首页 > 其他分享 >2022-08-17 第六小组 高佳誉 学习笔记

2022-08-17 第六小组 高佳誉 学习笔记

时间：2022-08-17 21:12:03浏览次数：53  
标签：JOIN NAME 17 08 score 2022 id SELECT LEFT

DQL查询语言

子查询

按照结果集的行列数不同，子查询可以分为以下几类：

  • 标量子查询：结果集只有一行一列（单行子查询）
  • 列子查询：结果集有一列多行
  • 行子查询：结果集有一行多列
  • 表子查询：结果集多行多列
-- 查询比小虎年龄大的所有学生
-- 标量子查询
SELECT
	* 
FROM
	student 
WHERE
	age > ( SELECT age FROM student WHERE NAME = '小虎' );

-- 查询有一门学科分数大于90分的学生信息
-- 列子查询
SELECT
	* 
FROM
	student 
WHERE
	id IN (
	SELECT
		s_id 
	FROM
		scores 
WHERE
	score > 90);

-- 查询男生且年龄最大的学生
-- 行子查询
SELECT
	* 
FROM
	student 
WHERE
	age = (
	SELECT
		max( age ) 
	FROM
		student 
	GROUP BY
		gender 
	HAVING
	gender = '男' 
	)
	
-- 优化
SELECT
	* 
FROM
	student 
WHERE
	( age, gender ) = (
	SELECT
		max( age ),
		gender 
	FROM
		student 
	GROUP BY
		gender 
	HAVING
	gender = '男' 
	)

总结：

  • where型子查询，如果是where 列 = （内层sql），则内层的sql返回的必须是单行单列，单个值。
  • where型子查询，如果是where (列1,列2) = （内层sql），内层的sql返回的必须是单列，可以是多行。
-- 取排名数学成绩前五的学生，正序排列
SELECT
	* 
FROM
	(
	SELECT
		s.*,
		sc.score score,
		c.NAME 科目 
	FROM
		student s
		LEFT JOIN scores sc ON s.id = sc.s_id
		LEFT JOIN course c ON c.id = sc.c_id 
	WHERE
		c.NAME = '数学' 
	ORDER BY
		score DESC 
		LIMIT 5 
	) t 
WHERE
	t.gender = '男';

经验分享：

  1. 分析需求
  2. 拆步骤
  3. 分步写sql
  4. 整合拼装sql
-- 查询每个老师的代课数
SELECT t.id, t.NAME,( SELECT count(*) FROM course c WHERE c.id = t.id ) AS 代课的数量 
FROM
	teacher t;
----------------------------------------------------------------------------
SELECT
	t.id,
	t.NAME,
	count(*) '代课的数量' 
FROM
	teacher t
	LEFT JOIN course c ON c.t_id = t.id 
GROUP BY
	t.id,
	t.NAME;

-- exists
SELECT
	* 
FROM
	teacher t 
WHERE
	EXISTS ( SELECT * FROM course c WHERE c.t_id = t.id );
----------------------------------------------------------------------------SELECT
	t.*,
	c.`name` 
FROM
	teacher t
	INNER JOIN course c ON t.id = c.t_id;

总结：如果一个需求可以不用子查询，尽量不使用。

sql可读性太低。

需求

-- 3.查询每个同学的最高成绩和科目名称****
SELECT
	t.id,
	t.NAME,
	c.id,
	c.NAME,
	r.score 
FROM
	(
	SELECT
		s.id,
		s.NAME,(
		SELECT
			max( score ) 
		FROM
			scores r 
		WHERE
			r.s_id = s.id 
		) score 
	FROM
		student s 
	) t
	LEFT JOIN scores r ON r.s_id = t.id 
	AND r.score = t.score
	LEFT JOIN course c ON r.c_id = c.id;
-- 5.查询每个课程的最高分的学生信息*****
SELECT
	* 
FROM
	student s 
WHERE
	id IN (
	SELECT DISTINCT
		r.s_id 
	FROM
		(
		SELECT
			c.id,
			c.NAME,
			max( score ) score 
		FROM
			student s
			LEFT JOIN scores r ON r.s_id = s.id
			LEFT JOIN course c ON c.id = r.c_id 
		GROUP BY
			c.id,
			c.NAME 
		) t
		LEFT JOIN scores r ON r.c_id = t.id 
	AND t.score = r.score 
	)

-- 6.查询名字中含有'张'或'李'字的学生的信息和各科成绩。
SELECT
	s.id,
	s.NAME sname,
	sc.score,
	c.NAME 
FROM
	student s
	LEFT JOIN scores sc ON s.id = sc.s_id
	LEFT JOIN course c ON sc.c_id = c.id 
WHERE
	s.NAME LIKE '%张%' 
	OR s.NAME LIKE '%李%';
-- 7.查询平均成绩及格的同学的信息。（子查询）
SELECT
	* 
FROM
	student 
WHERE
	id IN (
	SELECT
		sc.s_id 
	FROM
		scores sc 
	GROUP BY
		sc.s_id 
	HAVING
	avg( sc.score ) >= 70 
	)
-- 8.将学生按照总分数进行排名。（从高到低）
SELECT
	s.id,
	s.NAME,
	sum( sc.score ) score 
FROM
	student s
	LEFT JOIN scores sc ON s.id = sc.s_id 
GROUP BY
	s.id,
	s.NAME 
ORDER BY
	score DESC,
	s.id ASC;
-- 9.查询数学成绩的最高分、最低分、平均分。
SELECT
	c.NAME,
	max( sc.score ),
	min( sc.score ),
	avg( sc.score ) 
FROM
	course c
	LEFT JOIN scores sc ON c.id = sc.c_id 
WHERE
	c.NAME = '数学';
-- 10.将各科目按照平均分排序。
SELECT
	c.id,
	c.NAME,
	avg( sc.score ) score 
FROM
	course c
	LEFT JOIN scores sc ON c.id = sc.c_id 
GROUP BY
	c.id,
	c.NAME 
ORDER BY
	score DESC;

-- 11.查询老师的信息和他所带的科目的平均分
SELECT
	t.id,
	t.NAME,
	c.id cid,
	c.NAME cname,
	avg( r.score ) 
FROM
	teacher t
	LEFT JOIN course c ON t.id = c.t_id
	LEFT JOIN scores r ON r.c_id = c.id 
GROUP BY
	t.id,
	t.NAME,
	c.id,
	c.NAME;
-- 12.查询被"Tom"和"Jerry"教的课程的最高分和最低分
SELECT
	t.id,
	t.NAME,
	c.id cid,
	c.NAME cname,
	max( r.score ),
	min( r.score ) 
FROM
	teacher t
	LEFT JOIN course c ON t.id = c.t_id
	LEFT JOIN scores r ON r.c_id = c.id 
GROUP BY
	t.id,
	t.NAME,
	c.id,
	c.NAME 
HAVING
	t.NAME IN ( 'Tom', 'Jerry' );
-- 13.查询每个学生的最好成绩的科目名称（子查询）
SELECT
	t.id,
	t.sname,
	r.c_id,
	c.NAME,
	t.score 
FROM
	(
	SELECT
		s.id,
		s.NAME sname,
		max( r.score ) score 
	FROM
		student s
		LEFT JOIN scores r ON r.s_id = s.id 
	GROUP BY
		s.id,
		s.NAME 
	) t
	LEFT JOIN scores r ON r.s_id = t.id 
	AND r.score = t.score
	LEFT JOIN course c ON r.c_id = c.id;
-- 14.查询所有学生的课程及分数
SELECT
	s.id,
	s.NAME,
	c.id,
	c.NAME,
	r.score 
FROM
	student s
	LEFT JOIN scores r ON s.id = r.s_id
	LEFT JOIN course c ON c.id = r.c_id;
-- 15.查询课程编号为1且课程成绩在60分以上的学生的学号和姓名（子查询）
SELECT
	* 
FROM
	student s 
WHERE
	s.id IN (
	SELECT
		r.s_id 
	FROM
		scores r 
	WHERE
	r.c_id = 1 
	AND r.score > 60)
--------------------------------------------------------
SELECT
	s.*,
	r.* 
FROM
	student s
	LEFT JOIN scores r ON s.id = r.s_id 
WHERE
	r.c_id = 1 
	AND r.score > 60

-- 16. 查询平均成绩大于等于70的所有学生学号、姓名和平均成绩
SELECT
	s.id,
	s.NAME,
	t.score 
FROM
	student s
	LEFT JOIN ( SELECT r.s_id, avg( r.score ) score FROM scores r GROUP BY r.s_id ) t ON s.id = t.s_id 
WHERE
	t.score >= 70;
-- 17.查询有不及格课程的学生信息
SELECT
	* 
FROM
	student s 
WHERE
	id IN ( SELECT r.s_id FROM scores r GROUP BY r.s_id HAVING min( r.score ) < 60 );
-- 18.查询每门课程有成绩的学生人数
SELECT
	c.id,
	c.NAME,
	t.number 
FROM
	course c
	LEFT JOIN ( SELECT r.c_id, count(*) number FROM scores r GROUP BY r.c_id ) t ON c.id = t.c_id;
----------------------------------------------------
SELECT
	c.id,
	c.NAME,
	count(*) 
FROM
	course c
	LEFT JOIN scores r ON c.id = r.c_id 
GROUP BY
	c.id,
	c.NAME;
-- 19.查询每门课程的平均成绩，结果按照平均成绩降序排列，如果平均成绩相同，再按照课程编号升序排列
SELECT
	c.id,
	c.NAME,
	avg( score ) score 
FROM
	course c
	LEFT JOIN scores r ON c.id = r.c_id 
GROUP BY
	c.id,
	c.NAME 
ORDER BY
	score DESC,
	c.id ASC;
-- 20.查询平均成绩大于60分的同学的学生编号和学生姓名和平均成绩
SELECT
	s.id,
	s.NAME sname,
	avg( r.score ) score 
FROM
	student s
	LEFT JOIN scores r ON r.s_id = s.id
	LEFT JOIN course c ON c.id = r.c_id 
GROUP BY
	s.id,
	s.NAME 
HAVING
	score > 65;

-- 21.查询有且仅有一门课程成绩在80分以上的学生信息
SELECT
	* 
FROM
	student 
WHERE
	id IN ( SELECT r.s_id FROM scores r WHERE r.score > 80 GROUP BY r.s_id HAVING COUNT(*) = 1 );
----------------------------------------------------------------------------
SELECT
	s.id,
	s.NAME,
	s.gender 
FROM
	student s
	LEFT JOIN scores r ON s.id = r.s_id 
WHERE
	r.score > 80 
GROUP BY
	s.id,
	s.NAME,
	s.gender 
HAVING
	count(*) = 1
-- 22.查询出只有三门课程的学生的学号和姓名
SELECT
	* 
FROM
	student s 
WHERE
	id IN ( SELECT r.s_id FROM scores r GROUP BY r.s_id HAVING count(*) = 3 );
----------------------------------------------------------------------------
SELECT
	s.id,
	s.NAME,
	s.gender 
FROM
	student s
	LEFT JOIN scores r ON s.id = r.s_id 
GROUP BY
	s.id,
	s.NAME,
	s.gender 
HAVING
	count(*) = 3
-- 23.查询有不及格课程的课程信息
SELECT
	* 
FROM
	course c 
WHERE
	id IN (
	SELECT
		r.c_id 
	FROM
		scores r 
	GROUP BY
		r.c_id 
	HAVING
	min( r.score ) < 60 
	)
----------------------------------------------------------------------------
SELECT
	c.id,
	c.NAME 
FROM
	course c
	LEFT JOIN scores sc ON c.id = sc.c_id 
GROUP BY
	sc.c_id,
	c.NAME 
HAVING
	min( sc.score ) < 60;
-- 24.查询至少选择4门课程的学生信息
SELECT
	* 
FROM
	student 
WHERE
	id IN (
	SELECT
		r.s_id 
	FROM
		scores r 
	GROUP BY
		r.s_id 
	HAVING
	count(*) >= 4 
	)
----------------------------------------------------------------------------
SELECT
	s.id,
	s.NAME 
FROM
	student s
	LEFT JOIN scores r ON s.id = r.s_id 
GROUP BY
	s.id,
	s.NAME 
HAVING
	count(*) >= 4;
-- 25.查询没有选全所有课程的同学的信息
SELECT
	* 
FROM
	student 
WHERE
	id IN (
	SELECT
		r.s_id 
	FROM
		scores r 
	GROUP BY
		r.s_id 
	HAVING
	count(*) != 5
	)

-- 26.查询选全所有课程的同学的信息
SELECT
	s.id,
	s.NAME,
	count(*) number 
FROM
	student s
	LEFT JOIN scores r ON s.id = r.s_id 
GROUP BY
	s.id,
	s.NAME 
HAVING
	number = ( SELECT count(*) FROM course );
-- 27.查询各学生都选了多少门课
SELECT
	s.id,
	s.NAME,
	count(*) number 
FROM
	student s
	LEFT JOIN scores r ON s.id = r.s_id 
GROUP BY
	s.id,
	s.NAME
-- 28.查询课程名称为"java"，且分数低于60分的学生姓名和分数
SELECT
	s.id,
	s.NAME,
	r.score 
FROM
	student s
	LEFT JOIN scores r ON s.id = r.s_id
	LEFT JOIN course c ON r.c_id = c.id 
WHERE
	c.NAME = 'java' 
	AND r.score < 60;
-- 29.查询学过"Tony"老师授课的同学的信息
SELECT
	s.id,
	s.NAME 
FROM
	student s
	LEFT JOIN scores r ON r.s_id = s.id
	LEFT JOIN course c ON c.id = r.c_id
	LEFT JOIN teacher t ON t.id = c.t_id 
WHERE
	t.NAME = 'Tom';
-- 30.查询没学过"Tony"老师授课的学生信息
SELECT
	* 
FROM
	student 
WHERE
	id NOT IN (
	SELECT DISTINCT
		s.id 
	FROM
		student s
		LEFT JOIN scores r ON r.s_id = s.id
		LEFT JOIN course c ON c.id = r.c_id
		LEFT JOIN teacher t ON t.id = c.t_id 
	WHERE
	t.NAME = 'Tom' 
	)

日期格式

格式 描述
%a 缩写的星期名
%b 缩写月名
%c 月，数值
%D 带有英文前缀的月中的天
%d 月的天，数值（00-31）
%e 月的天，数值（0-31）
%f 微秒
%H 小时（00-23）
%h 小时（01-12）
%I 小时（01-12）
%i 分钟，数值（00-59）
%j 年的天（001-366）
%k 小时（0-23）
%l 小时（1-12）
%M 月名
%m 月，数值（00-12）
%p AM或PM
%r 时间，12-小时 （hh:mm:ss AM或PM）
%S 秒（00-59）
%s 秒（0-59）
%T 时间，24-小时（hh:mm:ss）
%U 周（00-53）星期日是一周的第一天
%u 周（00-53）星期一是一周的第一天
%W 星期名
%Y 年，2022
%y 年，22

标签：JOIN,NAME,17,08,score,2022,id,SELECT,LEFT
From： https://www.cnblogs.com/hanhandeshiqing/p/16596762.html

相关文章

  • acwing2022秋招每日一题 1302. 层数最深叶子节点的和
    题目给你一棵二叉树的根节点 root ，请你返回 层数最深的叶子节点的和 。思路根据层序遍历，访问所有节点。将每一层上的结点，进行统计，直到最后一层时，统计所有节点数。......
  • 2022-08-16 第五组 赖哲栋 学习笔记
    DQL数据库查询语言重点，DQL是我们每天都要接触编写最多也是最难的SQL，该语言用来查询记录，不会修改数据库和表结构。构建数据库创建一张student表：DROPTABLEIFEXISTSst......
  • 2022-08-17 第五组 赖哲栋 学习笔记
    DQL查询语言子查询按照结果集的行列数不同，子查询可以分为以下几类：标量子查询：结果集只有一行一列（单行子查询）列子查询：结果集有一列多行行子查询：结果集有一行多列表子......
  • 08. Qt绘图
    Qt绘图1.QPainter2D绘图离不开QPainter，可以把QPainter想象成一个画笔，开发人员拿着画笔理论上是可以绘制任何你想要的图形。QPainter一般在一个部件（widget）重绘事件（Pain......
  • 8.17总结
    自动刷题机\(solution\)二分答案找最大最小值考试时二分写错了ACCode#include<bits/stdc++.h>usingnamespacestd;#definelllonglonginlinellread(){ ll......
  • 2022/8/17 总结
    A.P4343[SHOI2015]自动刷题机啊对对对，算法都对了，二分写挂了：）Solution二分答案，每次\(\mathtt{O(n)}\)判断当前的\(mid\)是否可行，最大和最小分开二分；注意:......
  • 【2022-08-17】mysql基础知识(四)
    mysql基础知识(四)mysql之操作表的多条SQL语句修改表名普通方法：altertabletest1renametest;进阶方法：renametabletesttotest1;可同时修改多个：renam......
  • CF1719C Fighting Tournament 题解
    思路根据题意，很容易看出，每个人都完成一次比赛后，即完成\(n-1\)轮之后，力量值最大的人会留在第一的位置，且在第\(n-1\)轮完成后，除了力量值最大的人，其他人的胜场数都不会再......
  • CF1719A Chip Game 题解
    题目传送门。思路当其中一个人不能动的时候，这个人一定位于点\((n,m)\)上。令点\((n,m)\)为终点。当\(n\)和\(m\)都是奇数或当\(n\)和\(m\)都是偶数时，赢的人......
  • CF1719B Mathematical Circus 题解
    一道不错的构造题。思路先说一句废话，能被\(4\)整除的数在除以\(2\)之后得到的数还是一个偶数。我们可以根据\(k\)的奇偶性以及\(k\)除以\(2\)之后的奇偶性分......

赞助商

阅读排行

网站主页关于我们联系我们网站地图

本网站内容转载自其他媒体，侵权联系[admin##ips99.com]。

Copyright © 2020-2023 IPS99 版权所有 IPS99