-- 查询所有学生的信息
select * from tb_student;
-- 查询所有老师的信息
select * from tb_teacher;
-- 查询所有课程的信息
select * from tb_course;
-- 查询所有课程名称及学分(投影和别名)
select couname as 课程名称, coucredit as 学分 from tb_course;
-- 查询学生的姓名和性别
select stuname as 姓名, stusex as 性别 from tb_student;
select stuname as 姓名, case stusex when 1 then '男' else '女' end as 性别 from tb_student;
select stuname as 姓名, if(stusex, '男', '女') as 性别 from tb_student;
-- 查询所有女学生的姓名和出生日期(筛选)
select stuname, stubirth from tb_student where stusex=0;
-- 查询所有80后学生的姓名、性别和出生日期(筛选)
select stuname, stusex, stubirth from tb_student where stubirth >= '1980-1-1' and stubirth <= '1989-12-31';
select stuname, stusex, stubirth from tb_student where stubirth between '1980-1-1' and '1989-12-31';
-- 查询姓”杨“的学生姓名和性别(模糊)
select stuname, stusex from tb_student where stuname like '杨%';
-- 查询姓”杨“名字两个字的学生姓名和性别(模糊)
select stuname, stusex from tb_student where stuname like '杨_';
-- 查询姓”杨“名字三个字的学生姓名和性别(模糊)
select stuname, stusex from tb_student where stuname like '杨__';
-- 查询名字中有“不”字或“嫣”字的学生的姓名(模糊)
select stuname from tb_student where stuname like '%不%' or stuname like '%嫣%';
-- 查询没有录入家庭住址的学生姓名(空值)
select stuname from tb_student where stuaddr is null;
-- 查询录入了家庭住址的学生姓名(空值)
select stuname from tb_student where stuaddr is not null;
-- 查询学生选课的所有日期(去重)
select distinct seldate from tb_record;
-- 查询学生的家庭住址(去重)
select distinct stuaddr from tb_student where stuaddr is not null;
-- 查询男学生的姓名和生日按年龄从大到小排列(排序)
select stuname, stubirth from tb_student where stusex=1 order by stubirth asc, stuid desc;
select stuname as 姓名, datediff(curdate(), stubirth) div 365 as 年龄 from tb_student where stusex=1 order by 年龄 desc;
-- 查询年龄最大的学生的出生日期(聚合函数)
select min(stubirth) from tb_student;
-- 查询年龄最小的学生的出生日期(聚合函数)
select max(stubirth) from tb_student;
-- 聚合函数 - max / min / sum / avg / count
-- 查询学生总人数
select count(stuid) from tb_student;
-- 查询男女学生的人数(分组和聚合函数)
select if(stusex, '男', '女') as 性别, count(stuid) as 人数 from tb_student group by stusex order by 人数 desc;
-- 查询课程编号为1111的课程的平均成绩(筛选和聚合函数)
select round(avg(score), 1) as 平均分 from tb_record where cid=1111;
-- 查询学号为1001的学生所有课程的平均分(筛选和聚合函数)
select round(avg(score), 1) as 平均分 from tb_record where sid=1001;
-- 查询每个学生的学号和平均成绩(分组和聚合函数)
select sid as 学号, round(avg(score), 1) as 平均分 from tb_record group by sid;
-- 查询平均成绩大于等于90分的学生的学号和平均成绩
-- 注意：分组以前的筛选使用where子句 分组以后的筛选使用having子句
select sid as 学号, round(avg(score), 1) as 平均分 from tb_record group by sid having 平均分>=90;
-- 查询年龄最大的学生的姓名(子查询)
-- 嵌套查询：把一个查询的查询结果作为另外一个查询的一部分来使用
select stuname from tb_student where stubirth=(
select min(stubirth) from tb_student
);
-- 查询年龄最大的学生姓名和年龄(子查询+运算)
select stuname as 姓名, datediff(curdate(), stubirth) div 365 as 年龄 from tb_student where stubirth=(
select min(stubirth) from tb_student
);
-- 查询选了两门以上的课程的学生姓名(子查询/分组条件/集合运算)
select stuname from tb_student where stuid in (
select sid from tb_record
group by sid having count(sid)>2
);
-- 查询学生姓名、课程名称以及成绩(连接查询)
select stuname, couname, score from tb_student t1, tb_record t2, tb_course t3 where t1.stuid=t2.sid and t2.cid=t3.couid and score is not null;
select stuname, couname, score from tb_student inner join tb_record on stuid=sid inner join tb_course on couid=cid where score is not null;
-- 查询选课学生的姓名和平均成绩(子查询和连接查询)
select stuname, averagescore from tb_student as t1,
(select sid, avg(score) as averagescore from tb_record group by sid) as t2 where stuid=sid;
-- 查询每个学生的姓名和选课数量(左外连接和子查询)
select stuname, 选课数量 from tb_student,
(select sid, count(sid) as 选课数量 from tb_record group by sid) as t2 where stuid=sid;
-- 左外连接是指左边的表里面不满足条件的内容如也给选出来
select stuname, ifnull(选课数量, 0) as 选课数量 from tb_student left outer join
(select sid, count(sid) as 选课数量 from tb_record group by sid) as t2 on stuid=sid limit 5 offset 3;
select stuname, ifnull(选课数量, 0) as 选课数量 from tb_student left outer join
(select sid, count(sid) as 选课数量 from tb_record group by sid) as t2 on stuid=sid limit 5, 3;