写法1
case  column_name 
    when value1 then returnValue1  
    when value2 then returnValue2 
    ...
    else returnValueN end;
写法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
--学生表
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`)
);
--课程表
CREATE TABLE `Course`(
        `c_id`  VARCHAR(20),
        `c_name` VARCHAR(20) NOT NULL DEFAULT '',
        `t_id` VARCHAR(20) NOT NULL,
        PRIMARY KEY(`c_id`)
);
--教师表
CREATE TABLE `Teacher`(
        `t_id` VARCHAR(20),
        `t_name` VARCHAR(20) NOT NULL DEFAULT '',
        PRIMARY KEY(`t_id`)
);
--成绩表
CREATE TABLE `Score`(
        `s_id` VARCHAR(20),
        `c_id`  VARCHAR(20),
        `s_score` INT(3),
        PRIMARY KEY(`s_id`,`c_id`)
);
--插入学生表测试数据
insert into Student values('01' , '赵雷' , '1990-01-01' , '男');
insert into Student values('02' , '钱电' , '1990-12-21' , '男');
insert into Student values('03' , '孙风' , '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' , '吴兰' , '1992-03-01' , '女');
insert into Student values('07' , '郑竹' , '1989-07-01' , '女');
insert into Student values('08' , '王菊' , '1990-01-20' , '女');
--课程表测试数据
insert into Course values('01' , '语文' , '02');
insert into Course values('02' , '数学' , '01');
insert into Course values('03' , '英语' , '03');
insert into Course values('04' , '体育' , '01');
--教师表测试数据
insert into Teacher values('01' , '张三');
insert into Teacher values('02' , '李四');
insert into Teacher values('03' , '王五');
--成绩表测试数据
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、查询"01"课程比"02"课程成绩高的学生的信息及课程分数   
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、查询"01"课程比"02"课程成绩低的学生的信息及课程分数
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)  = 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、查询平均成绩小于60分的同学的学生编号和学生姓名和平均成绩
                 (包括有成绩的和无成绩的)
SELECT 
    st.s_id, 
    st.s_name, 
    AVG(sc.s_score) 平均成绩
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)  '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、查询和"01"号的同学学习的课程完全相同的其他同学的信息 
SELECT st.*
FROM Student st
JOIN (
    SELECT s_id,COUNT(*)AS cnt,                -- 该生修的课程门数
            GROUP_CONCAT(c_id ORDER BY c_id) AS courses -- 课程按字典序拼成串
    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')        -- 门数相同
  AND t.courses = (SELECT GROUP_CONCAT(c_id ORDER BY c_id)
                   FROM Score WHERE s_id = '01');                   -- 课程串相同
14、查询没学过"张三"老师讲授的任一门课程的学生姓名 
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='张三'));
			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='张三');
SELECT c_id 
FROM teacher t 
JOIN course c on t.t_id=c.t_id
WHERE t.t_name='张三';
15、查询两门及其以上不及格课程的同学的学号，姓名及其平均成绩
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=2;
SELECT s.s_id
FROM score s
WHERE s.s_score=2;
16、检索"01"课程分数小于60，按分数降序排列的学生信息
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