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2022-08-17 DQL----子查询，日期格式

2022-08-22 分类：野生技术 阅读(234) 评论(0)

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

来源：https://www.cnblogs.com/YQuicksilver/p/16610950.html
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