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2022-08-17 第四组王佳齐学习笔记

2022-08-17 19:04:55 作者：互联网

练习笔记

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：

分析需求

拆步骤

分步写sql

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

学习心得

今天做了很多练习，感觉比昨天掌握的好一点了。

标签：JOIN,王佳齐,17,score,第四组,LEFT,id,SELECT,NAME
来源： https://www.cnblogs.com/yiboxh-10000/p/16596400.html

2022-08-17 第四组 王佳齐 学习笔记

练习笔记

DQL查询语言

子查询

需求

学习心得

今天做了很多练习，感觉比昨天掌握的好一点了。

2022-08-17 第四组王佳齐学习笔记