python 数据库连表查询习题

# 多表查询
    # 连表查
        # 内连接  必须左表和右表中条件互相匹配的项才会被显示出来
            # 表1 inner join 表2 on 条件
        # 外链接 会显示条件不匹配的项
            # left join 左表显示全部,右表中的数据必须和左表条件互相匹配的项才会被显示出来
            # right join 右表显示全部,左表中的数据必须和右表条件互相匹配的项才会被显示出来
            # 全外连接
                # left join
                # union
                # right join
    # 子查询
        # select * from 表 where 字段 = (select 字段 from 表 where 条件)
        # select * from 表 where 字段 > (select 字段 from 表 where 条件)
        # select * from 表 where 字段 in (select 字段 from 表 where 条件)

# 5\查询至少有一门课与学号为1的同学所学课程相同的同学的学号和姓名；
    # select course_id from score where student_id =1;
    # select * from score where course_id in (1,2,4);
    # select distinct student_id from score where course_id in (select course_id from score where student_id =1) and student_id !=1;
    # select sid,sname from student right join (select distinct student_id from score where course_id in (select course_id from score where student_id =1) and student_id !=1) as t
    #               on student.sid = t.student_id;

    #  select course_id from score where student_id =1
    #  select * from student as t1 inner join score as t2 on t1.sid = t2.student_id where course_id in (select course_id from score where student_id =1);
    #  select distinct student_id,sname from student as t1 inner join score as t2 on t1.sid = t2.student_id where course_id in (select course_id from score where student_id =1);

# 8\查询课程编号“2”的成绩比课程编号“1”课程低的所有同学的学号、姓名；
    # 把课程编号2的所有人的学生id,成绩找出来
    # select student_id as sid2,num as n2 from score where course_id = 2;
    # 把课程编号1的所有人的学生id,成绩找出来
    # select student_id as sid1,num as n1 from score where course_id = 1;
    # 课程编号“2”的成绩比课程编号“1”课程低的所有同学的学号
    # select sid1 from (select student_id as sid2,num as n2 from score where course_id = 2) as t2
    #               inner join
    #               (select student_id as sid1,num as n1 from score where course_id = 1) as t1
    #               on t1.sid1 = t2.sid2 where n2<n1;
    # 连表student,取名字
    # select sid,sname from student right join (
    #               select sid1 from (select student_id as sid2,num as n2 from score where course_id = 2) as t2
    #               inner join
    #               (select student_id as sid1,num as n1 from score where course_id = 1) as t1
    #               on t1.sid1 = t2.sid2 where n2<n1
    #         ) as tmp
    #         on student.sid = tmp.sid1;

# 9\查询“生物”课程比“物理”课程成绩高的所有学生的学号；
    # 先获取生物课的id
    # select cid from course where cname = '生物';
    # 先获取生物理的id
    # select cid from course where cname = '物理';
    # 找到所有学物理的人
    # select student_id as sid2,num as n2 from score where course_id = (select cid from course where cname = '物理');
    # select student_id as sid2,num as n2 from score where course_id = (select cid from course where cname = '生物');

    # select sid1 from (select student_id as sid1,num as n1 from score where course_id = (select cid from course where cname = '物理')) t1
    #                 inner join
    #                 (select student_id as sid2,num as n2 from score where course_id = (select cid from course where cname = '生物')) t2
    #                 on t1.sid1 = t2.sid2 where n2>n1;

# 13\查询没学过“张磊老师”课的同学的学号、姓名；
    # 找一下张磊老师教什么课,取课程id
    # select cid from course inner join teacher on course.teacher_id = teacher.tid where tname = '张磊老师';
    # 再从score表中找到学习张磊老师课程的学生
    # select distinct student_id from score where course_id in (select cid from course inner join teacher on course.teacher_id = teacher.tid where tname = '张磊老师');
    # 再找不是这些学生的其他人
    # select sid,sname from student where sid not in (
    #     select distinct student_id from score where course_id in (select cid from course inner join teacher on course.teacher_id = teacher.tid where tname = '张磊老师'));

# 15\查询学过“李平老师”所教的所有课的同学的学号、姓名；
    # 找到李平老师教的课程
    # select cid from course inner join teacher on course.teacher_id = teacher.tid where tname = '李平老师';
    # select count(cid) from course inner join teacher on course.teacher_id = teacher.tid where tname = '李平老师';
    # # 找到所有学习李平老师课的学生
    # select * from score where course_id in (select cid from course inner join teacher on course.teacher_id = teacher.tid where tname = '李平老师');
    # # 查询学号
    # select student_id  from score where course_id in (
    #     select cid from course inner join teacher on course.teacher_id = teacher.tid where tname = '李平老师') group by student_id having count(*) = (
    #     select count(cid) from course inner join teacher on course.teacher_id = teacher.tid where tname = '李平老师'
    # );
    # 查询姓名
    # select sid,sname from student right join (
    #     select student_id  from score where course_id in (
    #         select cid from course inner join teacher on course.teacher_id = teacher.tid where tname = '李平老师') group by student_id having count(*) = (
    #         select count(cid) from course inner join teacher on course.teacher_id = teacher.tid where tname = '李平老师'
    #     )) as tmp on tmp.student_id =  student.sid;
来源：https://www.cnblogs.com/shaohuagu/p/12308307.html
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