举例：MySQL 使用DQL命令查询数据

DROP TABLE IF EXISTS `grade`; 

CREATE TABLE `grade` (
  `GradeID` INT(11) NOT NULL AUTO_INCREMENT COMMENT '年级编号',
  `GradeName` VARCHAR(50) NOT NULL COMMENT '年级名称',
  PRIMARY KEY (`GradeID`)
) ENGINE=INNODB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;
 
INSERT  INTO `grade`(`GradeID`,`GradeName`) VALUES (1,'大一'),(2,'大二'),(3,'大三'),(4,'大四'),(5,'预科班');
  
DROP TABLE IF EXISTS `result`;

CREATE TABLE `result` (
  `StudentNo` INT(4) NOT NULL COMMENT '学号',
  `SubjectNo` INT(4) NOT NULL COMMENT '课程编号',
  `ExamDate` DATETIME NOT NULL COMMENT '考试日期',
  `StudentResult` INT(4) NOT NULL COMMENT '考试成绩',
  KEY `SubjectNo` (`SubjectNo`)
) ENGINE=INNODB DEFAULT CHARSET=utf8;
 

INSERT  INTO `result`(`StudentNo`,`SubjectNo`,`ExamDate`,`StudentResult`) VALUES (1000,'111111','郭靖',1,1,'13500000001','北京海淀区中关村大街1号','1986-12-11 00:00:00','[email protected]','450323198612111234'),(1001,'123456','李文才',1,2,'13500000002','河南洛阳','1981-12-31 00:00:00','[email protected]','450323198112311234'),(1002,'111111','李斯文',1,1,'13500000003','天津市和平区','1986-11-30 00:00:00','[email protected]','450323198611301234'),(1003,'123456','武松',1,3,'13500000004','上海卢湾区','1986-12-31 00:00:00','[email protected]','450323198612314234'),(1004,'123456','张三',1,4,'13500000005','北京市通州','1989-12-31 00:00:00','[email protected]','450323198612311244'),(1005,'123456','张秋丽 ',2,1,'13500000006','广西桂林市灵川','1986-12-31 00:00:00','[email protected]','450323198612311214'),(1006,'123456','肖梅',2,4,'13500000007','地址不详','1986-12-31 00:00:00','[email protected]','450323198612311134'),(1007,'111111','欧阳峻峰',1,1,'13500000008','北京东城区','1986-12-31 00:00:00','[email protected]','450323198612311133'),(1008,'111111','梅超风',1,1,'13500000009','河南洛阳','1986-12-31 00:00:00','[email protected]','450323198612311221'),(1009,'123456','刘毅',1,2,'13500000011','安徽','1986-12-31 00:00:00','[email protected]','450323198612311231'),(1010,'111111','大凡',1,1,'13500000012','河南洛阳','1986-12-31 00:00:00','[email protected]','450323198612311044'),(1011,'111111','奥丹斯',1,1,'13500000013','北京海淀区中关村大街*号','1984-12-31 00:00:00','[email protected]','450323198412311234'),(1012,'123456','多伦',2,3,'13500000014','广西南宁中央大街','1986-12-31 00:00:00','[email protected]','450323198612311334'),(1013,'123456','李梅',2,1,'13500000015','上海卢湾区','1986-12-31 00:00:00','[email protected]','450323198612311534'),(1014,'123456','张得',2,4,'13500000016','北京海淀区中关村大街*号','1986-12-31 00:00:00','[email protected]','450323198612311264'),(1015,'123456','李东方',1,4,'13500000017','广西桂林市灵川','1976-12-31 00:00:00','[email protected]','450323197612311234'),(1016,'111111','刘奋斗',1,1,'13500000018','上海卢湾区','1986-12-31 00:00:00','[email protected]','450323198612311251'),(1017,'123456','可可',2,3,'13500000019','北京长安街1号','1981-09-10 00:00:00','[email protected]','450323198109108311'),(10066,'','Tom',1,1,'13500000000','','0000-00-00 00:00:00','[email protected]','33123123123123123');

DROP TABLE IF EXISTS `student`;

CREATE TABLE `student` (
  `StudentNo` INT(4) NOT NULL COMMENT '学号',
  `LoginPwd` VARCHAR(20) DEFAULT NULL,
  `StudentName` VARCHAR(20) DEFAULT NULL COMMENT '学生姓名',
  `Sex` TINYINT(1) DEFAULT NULL COMMENT '性别，取值0或1',
  `GradeId` INT(11) DEFAULT NULL COMMENT '年级编号',
  `Phone` VARCHAR(50) NOT NULL COMMENT '联系电话，允许为空，即可选输入',
  `Address` VARCHAR(255) NOT NULL COMMENT '地址，允许为空，即可选输入',
  `BornDate` DATETIME DEFAULT NULL COMMENT '出生时间',
  `Email` VARCHAR(50) NOT NULL COMMENT '邮箱账号，允许为空，即可选输入',
  `IdentityCard` VARCHAR(18) DEFAULT NULL COMMENT '身份证号',
  PRIMARY KEY (`StudentNo`),
  UNIQUE KEY `IdentityCard` (`IdentityCard`),
  KEY `Email` (`Email`)
) ENGINE=MYISAM DEFAULT CHARSET=utf8;
 
INSERT  INTO `student`(`StudentNo`,`LoginPwd`,`StudentName`,`Sex`,`GradeId`,`Phone`,`Address`,`BornDate`,`Email`,`IdentityCard`) VALUES (1000,'111111','郭靖',1,1,'13500000001','北京海淀区中关村大街1号','1986-12-11 00:00:00','[email protected]','450323198612111234'),(1001,'123456','李文才',1,2,'13500000002','河南洛阳','1981-12-31 00:00:00','[email protected]','450323198112311234'),(1002,'111111','李斯文',1,1,'13500000003','天津市和平区','1986-11-30 00:00:00','[email protected]','450323198611301234'),(1003,'123456','武松',1,3,'13500000004','上海卢湾区','1986-12-31 00:00:00','[email protected]','450323198612314234'),(1004,'123456','张三',1,4,'13500000005','北京市通州','1989-12-31 00:00:00','[email protected]','450323198612311244'),(1005,'123456','张秋丽 ',2,1,'13500000006','广西桂林市灵川','1986-12-31 00:00:00','[email protected]','450323198612311214'),(1006,'123456','肖梅',2,4,'13500000007','地址不详','1986-12-31 00:00:00','[email protected]','450323198612311134'),(1007,'111111','欧阳峻峰',1,1,'13500000008','北京东城区','1986-12-31 00:00:00','[email protected]','450323198612311133'),(1008,'111111','梅超风',1,1,'13500000009','河南洛阳','1986-12-31 00:00:00','[email protected]','450323198612311221'),(1009,'123456','刘毅',1,2,'13500000011','安徽','1986-12-31 00:00:00','[email protected]','450323198612311231'),(1010,'111111','大凡',1,1,'13500000012','河南洛阳','1986-12-31 00:00:00','[email protected]','450323198612311044'),(1011,'111111','奥丹斯',1,1,'13500000013','北京海淀区中关村大街*号','1984-12-31 00:00:00','[email protected]','450323198412311234'),(1012,'123456','多伦',2,3,'13500000014','广西南宁中央大街','1986-12-31 00:00:00','[email protected]','450323198612311334'),(1013,'123456','李梅',2,1,'13500000015','上海卢湾区','1986-12-31 00:00:00','[email protected]','450323198612311534'),(1014,'123456','张得',2,4,'13500000016','北京海淀区中关村大街*号','1986-12-31 00:00:00','[email protected]','450323198612311264'),(1015,'123456','李东方',1,4,'13500000017','广西桂林市灵川','1976-12-31 00:00:00','[email protected]','450323197612311234'),(1016,'111111','刘奋斗',1,1,'13500000018','上海卢湾区','1986-12-31 00:00:00','[email protected]','450323198612311251'),(1017,'123456','可可',2,3,'13500000019','北京长安街1号','1981-09-10 00:00:00','[email protected]','450323198109108311'),(10066,'','Tom',1,1,'13500000000','','0000-00-00 00:00:00','[email protected]','33123123123123123');

DROP TABLE IF EXISTS `subject`;

CREATE TABLE `subject` (
  `SubjectNo` INT(11) NOT NULL AUTO_INCREMENT COMMENT '课程编号',
  `SubjectName` VARCHAR(50) DEFAULT NULL COMMENT '课程名称',
  `ClassHour` INT(4) DEFAULT NULL COMMENT '学时',
  `GradeID` INT(4) DEFAULT NULL COMMENT '年级编号',
  PRIMARY KEY (`SubjectNo`)
) ENGINE=INNODB AUTO_INCREMENT=18 DEFAULT CHARSET=utf8;
  
INSERT  INTO `subject`(`SubjectNo`,`SubjectName`,`ClassHour`,`GradeID`) VALUES (1,'高等数学-1',110,1),(2,'高等数学-2',110,2),(3,'高等数学-3',100,3),(4,'高等数学-4',130,4),(5,'C语言-1',110,1),(6,'C语言-2',110,2),(7,'C语言-3',100,3),(8,'C语言-4',130,4),(9,'JAVA第一学年',110,1),(10,'JAVA第二学年',110,2),(11,'JAVA第三学年',100,3),(12,'JAVA第四学年',130,4),(13,'数据库结构-1',110,1),(14,'数据库结构-2',110,2),(15,'数据库结构-3',100,3),(16,'数据库结构-4',130,4),(17,'C#基础',130,1);
 

#select concat('学生:',studentname) 学生姓名 ,concat('+86',phone) 电话 from student;

# select count(studnetno) 学生总数 from student;

#DISTINCT关键字作用：去掉SELECT查询返回的记录结果中重复的记录（所有返回列 的值都相同），只返回一条 语法： SELECT DISTINCT 字段名1,字段名2... FROM 表名 
#注意：ALL 关键字是默认的，返回所有的记录，与之相反
#DISTINCT关键字的使用
#查询成绩表中的所包含的课程ID 
SELECT   DISTINCT  SubjectNo  FROM  result;

#BETWEEN AND查询
SELECT  *  FROM  SUBJECT  WHERE     ClassHour   BETWEEN  110   AND  120;
#等同于:
SELECT  *  FROM  SUBJECT  WHERE     ClassHour >= 110  AND  ClassHour <=120;

#查询课程名称、总课时及每天10课时计算的天数
SELECT  
SubjectName AS "课程名称",
ClassHour AS "总课时", 
(ClassHour/10)  AS "均课时/天" 
FROM  SUBJECT; 

#查询包含“数学”的所有课程
SELECT   *  FROM SUBJECT WHERE SubjectName  LIKE  "%数学%";
#查询所有姓名为“李**”三个字的学生信息
 SELECT  StudentNo,StudentName FROM student  WHERE StudentName 
LIKE "李__";

#案例 查找地址不为空的学生信息
SELECT  *
FROM  student
WHERE  Address IS NOT NUL

#内连接 左连接 右链接
SELECT s.studentno,s.studentname,g.Gradename FROM 
student s INNER JOIN `grade` g ON g.gradeid=s.GradeID;
SELECT s.studentno,s.studentname,g.Gradename FROM 
student s LEFT JOIN `grade` g ON g.gradeid=s.GradeID;
SELECT s.studentno,s.studentname,g.Gradename FROM 
student s  RIGHT JOIN `grade` g ON g.gradeid=s.GradeID;

#使用IN进行范围查询
SELECT  *  FROM  SUBJECT  WHERE    ClassHour = 100  OR ClassHour =110 OR ClassHour  = 120;  #普通组处理方式
SELECT  *  FROM  SUBJECT  WHERE ClassHour  IN ( 100, 110,120 );#使用IN进行查询方式，更为简洁，效率更高

#子查询   in
SELECT studentno,studentname
FROM student
WHERE StudentNo IN 
(SELECT studentno FROM result WHERE 
subjectno=(SELECT subjectno FROM `subject` WHERE subjectname='高等数学-2' )
AND StudentResult>=80
 ) ;

#常用统计函数
#COUNT(),MAX(),MIN(),AVG(),SUM
SELECT  COUNT(StudentResult) AS "成绩总个数", MAX(StudentResult) AS "最高分" ,  MIN(StudentResult) AS "最低分" , AVG(StudentResult) AS "平均分"
FROM result;

#按照不同的课程，分别算出其平均分、最高分和最低分，对于低于60分平均分的不予显示
SELECT 
s.SubjectName AS "课程名",MAX(StudentResult) AS "最高分" ,  MIN(StudentResult) AS "最低分" ,  AVG(StudentResult) AS "平均分" 
FROM 
result  AS r 
LEFT JOIN 
`subject` AS s ON s.SubjectNo = r.SubjectNo
GROUP BY  r.SubjectNo HAVING   AVG(StudentResult) >= 60 ;

# ORDER BY 排序
SELECT studentresult
 FROM result r WHERE 
 r.subjectno IN(SELECT subjectno FROM `subject` WHERE subjectname="数据库结构-1" )
ORDER BY studentresult DESC ; #默认ASC

#查询所有课程《数据库结构-2》的考试成绩，并按照由高到低的显示，同时把该成绩对应的学生的学号、姓名打印出来
#要求：
#第一次查询前10条记录
#第二次查询11-20条记录
SELECT s.studentno,s.studentname,c.subjectname,r.studentresult
FROM student s JOIN result r
ON s.StudentNo=r.StudentNo
JOIN SUBJECT c
ON r.SubjectNo=c.SubjectNo
WHERE c.subjectname="数据库结构-2"
ORDER BY r.StudentResult DESC LIMIT 0,5;  #limit 5,5

#要求：从subject和grade数据表查询课程名称和所属年级 名称
#非等值连接查询
SELECT SubjectName,GradeName FROM SUBJECT,grade;  
#等值查询
SELECT SubjectName,GradeName FROM SUBJECT,grade WHERE  subject.GradeID= grade.GradeID;  .GradeID;