sex, avg(grade) from student where(class_no is not null) group by class_no,stu_sex having(stu_sex = ‘男‘) order by class_no desc limit 1;
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3: [ in ](集合运算符)
需求: 查询出学号是2,3,5的学生
例: select * from student where stu_no = 2 or stu_no = 3 or stu_no =5; --使用or运算符
例: select * from student where stu_no in (2,3,5); --使用in集合运算符
例: select * from student where class_no in (2, null); --in集合运算符查询不到null值
--还可以有not in
例: select * from student where stu_no not in (2,3,5);
例: select * from student where class_no not in (2, null); --查询结果为空
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4: 子查询
需求: 获取student表里成绩成绩最高的学员
例: select * from student order by grade desc limit 1; --这里不满足需求
例: select * from student where grade = max(grade); --报错
例: select * from student where grade = (select max(grade) from student);
ps: 在查询里边还有其它的查询,那么我们就把里边的查询叫做子查询，子查询需要括号包起来, 子查询可以有多个
ps: 子查询其实也就是一个查询,所以它返回的结果有以下几种情况(单一值),(一列),(一行或者一行多列),(多行多列)
根据返回值的情况不同，可以把子查询分为四种情况
1: 单一值(标量子查询)
需求: 获取student表里成绩成绩最高的学员
例: select * from student where grade = (select max(grade) from student);
2: 一列(列子查询)
例: select stu_age from student where grade < 95;
需求: 查询出成绩小于95的学员年龄
[ in ](集合运算符)
例: select * from student where stu_age in (select stu_age from student where grade < 95); --in在集合中存在的
例: select * from student where stu_age not in (select stu_age from student where grade < 95); --not in在集合中不存在的
[ any ]
例: select * from student where stu_age = any(select stu_age from student where grade < 95); -- =any等于集合中任意一个就行
例: select * from student where stu_age != any(select stu_age from student where grade < 95); -- !=any不满足集合中任意一个就行
[ all ]
例: select * from student where stu_age = all(select stu_age from student where grade < 95); -- =all等于集合中所有元素
例: select * from student where stu_age != all(select stu_age from student where grade < 95); -- !=不等于集合中的所有元素(意思就是集合中不存在的)
3: 一行(行子查询)
需求: 查询出同一班中相同成绩的 学生的姓名,班级,成绩;
select stu_name, class_no, grade from student where (class_no, grade) = (select class_no,grade from student group by class_no, grade having count(*) > 1);
ps: (class_no, grade)意思是临时构造成一个行，根据子查询到一行去比较;
4: 多行多列(表子查询)
需求: 查询出表中的stu_name，stu_sex的字段，要使用子查询
例: select * from (select stu_name, stu_sex from student) as stu;
ps：from后面需要跟一个表,如果是跟着是一个子查询得的一个临时表,那么你需要给这个字查询起一个加名;