MySQL基础操作实战

作者: 执着的逗比 | 来源:发表于2020-04-20 22:40 被阅读0次

首先我们有一个school数据库，然后它有如下结构的四张表：

image.png

接下来通过SQL语句创建对应的数据库和数据表(英语字母统一用小写，好看一点)：

创建school数据库（指定字符集）；

create database school charset=utf8;

切换到school数据库；

use school;

创建student数据表；

create table student (
s_id varchar(10) collate utf8_bin default null,
s_name varchar(20) collate utf8_bin default null,
s_brithday date default null,
s_sex char(2) collate utf8_bin default null
) engine=innodb default charset=utf8 collate=utf8_bin;

向student表插入测试数据

insert into student values ('01', '赵雷', '1990-01-01', '男');
insert into student values ('02', '钱电', '1990-12-21', '男');
insert into student values ('03', '孙风', '1990-05-20', '男');
insert into student values ('04', '李云', '1990-08-06', '男');
insert into student values ('05', '周梅', '1991-12-01', '女');
insert into student values ('06', '吴兰', '1992-03-01', '女');
insert into student values ('07', '郑竹', '1989-07-01', '女');

创建course数据表

create table course (
c_id varchar(10) collate utf8_bin default null,
c_name varchar(20) collate utf8_bin default null,
t_id varchar(10) collate utf8_bin default null
) engine=innodb default charset=utf8 collate=utf8_bin;

向course表插入测试数据

insert into course values ('01', '语文', '02');
insert into course values ('02', '数学', '01');
insert into course values ('03', '英语', '03');

创建sc数据表

create table sc (
s_id varchar(10) collate utf8_bin default null,
c_id varchar(10) collate utf8_bin default null,
score decimal(4,0) default null
) engine=innodb default charset=utf8 collate=utf8_bin;

向sc表插入测试数据

insert into sc values ('01', '01', '80');
insert into sc values ('01', '02', '90');
insert into sc values ('01', '03', '99');
insert into sc values ('02', '01', '70');
insert into sc values ('02', '02', '60');
insert into sc values ('02', '03', '80');
insert into sc values ('03', '01', '80');
insert into sc values ('03', '02', '80');
insert into sc values ('03', '03', '80');
insert into sc values ('04', '01', '50');
insert into sc values ('04', '02', '30');
insert into sc values ('04', '03', '20');
insert into sc values ('05', '01', '76');
insert into sc values ('05', '02', '87');
insert into sc values ('06', '01', '31');
insert into sc values ('06', '03', '34');
insert into sc values ('07', '02', '89');
insert into sc values ('07', '03', '98');
insert into sc values ('07', '01', '99');

创建teacher数据表

create table teacher (
t_id varchar(10) collate utf8_bin default null,
t_name varchar(20) collate utf8_bin default null
) engine=innodb default charset=utf8 collate=utf8_bin;

向teacher插入测试数据

insert into teacher values('01', '张三');
insert into teacher values('02', '赵六');
insert into teacher values('03', '王五');

实际操作练习

统计同学的男女生个数

SELECT
    SUM(CASE WHEN s_sex = '男' THEN 1 ELSE 0 END) 男生个数,
    SUM(CASE WHEN s_sex = '女' THEN 1 ELSE 0 END) 女生个数
FROM
    student;

统计同学的男女生占比（保留两位小数）

SELECT
    cast(SUM(CASE WHEN s_sex = '男' THEN 1 ELSE 0 END)/count(1) AS DECIMAL(18,2)) 男生比例,
    cast(SUM(CASE WHEN s_sex = '女' THEN 1 ELSE 0 END)/count(1) AS DECIMAL(18,2)) 女生比例
FROM
    student;

查询"01"课程比"02"课程成绩高的学生的信息及课程分数

SELECT
    a.*, b.score 课程01的分数,
    c.score 课程02的分数
FROM
    student a,
    sc b,
    sc c
WHERE
    a.s_id = b.s_id
AND a.s_id = c.s_id
AND b.c_id = '01'
AND c.c_id = '02'
AND b.score > c.score;

查询"李"姓老师的数量

SELECT
    count(t_name) 李姓老师的数量
FROM
    teacher
WHERE
    t_name LIKE '李%'

查询语文课程的任职老师姓名

SELECT
    t_name
FROM
    teacher
WHERE
    t_id = (
        SELECT
            t_id
        FROM
            course
        WHERE
            c_name = '语文'
    );

查询"张三"老师教过同学的信息

SELECT DISTINCT
    student.*
FROM
    student,
    course,
    sc,
    teacher
WHERE
    student.s_id = sc.s_id
AND course.c_id = sc.c_id
AND course.t_id = teacher.t_id
AND teacher.t_name = '张三';

查询"张三"老师没教过的同学信息

SELECT
    *
FROM
    student
WHERE
    s_id NOT IN (
        SELECT DISTINCT
            student.s_id
        FROM
            student,
            course,
            sc,
            teacher
        WHERE
            student.s_id = sc.s_id
        AND course.c_id = sc.c_id
        AND course.t_id = teacher.t_id
        AND teacher.t_name = '张三'
    );

查询平均成绩大于60分的同学的学生编号和学生姓名以及平均分（保留两位小数）

SELECT
    student.s_id,
    student.s_name,
    cast(
        avg(sc.score) AS DECIMAL (18, 2)
    ) avg_score
FROM
    student,
    sc
WHERE
    student.s_id = sc.s_id
GROUP BY
    student.s_id
HAVING
    avg_score > 60;

查询任意一门课程都大于80分的学生信息

SELECT
    *
FROM
    student
WHERE
    s_id IN (
        SELECT
            s_id
        FROM
            sc
        GROUP BY
            s_id
        HAVING
            min(score) >= 80
    );

查询有一门课程分数超过80分的学生信息

SELECT
    *
FROM
    student
WHERE
    s_id IN (
        SELECT
            s_id
        FROM
            sc
        GROUP BY
            s_id
        HAVING
            max(score) >= 80
    );

查询没有学全所有课程的同学信息

SELECT
    s.*
FROM
    student s,
    sc
WHERE
    s.s_id = sc.s_id
GROUP BY
    s.s_id
HAVING
    count(c_id) < (
        SELECT
            count(c_id)
        FROM
            course
    );

统计每门课程各等级（60分以下 D等级，60分~75分（不含75分） C等级，75分~85分（不含85分） B等级，85分以上 A等级）分数的人数

SELECT
    c_id,  
  SUM(CASE WHEN score >=85 THEN 1 ELSE 0 end) A,
  SUM(CASE WHEN score>=75 AND score <85 THEN 1 ELSE 0 END) B,
  SUM(CASE WHEN score>=60 AND score<75 THEN 1 ELSE 0 END) C,
  SUM(case WHEN score < 60 THEN 1 ELSE 0 END) D
FROM
    sc
GROUP BY
    c_id;

查询数学课程的前三名的学生姓名

SELECT
    *
FROM
    student
WHERE
    s_id IN (
        SELECT
            s_id
        FROM
            (
                SELECT
                    sc.s_id
                FROM
                    sc,
                    course
                WHERE
                    sc.c_id = course.c_id
                AND course.c_name = '数学'
                ORDER BY
                    sc.score DESC
                LIMIT 3
            ) AS t
    );

查询每门课的前三名的同学的信息
待研究

未完待续

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