寫(xiě)法1
case  column_name 
    when value1 then returnValue1  
    when value2 then returnValue2 
    ...
    else returnValueN end;
寫(xiě)法2
case  
when condition1 then returnValue1  
when condition2 then returnValue2 
...
else returnValueN end;

window_function(expr) OVER ([PARTITION BY partition_expression, ...]
[ORDER BY sort_expression [ASC | DESC], ...])

select  column_name,column_name,.... from table_name 
union [all]
select  column_name,column_name,.... from table_name 

--建表
--學(xué)生表
CREATE TABLE `Student`(
        `s_id` VARCHAR(20),
        `s_name` VARCHAR(20) NOT NULL DEFAULT '',
        `s_birth` VARCHAR(20) NOT NULL DEFAULT '',
        `s_sex` VARCHAR(10) NOT NULL DEFAULT '',
        PRIMARY KEY(`s_id`)
);
--課程表
CREATE TABLE `Course`(
        `c_id`  VARCHAR(20),
        `c_name` VARCHAR(20) NOT NULL DEFAULT '',
        `t_id` VARCHAR(20) NOT NULL,
        PRIMARY KEY(`c_id`)
);
--教師表
CREATE TABLE `Teacher`(
        `t_id` VARCHAR(20),
        `t_name` VARCHAR(20) NOT NULL DEFAULT '',
        PRIMARY KEY(`t_id`)
);
--成績(jī)表
CREATE TABLE `Score`(
        `s_id` VARCHAR(20),
        `c_id`  VARCHAR(20),
        `s_score` INT(3),
        PRIMARY KEY(`s_id`,`c_id`)
);
--插入學(xué)生表測(cè)試數(shù)據(jù)
insert into Student values('01' , '趙雷' , '1990-01-01' , '男');
insert into Student values('02' , '錢電' , '1990-12-21' , '男');
insert into Student values('03' , '孫風(fēng)' , '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' , '吳蘭' , '1992-03-01' , '女');
insert into Student values('07' , '鄭竹' , '1989-07-01' , '女');
insert into Student values('08' , '王菊' , '1990-01-20' , '女');
--課程表測(cè)試數(shù)據(jù)
insert into Course values('01' , '語(yǔ)文' , '02');
insert into Course values('02' , '數(shù)學(xué)' , '01');
insert into Course values('03' , '英語(yǔ)' , '03');
insert into Course values('04' , '體育' , '01');
--教師表測(cè)試數(shù)據(jù)
insert into Teacher values('01' , '張三');
insert into Teacher values('02' , '李四');
insert into Teacher values('03' , '王五');
--成績(jī)表測(cè)試數(shù)據(jù)
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、查詢"01"課程比"02"課程成績(jī)高的學(xué)生的信息及課程分?jǐn)?shù)   
select st.s_id , st.s_name , s1.s_score a1 ,  s2.s_score a2
		from student st 
				LEFT join score s1 on st.s_id = s1.s_id AND s1.c_id = '01'   
				LEFT JOIN score s2 on st.s_id = s2.s_id AND s2.c_id = '02'
						where s1.s_score IS NOT NULL 
								AND	s1.s_score > COALESCE(s2.s_score, 0); 
-- 2、查詢"01"課程比"02"課程成績(jī)低的學(xué)生的信息及課程分?jǐn)?shù)
select st.s_id , st.s_name , s1.s_score a1,  s2.s_score a2
		from student st 
				LEFT join score s1 on st.s_id = s1.s_id AND s1.c_id = '01'
				LEFT JOIN score s2 on st.s_id = s2.s_id AND s2.c_id = '02'
						where COALESCE(s1.s_score, 0)  < COALESCE(s2.s_score, 0)
3、查詢平均成績(jī)大于等于60分的同學(xué)的學(xué)生編號(hào)和學(xué)生姓名和平均成績(jī)
select st.s_id , st.s_name , avg(sc.s_score)AS avg_score
		from student st
				JOIN score sc on st.s_id = sc.s_id
						group by st.s_id, st.s_name
								HAVING AVG(sc.s_score) >= 60;
SELECT 
    st.s_id, 
    st.s_name, 
    AVG(sc.s_score) AS avg_score
FROM 
    Student st
JOIN 
    Score sc ON st.s_id = sc.s_id
GROUP BY 
    st.s_id, st.s_name
HAVING 
    AVG(sc.s_score) >= 60;
4、查詢平均成績(jī)小于60分的同學(xué)的學(xué)生編號(hào)和學(xué)生姓名和平均成績(jī)
                 (包括有成績(jī)的和無(wú)成績(jī)的)
SELECT 
    st.s_id, 
    st.s_name, 
    AVG(sc.s_score) 平均成績(jī)
FROM 
    Student st
LEFT JOIN 
    Score sc ON st.s_id = sc.s_id
GROUP BY 
    st.s_id, st.s_name						 
HAVING 
    AVG(sc.s_score) < 60 or AVG(sc.s_score) is NULL ;						 
5、查詢所有同學(xué)的學(xué)生編號(hào)、學(xué)生姓名、選課總數(shù)、所有課程的總成績(jī)
SELECT  
	st.s_id,
	st.s_name,
	count(sc.c_id),
	SUM(sc.s_score)
FROM 
	student st LEFT JOIN score sc on st.s_id=sc.s_id
GROUP BY st.s_id;
6、查詢"李"姓老師的數(shù)量 
SELECT
		count(0)
from
		teacher
WHERE
		t_name LIKE '李%';
7、查詢學(xué)過(guò)"張三"老師授課的同學(xué)的信息 
SELECT
	st.s_id,st.s_name,st.s_birth,st.s_sex
FROM
	student st
	JOIN score  sc on st.s_id=sc.s_id
	JOIN course c on sc.c_id=c.c_id
	JOIN teacher t on c.t_id=t.t_id
WHERE
	t.t_name='張三'
GROUP BY st.s_id;
8、查詢沒(méi)學(xué)過(guò)"張三"老師授課的同學(xué)的信息 
SELECT *
FROM student 
WHERE s_id not in (SELECT sc.s_id
		FROM score  sc 
			JOIN course c on sc.c_id=c.c_id
			JOIN teacher t on c.t_id=t.t_id
		WHERE t.t_name ='張三' GROUP BY sc.s_id ); 
SELECT *
FROM student 
	WHERE s_id not in (SELECT st.s_id
FROM student st
	JOIN score  sc on st.s_id=sc.s_id
	JOIN course c on sc.c_id=c.c_id
	JOIN teacher t on c.t_id=t.t_id
WHERE
	t.t_name='張三'
GROUP BY st.s_id
	 ); 
9、查詢學(xué)過(guò)編號(hào)為"01"并且也學(xué)過(guò)編號(hào)為"02"的課程的同學(xué)的信息
SELECT
	st.s_id,st.s_name,st.s_birth,st.s_sex
FROM
	student st
	JOIN score s1 on st.s_id=s1.s_id and s1.c_id='01' 
	JOIN score s2 on st.s_id=s2.s_id and s2.c_id='02' 
10、查詢學(xué)過(guò)編號(hào)為"01"但是沒(méi)有學(xué)過(guò)編號(hào)為"02"的課程的同學(xué)的信息
SELECT st.* FROM
	student st
	JOIN score s1 on st.s_id=s1.s_id and s1.c_id='01'
	WHERE st.s_id NOT IN (
		SELECT
			st.s_id
		FROM
			student st
			JOIN score s2 on st.s_id=s2.s_id and s2.c_id='02');
11、查詢沒(méi)有學(xué)全所有課程的同學(xué)的信息
SELECT st.* FROM
	student st 
	WHERE st.s_id NOT IN (
	SELECT st.s_id FROM
	student st 
		JOIN score s1 on st.s_id=s1.s_id and s1.c_id='01'
	WHERE st.s_id IN (
		SELECT st.s_id FROM student st
			JOIN score s2 on st.s_id=s2.s_id and s2.c_id='02')
		AND st.s_id  in (
		SELECT st.s_id FROM student st
			JOIN score s3 on st.s_id=s3.s_id and s3.c_id='03'))
SELECT st.* 
FROM student st 
LEFT JOIN score sc on st.s_id=sc.s_id
GROUP BY st.s_id
HAVING COUNT(1)<3;
12、查詢至少有一門課與學(xué)號(hào)為"01"的同學(xué)所學(xué)相同的同學(xué)的信息 
SELECT *
FROM student st
JOIN score s2 on st.s_id=s2.s_id
WHERE st.s_id <> '01' AND s2.c_id in(
SELECT s.c_id
FROM score s WHERE s.s_id='01') 
GROUP BY st.s_id;                                                            
SELECT s.c_id
FROM score s WHERE s.s_id='01';
13、查詢和"01"號(hào)的同學(xué)學(xué)習(xí)的課程完全相同的其他同學(xué)的信息 
SELECT st.*
FROM Student st
JOIN (
    SELECT s_id,COUNT(*)AS cnt,                -- 該生修的課程門數(shù)
            GROUP_CONCAT(c_id ORDER BY c_id) AS courses -- 課程按字典序拼成串
    FROM Score
    GROUP BY s_id
     ) t ON st.s_id = t.s_id
WHERE st.s_id <> '01'                         -- 排除 01 自己
  AND t.cnt = (SELECT COUNT(*) FROM Score WHERE s_id = '01')        -- 門數(shù)相同
  AND t.courses = (SELECT GROUP_CONCAT(c_id ORDER BY c_id)
                   FROM Score WHERE s_id = '01');                   -- 課程串相同
14、查詢沒(méi)學(xué)過(guò)"張三"老師講授的任一門課程的學(xué)生姓名 
SELECT t.s_name
FROM student t
WHERE t.s_id not in(
			SELECT s.s_id
			FROM score s
			WHERE s.c_id in(
						SELECT c_id 
						FROM teacher t 
						JOIN course c on t.t_id=c.t_id
						WHERE t.t_name='張三'));
			SELECT s.s_id
			FROM score s
			WHERE s.c_id in(
						SELECT c_id 
						FROM teacher t 
						JOIN course c on t.t_id=c.t_id
						WHERE t.t_name='張三');
SELECT c_id 
FROM teacher t 
JOIN course c on t.t_id=c.t_id
WHERE t.t_name='張三';
15、查詢兩門及其以上不及格課程的同學(xué)的學(xué)號(hào)，姓名及其平均成績(jī)
SELECT t.s_id ,t.s_name,avg(s.s_score)
FROM student t
join score s on t.s_id=s.s_id
WHERE s.s_score<60
GROUP BY s.s_id
HAVING  count(0)>=2;
SELECT s.s_id
FROM score s
WHERE s.s_score<60
GROUP BY s.s_id
HAVING  count(0)>=2;
16、檢索"01"課程分?jǐn)?shù)小于60，按分?jǐn)?shù)降序排列的學(xué)生信息
SELECT t.*,s.s_score
FROM student t
join score s on t.s_id=s.s_id and c_id='01'
WHERE s.s_score<60
order by s.s_score DESC;
17、按平均成績(jī)從高到低顯示所有學(xué)生的所有課程的成績(jī)以及平均成績(jī)
SELECT
    st.s_id,
    st.s_name,
    MAX(CASE WHEN sc.c_id = '01' THEN sc.s_score END) AS 語(yǔ)文,
    MAX(CASE WHEN sc.c_id = '02' THEN sc.s_score END) AS 數(shù)學(xué),
    MAX(CASE WHEN sc.c_id = '03' THEN sc.s_score END) AS 英語(yǔ),
    ROUND(AVG(sc.s_score), 2) AS avg_score
FROM Student st
JOIN Score sc ON st.s_id = sc.s_id
GROUP BY st.s_id, st.s_name
ORDER BY avg_score DESC;