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2022-08-17 第二组刘禹彤学习笔记

2022-08-17 19:02:15 作者：互联网

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###学习内容

DQL数据库查询语言（续）

子查询（自连接）

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：1.where型的子查询，如果是where列=（内层sql），则内层的sql必须是单行单列，单个值

2.where型子查询，如果where（列1,列2）=（内层sql）,内层sql返回必须是单列,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：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;

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

-- 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;

-- 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 
	)

-- 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%';

-- 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 
	)

-- 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;

-- 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';

-- 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;

-- 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;

-- 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' );

-- 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;

-- 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;
--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

-- 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;

-- 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 );

-- 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;

-- 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;

--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;

-- 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

-- 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

-- 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;

-- 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;

-- 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
	)

-- 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 );

-- 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

-- 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;

--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';

-- 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	缩写的月名
%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（001-365）
%k	小时（0-23）
%l	小时1（1-12）
%M	月名
%m	月，数值（00-12）
%p	AM或PM
%r	时间，12小时（hhmmss 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，4位---2022
%y	年，2位----22

###学习心得

今天上课做了很多DQL的相关练习，感觉将知识点基本掌握了，练习也做得不错

###掌握情况：一般

###课上练习

SELECT
    t.`name`,
    c.`name`cname,
    MAX( score ),
    MIN( score ) 
FROM
    teacher t
    LEFT JOIN course c ON c.t_id = t.id
    LEFT JOIN scores sc ON sc.c_id = c.id 
WHERE
    t.`name` = 'Tom' 
    OR t.`name` = 'Jerry'
    GROUP BY
    c.`name`

###运行结果

标签：JOIN,NAME,17,08,score,刘禹彤,id,SELECT,LEFT
来源： https://www.cnblogs.com/lyt0612/p/16596415.html

2022-08-17 第二组刘禹彤 学习笔记

DQL数据库查询语言（续）

子查询（自连接）

需求

日期格式

2022-08-17 第二组刘禹彤学习笔记