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@@ -0,0 +1,494 @@
+1. ```text
+   *
+   创建表
+   --1.学生表
+   Student(SID,Sname,Sage,Ssex) --SID 学生编号,Sname 学生姓名,Sage 出生年月,Ssex 学生性别
+   --2.课程表
+   Course(CID,Cname,TID) --CID --课程编号,Cname 课程名称,TID 教师编号
+   --3.教师表
+   Teacher(TID,Tname) --TID 教师编号,Tname 教师姓名
+   --4.成绩表
+   SC(SID,CID,score) --SID 学生编号,CID 课程编号,score 分数
+   */
+   
+   ALTER DATABASE students CHARACTER SET utf8;
+   
+   CREATE TABLE student 
+   (SID VARCHAR(10),Sname VARCHAR(20),Sage DATETIME,Ssex VARCHAR(10));
+   # 插入数据
+   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');
+   
+   CREATE TABLE Course(CID VARCHAR(10),Cname VARCHAR(10),TID VARCHAR(10));
+   INSERT INTO Course VALUES('01' , '语文' , '02');
+   INSERT INTO Course VALUES('02' , '数学' , '01');
+   INSERT INTO Course VALUES('03' , '英语' , '03');
+   
+   CREATE TABLE Teacher(TID VARCHAR(10),Tname VARCHAR(10));
+   INSERT INTO Teacher VALUES('01' , '张三');
+   INSERT INTO Teacher VALUES('02' , '李四');
+   INSERT INTO Teacher VALUES('03' , '王五');
+   
+   CREATE TABLE SC(SID VARCHAR(10),CID VARCHAR(10),score DECIMAL(18,1));
+   INSERT INTO SC VALUES('01' , '01' , 80);
+   INSERT INTO SC VALUES('01' , '02' , 90);
+   INSERT INTO SC VALUES('01' , '03' , 99);
+   INSERT INTO SC VALUES('02' , '01' , 70);
+   INSERT INTO SC VALUES('02' , '02' , 60);
+   INSERT INTO SC VALUES('02' , '03' , 80);
+   INSERT INTO SC VALUES('03' , '01' , 80);
+   INSERT INTO SC VALUES('03' , '02' , 80);
+   INSERT INTO SC VALUES('03' , '03' , 80);
+   INSERT INTO SC VALUES('04' , '01' , 50);
+   INSERT INTO SC VALUES('04' , '02' , 30);
+   INSERT INTO SC VALUES('04' , '03' , 20);
+   INSERT INTO SC VALUES('05' , '01' , 76);
+   INSERT INTO SC VALUES('05' , '02' , 87);
+   INSERT INTO SC VALUES('06' , '01' , 31);
+   INSERT INTO SC VALUES('06' , '03' , 34);
+   INSERT INTO SC VALUES('07' , '02' , 89);
+   INSERT INTO SC 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 s.*,a.`score` AS score01,b.`score` AS score02
+   FROM student AS s
+   JOIN SC AS a ON  s.`SID`=a.`SID` AND a.`CID`="01"
+   LEFT JOIN SC AS b ON  s.`SID`=b.`SID` AND b.`CID`="02"
+   OR b.`CID` = NULL 
+   WHERE a.`score`>b.`score`;
+   
+   #--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 s.*,a.`score` AS score01,b.`score` AS score02
+   FROM student AS s
+   JOIN SC AS a ON  s.`SID`=a.`SID` AND a.`CID`="01"
+   LEFT JOIN SC AS b ON  s.`SID`=b.`SID` AND b.`CID`="02"
+     OR b.`CID` = NULL 
+   WHERE a.`score`<b.`score`;
+   
+   # 3、查询平均成绩大于等于60分的同学的学生编号和学生姓名和平均成绩
+   SELECT s.SID,s.Sname,AVG(a.score) AS avg_score
+   FROM student AS s
+   JOIN SC AS a ON s.`SID`=a.`SID`
+   GROUP BY a.`SID`
+   HAVING avg_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
+   SELECT s.SID,s.Sname,AVG(a.score) AS avg_score
+   FROM student AS s
+   JOIN SC AS a ON s.`SID`=a.`SID`
+   GROUP BY a.`SID`
+   HAVING avg_score < 60;
+   
+   # 5、查询所有同学的学生编号、学生姓名、选课总数、所有课程的总成绩
+   #【这里一定要用外连接，因为有的同学没有选课，不然用内连接筛选不出来】
+   SELECT s.`SID`,s.`Sname`,COUNT(a.`CID`) AS 选课总数,SUM(a.`score`) AS 总成绩
+   FROM student AS s
+   LEFT JOIN SC AS a ON s.`SID`=a.`SID`
+   GROUP BY a.`SID`,s.`Sname`;
+   
+   #--6、查询"李"姓老师的数量
+   SELECT COUNT(*)
+   FROM teacher
+   WHERE Tname LIKE '李%';
+   #7、查询学过"张三"老师授课的同学的信息
+   #方法一：
+   SELECT s.*
+   FROM student AS s
+   JOIN SC AS a ON s.`SID`=a.`SID`
+   JOIN course AS c ON c.`CID`=a.`CID`
+   JOIN teacher AS t ON t.`TID`=c.`TID`
+   WHERE t.`Tname` = '张三';
+   # 方法二：
+   SELECT s.*
+   FROM student AS s
+   JOIN SC AS a ON s.`SID`=a.`SID`
+   WHERE a.`CID` IN(
+   SELECT CID FROM course AS c
+   WHERE TID=(
+   SELECT TID FROM teacher AS t 
+   WHERE t.`Tname`='张三')
+   );
+   
+   # --8、查询没学过"张三"老师授课的同学的信息
+   SELECT s.*
+   FROM student AS s
+   WHERE s.`SID` NOT IN (
+   SELECT a.`SID` FROM student AS a JOIN SC AS b ON a.`SID`=b.`SID`
+   WHERE b.`CID` IN (
+   SELECT c.`CID`  FROM course AS c JOIN teacher AS t ON c.`TID`=t.`TID`
+   WHERE t.`Tname`='张三')
+   );
+   
+   #--9、查询学过编号为"01"并且也学过编号为"02"的课程的同学的信息
+   SELECT s.*,a.`CID`,b.`CID`
+   FROM student AS s,SC AS a,SC AS b
+   WHERE s.`SID`=a.`SID`
+   AND s.`SID`=b.`SID`
+   AND a.`CID`='01' 
+   AND b.`CID`='02';
+   
+   #--10、查询学过编号为"01"但是没有学过编号为"02"的课程的同学的信息
+   SELECT s.*
+   FROM student AS s
+   WHERE s.`SID` IN (
+   SELECT SID FROM SC WHERE CID='01'
+   )
+   AND s.`SID` NOT IN(
+   SELECT SID FROM SC WHERE CID='02'
+   );
+   
+   #--11、查询没有学全所有课程的同学的信息
+   SELECT s.*
+   FROM student AS s
+   LEFT JOIN SC AS a ON s.`SID`=a.`SID`
+   GROUP BY s.`SID`
+   HAVING COUNT(s.`SID`)< (SELECT COUNT(*) FROM course);
+   # --12、查询至少有一门课与学号为"01"的同学所学相同的同学的信息
+   # 方法一：
+   SELECT DISTINCT s.*
+   FROM student AS s
+   JOIN SC AS a ON s.`SID`=a.`SID`
+   WHERE a.`CID` IN (
+   SELECT CID
+   FROM student AS s
+   JOIN SC AS a ON s.`SID`=a.`SID`
+   WHERE s.`SID`='01'
+   );
+   # 方法二：
+   SELECT * FROM student 
+   WHERE SID IN(
+   SELECT DISTINCT a.SID FROM SC a
+                     WHERE a.CID IN(SELECT a.CID FROM SC a WHERE a.SID='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 s.*
+   FROM student AS s
+   JOIN SC AS a ON s.`SID`=a.`SID`
+   GROUP BY a.`SID`
+   HAVING GROUP_CONCAT(a.`CID`) = (
+   SELECT GROUP_CONCAT(CID)
+   FROM SC
+   GROUP BY SID
+   HAVING SID = '01')
+   AND a.`SID` != '01';
+   
+   #--14、查询没学过"张三"老师讲授的任一门课程的学生姓名
+   SELECT  s.`Sname`
+   FROM student AS s
+   WHERE s.`SID` NOT IN (
+   SELECT SID FROM SC AS a
+   WHERE CID = (
+   SELECT CID FROM course AS c
+   JOIN teacher AS t ON c.`TID`=t.`TID`
+   WHERE t.`Tname`='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 s.`SID`,s.`Sname`,AVG(a.`score`) AS 平均成绩
+   FROM student AS s
+   LEFT JOIN SC AS a ON s.`SID`=a.`SID`
+   WHERE s.`SID` IN (
+   SELECT SID FROM SC WHERE score<60 
+   GROUP BY SID 
+   HAVING COUNT(*)>=2
+   )
+   GROUP BY s.`SID`,s.`Sname`;
+   
+   #--16、检索"01"课程分数小于60，按分数降序排列的学生信息
+   # 方法一：
+   SELECT s.*,a.`score`
+   FROM student AS s,SC AS a
+   WHERE s.`SID`=a.`SID`
+   AND a.`CID`='01'
+   AND a.`score`<60
+   ORDER BY score DESC;
+   
+   # 方法二：
+   SELECT s.*,a.`score`
+   FROM student AS s
+   LEFT JOIN SC AS a ON s.`SID`=a.`SID`
+   WHERE a.`CID`='01'
+   AND a.`score`<60
+   ORDER BY score DESC;
+   
+   # --17、按平均成绩从高到低显示所有学生的所有课程的成绩以及平均成绩
+   # 这个题做得我头挺大的   最后参考了别人写的代码
+   SELECT s.`SID`,s.`Sname`,
+   SUM(CASE WHEN a.`CID`='01'THEN IFNULL(a.`score`,0)ELSE 0 END) AS '语文',
+   SUM(CASE WHEN a.`CID`='02'THEN IFNULL(a.`score`,0)ELSE 0 END) AS '数学',
+   SUM(CASE WHEN a.`CID`='02'THEN IFNULL(a.`score`,0)ELSE 0 END) AS '英语',
+   ROUND(AVG(a.`score`),2) AS 平均成绩
+   FROM SC AS a
+   LEFT JOIN student AS s ON a.`SID`=s.`SID`
+   GROUP BY a.`SID`
+   UNION
+   SELECT s1.SID,s1.Sname,0 AS '语文',0 AS '数学',0 AS '英语',0 AS '平均成绩'
+   FROM student AS s1
+   WHERE s1.SID NOT IN (SELECT DISTINCT SID FROM SC)
+   ORDER BY 平均成绩 DESC;
+   
+   #--18、查询各科成绩最高分、最低分和平均分：以如下形式显示：课程ID，课程name，最高分，最低分，平均分，及格率，中等率，优良率，优秀率
+   # 及格为>=60，中等为：70-80，优良为：80-90，优秀为：>=90
+   SELECT a.`CID`,b.`Cname`,MAX(a.`score`) AS 最高分,MIN(a.`score`) AS 最低分,AVG(a.`score`) AS 平均分,
+   ROUND(100*(SUM(CASE WHEN a.`score`>=60 THEN 1 ELSE 0 END)/SUM(CASE WHEN a.`score` THEN 1 ELSE 0 END)),2) AS 及格率,
+   ROUND(100*(SUM(CASE WHEN a.`score`>=70 AND a.`score`<80  THEN 1 ELSE 0 END)/SUM(CASE WHEN a.`score` THEN 1 ELSE 0 END)),2) AS 中等率,
+   ROUND(100*(SUM(CASE WHEN a.`score`>=80 AND a.`score`<90  THEN 1 ELSE 0 END)/SUM(CASE WHEN a.`score` THEN 1 ELSE 0 END)),2) AS 优良率,
+   ROUND(100*(SUM(CASE WHEN a.`score`>=90 THEN 1 ELSE 0 END)/SUM(CASE WHEN a.`score` THEN 1 ELSE 0 END)),2) AS 优秀率
+   FROM SC AS a 
+   LEFT JOIN course AS b ON a.`CID`=b.`CID`
+   GROUP BY a.`CID`,b.`Cname`;
+   
+   # --19、按各科成绩进行排序，并显示排名
+   #(我觉得这个题使用开窗函数蛮好的，但我用的mysql5.5不支持开窗函数，我就不用这个方法了)
+   SELECT a.* ,  (SELECT COUNT(DISTINCT score) FROM SC WHERE CID = a.CID AND score >a.score) +1 
+   AS 排名 FROM SC AS a ORDER BY a.cid , 排名;
+   
+   # --20、查询学生的总成绩并进行排名（没做完）
+   SELECT s.`SID`,s.`Sname`,SUM(a.`score`) AS 总成绩
+   FROM student AS s
+   LEFT JOIN SC AS a ON s.`SID`=a.`SID`
+   GROUP BY s.`SID`,s.`Sname`
+   ORDER BY 总成绩;
+   
+   # --21、查询不同老师所教不同课程平均分从高到低显示 
+   SELECT t.`TID`,t.`Tname`,ROUND(AVG(a.`score`),2) AS 平均分,c.`CID`
+   FROM course AS c
+   LEFT JOIN teacher AS t ON t.`TID`=c.`TID`
+   LEFT JOIN SC AS a ON a.`CID`=c.`CID`
+   GROUP BY t.`TID`
+   ORDER BY 平均分 DESC;
+   
+   # --22、查询所有课程的成绩第2名到第3名的学生信息及该课程成绩
+   SELECT a.* , (SELECT COUNT(DISTINCT score) FROM SC WHERE CID = a.CID AND score > a.score) + 1 AS 排名
+   FROM SC AS a HAVING 排名 BETWEEN 2 AND 3 ORDER BY a.CID,排名;
+   
+   # --23、统计各科成绩各分数段人数：课程编号,课程名称, 100-85 , 85-70 , 70-60 , 0-60 及所占百分比 
+   SELECT DISTINCT f.Cname,a.CID,b.`85-100`,b.百分比,c.`70-85`,c.百分比,d.`60-70`,d.百分比,e.`0-60`,e.百分比 FROM SC AS a
+   LEFT JOIN (SELECT CID,SUM(CASE WHEN score >85 AND score <=100 THEN 1 ELSE 0 END) AS `85-100`,
+   ROUND(100*(SUM(CASE WHEN score >85 AND score <=100 THEN 1 ELSE 0 END)/COUNT(*)),2) AS 百分比
+   FROM SC GROUP BY CID) AS b ON a.CID=b.CID
+   LEFT JOIN (SELECT CID,SUM(CASE WHEN score >70 AND score <=85 THEN 1 ELSE 0 END) AS `70-85`,
+   ROUND(100*(SUM(CASE WHEN score >70 AND score <=85 THEN 1 ELSE 0 END)/COUNT(*)),2) AS 百分比
+   FROM SC GROUP BY CID) AS c ON a.CID=c.CID
+   LEFT JOIN (SELECT CID,SUM(CASE WHEN score >60 AND score <=70 THEN 1 ELSE 0 END) AS `60-70`,
+   ROUND(100*(SUM(CASE WHEN score >60 AND score <=70 THEN 1 ELSE 0 END)/COUNT(*)),2) AS 百分比
+   FROM SC GROUP BY CID) AS d ON a.CID=d.CID
+   LEFT JOIN (SELECT CID,SUM(CASE WHEN score >=0 AND score <=60 THEN 1 ELSE 0 END) AS `0-60`,
+   ROUND(100*(SUM(CASE WHEN score >=0 AND score <=60 THEN 1 ELSE 0 END)/COUNT(*)),2) AS 百分比
+   FROM SC GROUP BY CID)e ON a.CID=e.CID
+   LEFT JOIN course AS f ON a.CID = f.CID;
+   
+   # --24、查询学生平均成绩及其名次
+   SELECT t1.*,(SELECT COUNT(1) FROM (
+   SELECT  s.SID,s.Sname,AVG(a.score) AS 平均成绩
+   FROM student AS s
+   LEFT JOIN SC AS a ON s.SID = a.SID
+   GROUP BY s.SID,s.Sname) AS t2
+   WHERE 平均成绩>t1.平均成绩)+1 AS 排名
+   FROM
+   (SELECT  s.SID,s.Sname,AVG(a.score) AS 平均成绩
+   FROM student AS s
+   LEFT JOIN SC AS a ON s.SID = a.SID
+   GROUP BY s.SID,s.Sname) AS t1
+   ORDER BY 排名;
+   
+   # --25、查询各科成绩前三名的记录
+   SELECT a.`SID`,a.`CID`,a.`score`,COUNT(b.score) +1 AS 排名
+   FROM SC AS a
+   LEFT JOIN SC AS b ON a.`CID`=b.`CID` AND a.`score`<b.`score`
+   GROUP BY a.`SID`,a.`CID`,a.`score`
+   HAVING COUNT(b.`SID`)<3
+   ORDER BY a.`CID`,排名 ;
+   
+   # --26、查询每门课程被选修的学生数
+   SELECT a.`CID`,COUNT(a.`SID`)
+   FROM SC AS a 
+   GROUP BY a.`CID`;
+   
+   # --27、查询出只有两门课程的全部学生的学号和姓名
+   SELECT s.`SID`,s.`Sname`
+   FROM student AS s
+   JOIN SC AS a ON s.`SID`=a.`SID`
+   GROUP BY s.`SID`,s.`Sname`
+   HAVING COUNT(a.`CID`)=2;
+   
+   #--28、查询男生、女生人数
+   SELECT s.`Ssex`,COUNT(s.`Ssex`) AS 人数
+   FROM student AS s
+   GROUP BY s.`Ssex`;
+   
+   # --29、查询名字中含有"风"字的学生信息
+   SELECT s.*
+   FROM student AS s
+   WHERE s.`Sname` LIKE '%风%';
+   #--30、查询同名同性学生名单，并统计同名人数
+   SELECT a.`Sname`,a.`Ssex`
+   FROM student AS a
+   JOIN student AS b ON a.`SID`=b.`SID` AND a.`Sname`=b.`Sname` AND a.`Ssex`=b.`Ssex`
+   GROUP BY a.`SID`,a.`Sname`
+   HAVING COUNT(*)>1;
+   
+   # --31、查询1990年出生的学生名单(注：Student表中Sage列的类型是datetime)
+   SELECT s.*
+   FROM student AS s
+   WHERE s.`Sage` LIKE '1990%';
+   
+   # --32、查询每门课程的平均成绩，结果按平均成绩降序排列，平均成绩相同时，按课程编号升序排列
+   SELECT a.`CID`,ROUND(AVG(a.`score`),2) AS 平均成绩
+   FROM SC AS a
+   GROUP BY a.`CID`
+   ORDER BY 平均成绩 DESC,a.`CID` ASC;
+   
+   # --33、查询平均成绩大于等于85的所有学生的学号、姓名和平均成绩
+   SELECT s.`SID`,s.`Sname`,ROUND(AVG(a.`score`),2) AS 平均成绩
+   FROM student AS s
+   LEFT JOIN SC AS a ON s.`SID`=a.`SID`
+   GROUP BY s.`SID`,s.`Sname`
+   HAVING 平均成绩 >= 85;
+   
+   
+   #--34、查询课程名称为"数学"，且分数低于60的学生姓名和分数
+   SELECT s.`Sname`,a.`score`
+   FROM SC AS a
+   LEFT JOIN student AS s ON a.`SID`=s.`SID`
+   LEFT JOIN course AS c ON a.`CID`=c.`CID`
+   WHERE c.`Cname`='数学'
+   AND a.`score`<60;
+   
+   # --35、查询所有学生的课程及分数情况；
+   SELECT s.*,a.`CID`,c.`Cname`,a.`score`
+   FROM student AS s,SC AS a,course AS c
+   WHERE s.`SID`=a.`SID` AND a.`CID`=c.`CID`
+   GROUP BY s.`SID`,a.`CID`
+   ORDER BY s.`SID`,a.`CID`;
+   
+   #--36、查询任何一门课程成绩在70分以上的姓名、课程名称和分数；
+   SELECT s.`Sname`,a.`score`,c.`Cname`
+   FROM SC AS a
+   LEFT JOIN student AS s ON a.`SID`=s.`SID`
+   LEFT JOIN course AS c ON a.`CID`=c.`CID`
+   WHERE a.`score`>70
+   GROUP BY s.`SID`,a.`CID`;
+   
+   # --37、查询不及格的课程
+   SELECT s.*,a.`CID`,c.`Cname`,a.`score`
+   FROM SC AS a
+   JOIN course AS c ON a.`CID`=c.`CID`
+   JOIN student AS s ON a.`SID`=s.`SID`
+   WHERE a.`score`<60;
+   
+   #--38、查询课程编号为01且课程成绩在80分以上的学生的学号和姓名；
+   SELECT s.`SID`,s.`Sname`,a.`CID`,a.`score`
+   FROM student AS s
+   LEFT JOIN SC AS a ON s.`SID`=a.`SID`
+   WHERE a.`CID`=01
+   AND a.`score`>=80;
+   
+   # --39、求每门课程的学生人数
+   SELECT a.`CID`,COUNT(a.`CID`)AS 学生人数
+   FROM SC AS a
+   GROUP BY a.`CID`;
+   
+   # --40、查询选修"张三"老师所授课程的学生中，成绩最高的学生信息及其成绩
+   SELECT s.*,a.`CID`,a.`score`,t.`Tname`
+   FROM student AS s
+   JOIN SC AS a ON a.`SID`=s.`SID`
+   JOIN course AS c ON c.`CID`=a.`CID`
+   JOIN teacher AS t ON t.`TID`=c.`TID`
+   WHERE t.`Tname`='张三'
+   AND a.`score` IN (
+   SELECT MAX(score) FROM  SC AS a 
+   JOIN course AS c ON c.`CID`=a.`CID`
+   JOIN teacher AS t ON t.`TID`=c.`TID`
+   WHERE t.`Tname`='张三'
+   );
+   
+   #--41、查询不同课程成绩相同的学生的学生编号、课程编号、学生成绩
+   SELECT a.*
+   FROM SC AS a,(
+   SELECT CID,score 
+   FROM SC
+   GROUP BY CID ,score
+   HAVING COUNT(1)>1
+   ) AS b
+   WHERE a.`CID`=b.CID
+   AND a.`score`=b.score
+   ORDER BY a.`CID`,a.`SID`,a.`score`;
+   
+   #--42、查询每门功课成绩最好的前两名
+   SELECT a.`CID`,a.`SID`,a.`score`
+   FROM SC AS a
+   WHERE (
+   SELECT COUNT(1) FROM SC AS b 
+   WHERE b.`CID`=a.`CID`
+   AND b.`score`>=a.`score`
+   ) <= 2
+   ORDER BY a.`CID`;
+   
+   # --43、统计每门课程的学生选修人数（超过5人的课程才统计）。要求输出课程号和选修人数，
+   # 查询结果按人数降序排列，若人数相同，按课程号升序排列 
+   SELECT a.`CID`,COUNT(*) AS 选修人数
+   FROM SC AS a
+   GROUP BY a.`CID`
+   HAVING COUNT(*)>5
+   ORDER BY 选修人数 DESC,a.`CID` ASC;
+   
+   
+   # --44、检索至少选修两门课程的学生学号
+   SELECT a.`SID`,COUNT(*) AS 选修课程数
+   FROM SC AS a
+   GROUP BY a.`SID`
+   HAVING COUNT(*)>=2;
+   
+   # --45、查询选修了全部课程的学生信息
+   SELECT s.*,COUNT(*) AS 选修课程数
+   FROM student AS s,SC AS a
+   WHERE s.`SID`=a.`SID`
+   GROUP BY a.`SID`
+   HAVING COUNT(*)>=3;
+   
+   SELECT *
+   FROM student 
+   WHERE SID IN (
+   SELECT SID FROM SC 
+   GROUP BY SID
+   HAVING COUNT(*)=(SELECT COUNT(*) FROM course)
+   );
+   
+   # --46、查询各学生的年龄
+   #1.只按年份来算
+   SELECT SID,Sname,YEAR(NOW())-YEAR(Sage) AS 年龄
+   FROM student AS s；
+   #2.按出生日期来算，过了生日那一天肯定就是大一岁了
+   SELECT SID,Sname,Sage,(DATE_FORMAT(NOW(),'%Y')-DATE_FORMAT(Sage,'%Y')-
+   (CASE
+   WHEN DATE_FORMAT(NOW(),'%m%d')>DATE_FORMAT(Sage,'%m%d')
+   THEN 0
+   ELSE 1
+   END)) AS 年龄
+   FROM student;
+   
+   # --47、查询本周过生日的学生
+   SELECT *
+   FROM student
+   WHERE WEEK(DATE_FORMAT(NOW(),'%Y%m%d'))=WEEK(Sage);
+   
+   # --48、查询下周过生日的学生
+   SELECT *
+   FROM student
+   WHERE WEEK(DATE_FORMAT(NOW(),'%Y%m%d'))+1=WEEK(Sage);
+   
+   # --49、查询本月过生日的学生
+   SELECT *
+   FROM student
+   WHERE MONTH(DATE_FORMAT(NOW(),'%Y%m%d'))=MONTH(Sage);
+   
+   # --50、查询下月过生日的学生
+   SELECT *
+   FROM student
+   WHERE MONTH(DATE_FORMAT(NOW(),'%Y%m%d'))+1=MONTH(Sage);
+   ```
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