學習鏈接：https://blog.csdn.net/qq_43119297/article/details/82559687

1：創建book表，並插入數據

CREATE TABLE book(
         bookID VARCHAR(20) PRIMARY KEY ,
          bookName VARCHAR(50) DEFAULT NULL
   );

INSERT INTO book VALUES
(1,‘java基礎‘),
(2,‘javaweb‘),
(3,‘JDBC‘),
(4,‘HTML高級‘);

2:創建loan表

CREATE TABLE loan(
         uid INT,
         bookID VARCHAR(32),
         lnum INT,
         PRIMARY KEY(uid,bookID)
       );

INSERT INTO loan VALUES
(1,1,8),
(2,4,2),
(3,3,3),
(4,2,7),
(5,1,1),
(2,3,10),
(2,2,3),
(3,1,5);

3.創建user表，並插入數據

 CREATE TABLE USER(
    uid INT AUTO_INCREMENT PRIMARY KEY,
    username VARCHAR(20) UNIQUE,
    age INT,
    sex VARCHAR(2)
 );

INSERT INTO USER VALUES 
(NULL,‘周瑜‘,22,‘男‘),
(NULL,‘小喬‘,18,‘女‘),
(NULL,‘甄宓‘,23,‘女‘),
(NULL,‘曹操‘,30,‘男‘),
(NULL,‘貂蟬‘,26,‘女‘);  

4，問題

– 1.查詢年齡小於 25的學生姓名、學生年齡

– 2.查詢年齡在18-22之間（包含18和22）的學生信息

–3. 統計學生表中男女的數量分別是多少

– 4.查詢學生的總人數、平均年齡、最小年齡

– 5.查詢借閱過圖書的每個學生ID借閱的總次數

– 6.查詢圖書名稱包含”java”的圖書數量

–7. 查詢借閱總次數大於50的學生學號

–8. 查詢借閱了’HTML高級’的學生學號

–9. 查詢沒有借閱過圖書的學生學號、學生姓名

– 10.查詢借閱過’javaWeb’圖書的學生學號、借閱次數

– 11.查詢年齡比”周瑜”大的所有學生姓名、學生年齡

– 12.查詢年齡最大的學生ID、學生姓名，學生年齡

–13. 查詢借閱了圖書的學生姓名、借閱的不同圖書總數、借閱所有圖書總次

–14. 查詢被借閱的每本圖書的圖書名稱、借閱總次數

– 15.查詢借閱次數最多的圖書ID和借閱的總次數
--------------------- 

5，答案

1：
SELECT username,age
FROM USER
WHERE age<25;

2：
SELECT username,age
FROM USER
WHERE age BETWEEN 18 AND 22;

3：
SELECT sex,COUNT(username) 數量
FROM USER
GROUP BY sex;

4：
SELECT COUNT(uid) 總人數,AVG(age) 平均年齡,MIN(age) 最小年齡
FROM USER;

5：
SELECT uid,COUNT(lnum)借閱次數
FROM loan
GROUP BY uid;

6：
SELECT SUM(b.`lnum`) 總數
FROM book a,loan b
AND bookname LIKE "java%";

7：
SELECT uid
FROM loan
GROUP BY uid HAVING SUM(lnum)>5;

8：
 SELECT user.`uid`
 FROM book,USER,loan
 WHERE loan.`bookID`=book.`bookID` AND 
 loan.`uid` =user.`uid` AND
 book.`bookName`=‘HTML高級‘;

9：
 SELECT user.`uid`,user.`username`
 FROM book,USER,loan
 WHERE loan.`bookID`=book.`bookID` AND 
 loan.`uid` =user.`uid` AND
 book.`bookName`NOT IN
 (‘HTML高級‘,‘JDBC‘,‘javaweb‘,‘java基礎‘);
--------------------- 

10：
SELECT user.`uid`,SUM(loan.`lnum`) 次數
 FROM book,USER,loan
 WHERE loan.`bookID`=book.`bookID` AND 
 loan.`uid` =user.`uid` AND
 book.`bookName`=‘javaWeb‘
 GROUP BY book.`bookName`;

11：
 SELECT username,age
WHERE username=‘周瑜‘) t
 WHERE a.`age`>t.b;

12：
SELECT uid, username,age
 FROM USER a , (SELECT MAX(age) b FROM USER ) t
 WHERE a.`age`=t.b;

13：
 SELECT a.`username`,s.e 借閱的不同圖書總數,s.b 借閱所有圖書總次
 FROM USER a,(SELECT d.`uid` k,SUM(d.`lnum`) e ,
 COUNT(d.`bookID`) b FROM loan d GROUP BY uid ) s
 WHERE a.`uid`=s.k;


14：
 SELECT  book.bookname 書名,n.v 借閱次數  
 FROM book,(SELECT bookid s,SUM(lnum) v FROM
  loan GROUP BY bookid)  n
 WHERE n.s=book.`bookID`;

15：
SELECT  bookID,MAX(n.a)  
FROM (SELECT bookid,SUM(lnum) a 
FROM loan GROUP BY bookid) n;

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