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mysql 50道经典题

mysql 50道经典题

作者: knock | 来源:发表于2021-12-22 16:22 被阅读0次

    转:https://blog.csdn.net/csdnluolei/article/details/83507312

    ---------创建数据库、表、插入数据----------------------
     
    -- 建表
    -- 学生表
    CREATE TABLE student(
        s_id VARCHAR(20) COMMENT '学生编号',
        s_name VARCHAR(20) NOT NULL DEFAULT '' COMMENT '学生姓名',
        s_birth VARCHAR(20) NOT NULL DEFAULT '' COMMENT '出生年月',
        s_sex VARCHAR(10) NOT NULL DEFAULT '' COMMENT '学生性别',
        PRIMARY KEY(s_id)
    )ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT '学生表';
    -- 课程表
    CREATE TABLE course(
        c_id  VARCHAR(20) COMMENT '课程编号',
        c_name VARCHAR(20) NOT NULL DEFAULT '' COMMENT '课程名称',
        t_id VARCHAR(20) NOT NULL COMMENT '教师编号',
        PRIMARY KEY(c_id)
    )ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT '课程表';
    -- 教师表
    CREATE TABLE teacher(
        t_id VARCHAR(20) COMMENT '教师编号',
        t_name VARCHAR(20) NOT NULL DEFAULT '' COMMENT '教师姓名',
        PRIMARY KEY(t_id)
    )ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT '教师表';
    -- 成绩表
    CREATE TABLE score(
        s_id VARCHAR(20) COMMENT '学生编号',
        c_id  VARCHAR(20) COMMENT '课程编号',
        s_score INT(3) COMMENT '分数',
        PRIMARY KEY(s_id,c_id)
    )ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT '成绩表';
     
    -- 插入学生表测试数据
    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');
    insert into student values('08' , '王菊' , '1990-01-20' , '女');
    -- 课程表测试数据
    insert into course values('01' , '语文' , '02');
    insert into course values('02' , '数学' , '01');
    insert into course values('03' , '英语' , '03');
     
    -- 教师表测试数据
    insert into teacher values('01' , '张三');
    insert into teacher values('02' , '李四');
    insert into teacher values('03' , '王五');
     
    -- 成绩表测试数据
    insert into score values('01' , '01' , 80);
    insert into score values('01' , '02' , 90);
    insert into score values('01' , '03' , 99);
    insert into score values('02' , '01' , 70);
    insert into score values('02' , '02' , 60);
    insert into score values('02' , '03' , 80);
    insert into score values('03' , '01' , 80);
    insert into score values('03' , '02' , 80);
    insert into score values('03' , '03' , 80);
    insert into score values('04' , '01' , 50);
    insert into score values('04' , '02' , 30);
    insert into score values('04' , '03' , 20);
    insert into score values('05' , '01' , 76);
    insert into score values('05' , '02' , 87);
    insert into score values('06' , '01' , 31);
    insert into score values('06' , '03' , 34);
    insert into score values('07' , '02' , 89);
    insert into score values('07' , '03' , 98);
     
    --------------------------------------------------
     
    -- 1、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.s_score AS score1,
        c.s_score AS score2
    FROM
        student a
    LEFT JOIN score b ON a.s_id = b.s_id
    AND b.c_id = '01'
    LEFT JOIN score c ON a.s_id = c.s_id
    AND (c.c_id = '02' OR c.c_id =NULL)
    WHERE
        b.s_score > c.s_score ;
     
    -- 2、查询"01"课程比"02"课程成绩低的学生的信息及课程分数
    SELECT
        a.*, b.s_score AS score1,
        c.s_score AS score2
    FROM
        student a
    LEFT JOIN score b ON a.s_id = b.s_id
    AND (b.c_id = '01' OR b.c_id =NULL)
    LEFT JOIN score c ON a.s_id = c.s_id
    AND c.c_id = '02' 
    WHERE
        b.s_score < c.s_score ;
     
    -- 3、查询平均成绩大于等于60分的同学的学生编号和学生姓名和平均成绩
    SELECT
        a.s_id,
        a.s_name,
        ROUND(AVG(b.s_score), 1) AS 平均成绩
    FROM
        student a
    LEFT JOIN score b ON a.s_id = b.s_id
    GROUP BY
        a.s_id
    HAVING
        AVG(b.s_score) >= 60;
     
    -- 4、查询平均成绩小于60分的同学的学生编号和学生姓名和平均成绩
    -- (包括有成绩的和无成绩的)
    -- 方法一:
    SELECT
        a.s_id,
        a.s_name,
        ROUND(AVG(b.s_score), 1) AS 平均成绩
    FROM
        student a
    LEFT JOIN score b ON a.s_id = b.s_id
    GROUP BY
        a.s_id
    HAVING
        AVG(b.s_score) < 60
    OR
    a.s_id NOT IN(SELECT DISTINCT a.s_id FROM student a JOIN score b WHERE a.s_id=b.s_id);
     
    -- 方法二:
    SELECT
        a.s_id,a.s_name,
        ROUND(AVG(b.s_score),2) AS avg_score
    FROM
        student a
    JOIN score b ON a.s_id = b.s_id
    GROUP BY
        a.s_id
    HAVING AVG(b.s_score) < 60
    UNION
    SELECT a.s_id,a.s_name,0 AS avg_score FROM 
    student a
    WHERE a.s_id NOT IN (SELECT DISTINCT s_id FROM score);
     
    -- 5、查询所有同学的学生编号、学生姓名、选课总数、所有课程的总成绩,并从高到低排序
    SELECT
        a.s_id,
        a.s_name,
        COUNT(b.c_id) AS 选课总数 ,
        SUM(b.s_score) AS 总成绩
    FROM
        student a
    LEFT JOIN score b
    ON
        a.s_id = b.s_id
    GROUP BY a.s_id
    ORDER BY SUM(b.s_score) DESC;
     
    -- 6、查询"李"姓老师的数量 
    SELECT COUNT(t_id) FROM teacher WHERE t_name LIKE '李%';
     
    -- 7、查询学过"张三"老师授课的同学的信息 
    #张三编号
    SELECT t_id FROM teacher WHERE t_name = '张三'
    #张三代课的课程编号
    SELECT c_id FROM course WHERE t_id = (SELECT t_id FROM teacher WHERE t_name = '张三')
    #学张三课程的学生编号
    SELECT s_id FROM score WHERE c_id = (SELECT c_id FROM course WHERE t_id = (SELECT t_id FROM teacher WHERE t_name = '张三'))
    -- 方法一:
    SELECT *FROM student WHERE 
    s_id IN (SELECT s_id FROM score WHERE 
    c_id = (SELECT c_id FROM course WHERE
    t_id = (SELECT t_id FROM teacher WHERE t_name = '张三'))
    );
     
    -- 方法二:
    SELECT a.* FROM student a
    JOIN score b ON a.s_id = b.s_id
    WHERE 
    b.c_id IN (SELECT c_id FROM course WHERE t_id = (SELECT t_id FROM teacher WHERE t_name = '张三'));
     
    -- 8、查询没学过"张三"老师授课的同学的信息 
    -- 方法一:
    SELECT *FROM student WHERE 
    s_id NOT IN (SELECT s_id FROM score WHERE 
    c_id = (SELECT c_id FROM course WHERE
    t_id = (SELECT t_id FROM teacher WHERE t_name = '张三'))
    );
     
    -- 9、查询学过编号为"01"并且也学过编号为"02"的课程的同学的信息,及两门课程成绩
    -- 方法一
    SELECT
        a.*, b.s_score,
        c.s_score 
    FROM
        student a
    JOIN score b ON a.s_id = b.s_id
    AND b.c_id = '01'
    JOIN score c ON a.s_id = c.s_id
    AND c.c_id = '02';
     
    -- 方法二:
    SELECT
        a.*, b.s_score,
        c.s_score
    FROM
        student a,
        score b,
        score 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';
     
    -- 10、查询学过编号为"01"但是没有学过编号为"02"的课程的同学的信息
    SELECT s_id FROM score  WHERE c_id = '01'
    SELECT s_id FROM score  WHERE c_id = '02'
     
    SELECT *FROM student a
    WHERE
    a.s_id IN (SELECT s_id FROM score  WHERE c_id = '01')
    AND a.s_id NOT IN (SELECT s_id FROM score  WHERE c_id = '02');
     
     
    -- 11、查询没有学全所有课程的同学的信息 
     
    SELECT s_id FROM score
    GROUP BY s_id
    HAVING COUNT(s_id) != 3
     
    #方法一:
    SELECT *FROM student WHERE s_id IN(
        SELECT s_id FROM score
        GROUP BY s_id
        HAVING COUNT(s_id) != 3
    );
     
    #方法二:
    SELECT a.s_id FROM score a 
        JOIN score b ON a.s_id=b.s_id AND b.c_id='02'
        JOIN score c ON a.s_id=c.s_id AND c.c_id='03'
        WHERE a.c_id='01'
     
    SELECT *FROM student d WHERE d.s_id IN(
        SELECT e.s_id FROM score e WHERE e.s_id NOT IN(
            SELECT a.s_id FROM score a 
                JOIN score b ON a.s_id=b.s_id AND b.c_id='02'
                JOIN score c ON a.s_id=c.s_id AND c.c_id='03'
                WHERE a.c_id='01')
    );
     
    #--------------
     
    # 上述两种方法结果都少了没有选课的8号学生,但看具体条件是否需要查出
     
    # 学全选取所有课程的同学的id 
    SELECT s_id FROM score
        GROUP BY s_id
        HAVING COUNT(s_id) = 3
     
    #方法一:
    SELECT *FROM student WHERE s_id NOT IN(
        SELECT s_id FROM score
        GROUP BY s_id
        HAVING COUNT(s_id) = 3
    );
     
    -- 12、查询至少有一门课与学号为"01"的同学所学相同的同学的信息 
     
    SELECT c_id FROM score WHERE s_id ='01'
     
    SELECT DISTINCT s_id FROM score WHERE c_id IN (SELECT c_id FROM score WHERE s_id ='01')
     
    SELECT * FROM student a
    WHERE
    a.s_id IN (SELECT DISTINCT b.s_id FROM score b WHERE 
    b.c_id IN (SELECT c.c_id FROM score c WHERE c.s_id ='01')
    );
     
    -- 13、查询和"01"号的同学学习的课程完全相同的其他同学的信息 
     
    SELECT * FROM student WHERE s_id IN(
        SELECT DISTINCT s_id FROM score WHERE s_id!='01' AND c_id IN (SELECT c_id FROM score WHERE s_id ='01') 
        GROUP BY s_id
        HAVING COUNT(1)=(SELECT COUNT(1) FROM score WHERE s_id='01')
    )
     
    -- 15、查询两门及其以上不及格课程的同学的学号,姓名及其平均成绩 
    SELECT s_id FROM score WHERE s_score<60 GROUP BY s_id HAVING COUNT(1) >= 2
     
    SELECT
        a.s_id,
        a.s_name,
        ROUND(AVG(b.s_score), 1) AS 平均成绩 
    FROM
        student a
    LEFT JOIN score b ON a.s_id = b.s_id
    GROUP BY a.s_id
    HAVING
    a.s_id IN(SELECT s_id FROM score WHERE s_score<60 GROUP BY s_id HAVING COUNT(1) >= 2)
     
     
    -- 16、检索"01"课程分数小于60,按分数降序排列的学生信息及01分数
    -- 方法一:
    SELECT
        a.*, b.s_score
    FROM
        student a
    LEFT JOIN score b ON a.s_id = b.s_id
    WHERE
        b.c_id = '01'
    AND b.s_score < 60
    ORDER BY
        b.s_score DESC;
     
    -- 方法二:
    SELECT
        a.*, b.s_score
    FROM
        student a,score b
    WHERE
    a.s_id = b.s_id AND b.c_id='01' AND b.s_score < 60
    ORDER BY
        b.s_score DESC;
     
    -- 方法三(有点瑕疵):
    SELECT
        a.*, b.s_score
    FROM
        student a
    LEFT JOIN score b ON a.s_id = b.s_id
    AND    b.c_id = '01'
    AND b.s_score < 60
    ORDER BY
        b.s_score DESC;
     
    -- 17、按平均成绩从高到低显示所有学生的所有课程的成绩以及平均成绩
     
    -- 方法一(自连接):
    SELECT
     a.s_id,
    (SELECT s_score FROM score WHERE s_id=a.s_id AND c_id='01') AS score1,
    (SELECT s_score FROM score WHERE s_id=a.s_id AND c_id='02') AS score2,
    (SELECT s_score FROM score WHERE s_id=a.s_id AND c_id='03') AS score3,
    ROUND(avg(a.s_score), 2) AS 平均分
    FROM score a
    GROUP BY a.s_id
    ORDER BY 平均分 DESC;
     
    -- 方法二(自连接):
    SELECT 
     a.s_id,
     b.s_score,
     c.s_score,
     d.s_score,
     ROUND(avg(a.s_score), 2) AS 平均分
    FROM
        score a
    LEFT JOIN score b ON a.s_id = b.s_id AND b.c_id='01'
    LEFT JOIN score c ON a.s_id = c.s_id AND c.c_id='02'
    LEFT JOIN score d ON a.s_id = d.s_id AND d.c_id='03'
    GROUP BY a.s_id
    ORDER BY 平均分 DESC;
     
    -- 18.查询各科成绩最高分、最低分和平均分:以如下形式显示:课程ID,课程name,最高分,最低分,平均分,及格率,中等率,优良率,优秀率
    -- 及格为>=60,中等为:70-80,优良为:80-90,优秀为:>=90
     
    SELECT 
    a.c_id,
    MAX(a.s_score),
     MIN(a.s_score),
     AVG(a.s_score)
    FROM
        score a
    GROUP BY a.c_id;
     
    -- 方法一:
    SELECT
        a.c_id AS 课程ID,
        b.c_name AS 课程name,
        MAX(a.s_score) AS 最高分,
        MIN(a.s_score) AS 最低分,
        ROUND(AVG(a.s_score),2) AS 平均分,
      ROUND(100*(SUM(CASE WHEN a.s_score >= 60 THEN 1 ELSE 0 END) / COUNT(1)) , 2)  AS '及格率',
        ROUND(100*(SUM(CASE WHEN a.s_score >= 70 AND  a.s_score <80 THEN 1 ELSE 0 END) / COUNT(1)) , 2)  AS '中等率',
      ROUND(100*(SUM(CASE WHEN a.s_score >= 80 AND  a.s_score <90 THEN 1 ELSE 0 END) / COUNT(1)) , 2)  AS '优良率',
      ROUND(100*(SUM(CASE WHEN a.s_score >= 90 THEN 1 ELSE 0 END) / COUNT(1)) , 2)  AS '优秀率'
    FROM
        score a
    LEFT JOIN course b ON a.c_id = b.c_id
    GROUP BY
        b.c_id;
     
    -- 19、按各科成绩进行排序,并显示排名
    -- mysql没有rank顺序函数
     select a.s_id,a.c_id,
            @i:=@i +1 as i保留排名,
            @k:=(case when @score=a.s_score then @k else @i end) as rank不保留排名,
            @score:=a.s_score as score
        from (
            select s_id,c_id,s_score from score WHERE c_id='01' GROUP BY s_id,c_id,s_score ORDER BY s_score DESC
    )a,(select @k:=0,@i:=0,@score:=0)s
        union
        select a.s_id,a.c_id,
            @i:=@i +1 as i,
            @k:=(case when @score=a.s_score then @k else @i end) as rank,
            @score:=a.s_score as score
        from (
            select s_id,c_id,s_score from score WHERE c_id='02' GROUP BY s_id,c_id,s_score ORDER BY s_score DESC
    )a,(select @k:=0,@i:=0,@score:=0)s
        union
        select a.s_id,a.c_id,
            @i:=@i +1 as i,
            @k:=(case when @score=a.s_score then @k else @i end) as rank,
            @score:=a.s_score as score
        from (
            select s_id,c_id,s_score from score WHERE c_id='03' GROUP BY s_id,c_id,s_score ORDER BY s_score DESC
    )a,(select @k:=0,@i:=0,@score:=0)s
     
    -- 20、查询学生的总成绩并进行排名
    SELECT a.s_id,
        @i:=@i+1 AS i,
        @k:=(CASE WHEN @score=a.sum_score THEN @k ELSE @i END) AS rank,
        @score:=a.sum_score AS score
    FROM (SELECT s_id,SUM(s_score) AS sum_score FROM score GROUP BY s_id ORDER BY sum_score DESC) AS a,
    (SELECT @i:=0,@score:=0) AS b
     
    -- 21、查询不同老师所教不同课程平均分从高到低显示 
    SELECT
        a.t_name,
        b.c_id,
        b.c_name,
        ROUND(AVG(c.s_score) ,2) AS 平均分
    FROM
        teacher a
    LEFT JOIN course b ON a.t_id = b.t_id
    LEFT JOIN score c ON b.c_id=c.c_id
    GROUP BY c.c_id
    ORDER BY AVG(c.s_score) DESC;
     
    -- 22、查询所有课程的成绩第2名到第3名的学生信息及该课程成绩
    SELECT c.*,d.s_name,d.s_birth,d.s_sex FROM 
    (SELECT a.s_id,a.s_score,a.c_id,@i:=@i+1 AS 排名 FROM score a,(SELECT @i:=0)b WHERE a.c_id='01') c 
    LEFT JOIN student d ON c.s_id=d.s_id
    WHERE 排名 BETWEEN 2 AND 3
    UNION
    SELECT c.*,d.s_name,d.s_birth,d.s_sex FROM 
    (SELECT a.s_id,a.s_score,a.c_id,@j:=@j+1 AS 排名 FROM score a,(SELECT @j:=0)b WHERE a.c_id='02') c 
    LEFT JOIN student d ON c.s_id=d.s_id
    WHERE 排名 BETWEEN 2 AND 3
    UNION
    SELECT c.*,d.s_name,d.s_birth,d.s_sex FROM 
    (SELECT a.s_id,a.s_score,a.c_id,@k:=@k+1 AS 排名 FROM score a,(SELECT @k:=0)b WHERE a.c_id='03') c 
    LEFT JOIN student d ON c.s_id=d.s_id
    WHERE 排名 BETWEEN 2 AND 3;
     
    -- 23、统计各科成绩各分数段人数:课程编号,课程名称,[100-85],[85-70],[70-60],[0-60]及所占百分比
      
    SELECT
        a.c_id AS 课程编号, a.c_name AS 课程名称,
      c.`[100-85]的人数`, c.`[100-85]所占百分比`,
        d.`[85-70]的人数`, d.`[85-70]所占百分比`,
        e.`[70-60]的人数`, e.`[70-60]所占百分比`,
        f.`[0-60]的人数`, f.`[0-60]所占百分比`
    FROM
        course a
    LEFT JOIN score b ON a.c_id = b.c_id
    LEFT JOIN
        (SELECT *,SUM(CASE WHEN s_score >85 AND s_score <=100 THEN 1 ELSE 0 END) AS '[100-85]的人数' ,
        ROUND(SUM(CASE WHEN s_score >85 AND s_score <=100 THEN 1 ELSE 0 END)/COUNT(1)*100,2) AS '[100-85]所占百分比'
        FROM score GROUP BY c_id) c ON a.c_id=c.c_id
    LEFT JOIN
        (SELECT*,SUM(CASE WHEN s_score >70 AND s_score <=85 THEN 1 ELSE 0 END) AS '[85-70]的人数' ,
        ROUND(SUM(CASE WHEN s_score >70 AND s_score <=85 THEN 1 ELSE 0 END)/COUNT(1)*100,2) AS '[85-70]所占百分比'
        FROM score GROUP BY c_id) d ON a.c_id=d.c_id
    LEFT JOIN
        (SELECT*,SUM(CASE WHEN s_score >60 AND s_score <=70 THEN 1 ELSE 0 END) AS '[70-60]的人数' ,
        ROUND(SUM(CASE WHEN s_score >60 AND s_score <=70 THEN 1 ELSE 0 END)/COUNT(1)*100,2) AS '[70-60]所占百分比'
        FROM score GROUP BY c_id) e ON a.c_id=e.c_id
    LEFT JOIN
        (SELECT *,SUM(CASE WHEN s_score >0 AND s_score <=60 THEN 1 ELSE 0 END) AS '[0-60]的人数' ,
        ROUND(SUM(CASE WHEN s_score >0 AND s_score <=60 THEN 1 ELSE 0 END)/COUNT(1)*100,2) AS '[0-60]所占百分比'
        FROM score GROUP BY c_id) f ON a.c_id=f.c_id
    GROUP BY a.c_id
     
    -- 24、查询学生平均成绩及其名次 
    SELECT
        b.s_id,
        @i:=@i+1 AS 相同分数的不同名次,
        @k:=(CASE WHEN @avg_s=b.avg_score THEN @k ELSE @i END) AS 相同分数的相同名次,
        @avg_s:=b.avg_score AS 平均成绩
    FROM
    (SELECT
        a.s_id,
        ROUND(AVG(a.s_score), 2) AS avg_score
    FROM
        score a
    GROUP BY
        a.s_id
    ORDER BY AVG(a.s_score) DESC) b,(SELECT @i:=0,@avg_s:=0,@k:=0) c
     
     
    -- 24.1添加名次rank,(相同分数的相同名次,并列排名)
    -- 上面24难以看出并列排名
    SELECT
        b.s_id,    b.c_id,
        -- 顺序一直在变大
        @i:=@i+1 AS 相同分数的不同名次,
      -- 只有在前后二次分数不同时才会使用顺序号
        @k:=(CASE WHEN @s=b.s_score THEN @k ELSE @i END) AS 相同分数的相同名次,
        @s:=b.s_score AS 成绩
    FROM
    (SELECT *FROM score  WHERE s_id='03' ORDER BY s_score DESC)b,
    (SELECT @i:=0,@k:=0,@s:=0)c
     
    -- 25、查询各科成绩前三名的记录
    -- 1.选出b表比a表成绩大的所有组
    -- 2.选出比当前id成绩大的 小于三个的
    -- SELECT  a.s_id,a.c_id,a.s_score FROM score a 
    -- LEFT JOIN score b ON a.c_id=b.c_id AND a.s_score<b.s_score 
    -- GROUP BY a.s_id,a.c_id,a.s_score 
    -- HAVING COUNT(b.s_id)<3
    -- ORDER BY a.c_id,a.s_score DESC
     
    -- 26、查询每门课程被选修的学生数 
    SELECT c_id,COUNT(1) FROM  score GROUP BY c_id
     
    -- 27、查询出只有两门课程的全部学生的学号和姓名 
    -- 方法一:
    SELECT a.s_id,a.s_name FROM student a 
    LEFT JOIN score b ON a.s_id=b.s_id
    GROUP BY a.s_id
    HAVING COUNT(1)=2
     
    -- 方法二:
    SELECT a.s_id,a.s_name FROM student a WHERE a.s_id IN 
    (SELECT s_id FROM score GROUP BY s_id HAVING COUNT(1)=2)
     
    -- 28、查询男生、女生人数 
    SELECT s_sex,COUNT(1) FROM student GROUP BY s_sex
     
    -- 29、查询名字中含有"风"字的学生信息
    SELECT *FROM student WHERE s_name LIKE '%风%' 
     
    -- 30、查询同名同性学生名单,并统计同名人数 
    SELECT a.s_name,a.s_sex,COUNT(1) AS 人数 FROM student a 
    JOIN student b ON a.s_name=b.s_name AND a.s_sex=b.s_sex AND a.s_id!=b.s_id
    GROUP BY a.s_name,a.s_sex
     
    -- 31、查询1990年出生的学生名单
    -- 方法一
    SELECT s_name FROM student WHERE YEAR(s_birth)='1990'
    -- 方法二
    SELECT s_name FROM student WHERE s_birth LIKE '1990%'
     
    -- 32、查询每门课程的平均成绩,结果按平均成绩降序排列,平均成绩相同时,按课程编号升序排列
    SELECT c_id,ROUND(avg(s_score),2)FROM score 
    GROUP BY c_id
    ORDER BY avg(s_score) DESC,c_id ASC 
     
    -- 33、查询平均成绩大于等于85的所有学生的学号、姓名和平均成绩 
    SELECT a.s_id,a.s_name,ROUND(avg(b.s_score),2) AS 平均成绩 FROM student a 
    LEFT JOIN score b ON a.s_id=b.s_id
    GROUP BY a.s_id
    HAVING avg(b.s_score) >= 85
     
    -- 34、查询课程名称为"数学",且分数低于60的学生姓名和分数 
    SELECT a.s_name,b.s_score FROM student a 
    LEFT JOIN score b ON a.s_id=b.s_id
    WHERE c_id=(
    SELECT c_id FROM course WHERE c_name='数学'
    ) AND b.s_score < 60
     
    -- 35、查询所有学生的课程及分数情况; 
    -- 方法一:
    SELECT a.s_id,a.s_name,b.s_score,c.s_score,d.s_score FROM student a 
    LEFT JOIN score b ON a.s_id=b.s_id AND b.c_id='01'
    LEFT JOIN score c ON a.s_id=c.s_id AND c.c_id='02'
    LEFT JOIN score d ON a.s_id=d.s_id AND d.c_id='03'
    GROUP BY a.s_id
     
    -- 方法二:
     SELECT a.s_id,a.s_name,
    SUM(CASE c.c_name WHEN '语文' THEN b.s_score ELSE 0 END) AS '语文',
    SUM(CASE c.c_name WHEN '数学' THEN b.s_score ELSE 0 END) AS '数学',
    SUM(CASE c.c_name WHEN '英语' THEN b.s_score ELSE 0 END) AS '英语',
    SUM(b.s_score) as  '总分'
    FROM student a 
    LEFT JOIN score b ON a.s_id = b.s_id 
    LEFT JOIN course c ON b.c_id = c.c_id 
    GROUP BY a.s_id,a.s_name
     
     -- 36、查询任何一门课程成绩在70分以上的姓名、课程名称和分数; 
     
    SELECT a.s_name,c.c_name,b.s_score FROM student a
    LEFT JOIN score b ON a.s_id=b.s_id
    LEFT JOIN course c ON b.c_id=c.c_id
    HAVING b.s_score > 70
     
    -- 37、查询不及格的学生id,姓名,及其课程名称,分数
    SELECT a.s_id,a.s_name,c.c_name,b.s_score FROM student a 
    LEFT JOIN score b ON a.s_id=b.s_id
    LEFT JOIN course c ON b.c_id=c.c_id
    WHERE b.s_score < 60
     
     
    -- 38、查询课程编号为01且课程成绩在80分以上的学生的学号和姓名; 
    SELECT b.s_id,b.s_name FROM score a
    LEFT JOIN student b ON a.s_id=b.s_id
    WHERE a.c_id='01' AND a.s_score>80
     
    -- 39、求每门课程的学生人数 
    SELECT c_id,COUNT(1) FROM score GROUP BY c_id
     
     
    -- 40、查询选修"张三"老师所授课程的学生中,成绩最高的学生信息及其成绩
     
    SELECT t_id FROM teacher a WHERE a.t_name='张三'
     
    SELECT c_id FROM course b WHERE t_id=(SELECT t_id FROM teacher a WHERE a.t_name='张三')
     
    SELECT c.*,d.s_score FROM student c
    LEFT JOIN score d ON c.s_id=d.s_id AND d.c_id=
    (SELECT c_id FROM course b WHERE t_id=(SELECT t_id FROM teacher a WHERE a.t_name='张三'))
    HAVING  MAX(d.s_score)
     
     
    -- 41、查询不同课程成绩相同的学生的学生编号、课程编号、学生成绩 
    SELECT DISTINCT a.s_id,a.c_id,a.s_score 
    FROM score a,score b WHERE a.s_score=b.s_score AND a.c_id!=b.c_id
     
    -- 42、查询每门功课成绩最好的前两名 
    -- 方法一
    SELECT *FROM
    (SELECT a.s_id,a.c_id,a.s_score, @i:=@i+1 as 排名 FROM score a,(SELECT @i:=0)b WHERE a.c_id='01' ORDER BY a.s_score DESC
    ) c
    WHERE 排名 BETWEEN 1 AND 2
    UNION
    SELECT *FROM
    (SELECT a.s_id,a.c_id,a.s_score, @j:=@j+1 as 排名 FROM score a,(SELECT @j:=0)b WHERE a.c_id='02' ORDER BY a.s_score DESC
    ) c
    WHERE 排名 BETWEEN 1 AND 2
    UNION
    SELECT *FROM
    (SELECT a.s_id,a.c_id,a.s_score, @k:=@k+1 as 排名 FROM score a,(SELECT @k:=0)b WHERE a.c_id='03' ORDER BY a.s_score DESC
    ) c
    WHERE 排名 BETWEEN 1 AND 2
     
    -- 方法二
    -- SELECT a.s_id,a.c_id,a.s_score FROM score a
    -- WHERE (SELECT COUNT(1) FROM score b WHERE b.c_id=a.c_id AND b.s_score>=a.s_score)<=2 ORDER BY a.c_id
     
    -- 43、统计每门课程的学生选修人数(超过5人的课程才统计)。要求输出课程号和选修人数,查询结果按人数降序排列,若人数相同,按课程号升序排列  
    SELECT c_id AS 课程号,COUNT(1) AS 选修人数 FROM score 
    GROUP BY c_id 
    HAVING COUNT(1)>5 
    ORDER BY COUNT(1) DESC,c_id
     
    -- 44、检索至少选修两门课程的学生学号 
    SELECT s_id,COUNT(1) FROM score GROUP BY s_id HAVING COUNT(1)>=2
     
     
    -- 45、查询选修了全部课程的学生信息 
    SELECT COUNT(1) FROM course
     
    SELECT b.* FROM score a
    LEFT JOIN student b ON a.s_id=b.s_id
    GROUP BY a.s_id 
    HAVING COUNT(1)=(SELECT COUNT(1) FROM course)
     
    -- 46、查询各学生的年龄
     -- 按照出生日期来算,当前月日<出生年月的月日则,年龄减一
    -- 方法一
    SELECT s_id,s_birth,(DATE_FORMAT(NOW(),'%Y')-DATE_FORMAT(s_birth,'%Y'))-
    (CASE WHEN DATE_FORMAT(NOW(),'%m%d')< DATE_FORMAT(s_birth,'%m%d') THEN 1 ELSE 0 END) AS age
    FROM student
     
    -- 47、查询本周过生日的学生
    -- 方法一
    SELECT * FROM student WHERE WEEK(CURRENT_DATE)=WEEK(s_birth)
    -- 方法二
    SELECT * FROM student WHERE WEEK(DATE_FORMAT(NOW(),'%Y%m%d'))=WEEK(s_birth)
     
    -- 48、查询下周过生日的学生
    SELECT * FROM student WHERE WEEK(DATE_FORMAT(NOW(),'%Y%m%d'))+1=WEEK(s_birth)
     
    -- 49、查询本月过生日的学生
    SELECT * FROM student WHERE MONTH(NOW())=MONTH(s_birth)
     
    -- 50、查询下月过生日的学生
    SELECT * FROM student WHERE MONTH(NOW())+1=MONTH(s_birth)
     
    SELECT DATE_FORMAT(NOW(),'%Y')
    SELECT DATE_FORMAT(NOW(),'%Y%m%d')
    

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