Mysql的学习4____数据的单标查询，连接查询，自连接查询，子查询
DQL（Data Query Language）：数据库查询语言
1.进行简单的单表查询，复杂的多表查询，嵌套查询；
/* SELECT [ALL | DISTINCT] {* | table.* | [table.field1[as alias1][,table.field2[as alias2]][,...]]} FROM table_name [as table_alias] [left | right | inner join table_name2] #联合查询 [WHERE ...] #指定结果需满足的条件 [GROUP BY ...] #指定结果按照哪几个字段来分组 [HAVING] #过滤分组的记录必须满足的次要条件 [ORDER BY ...] #指定查询记录按一个或多个条件排序 [LIMIT {[offset,]row_count | row_countOFFSET offset}]; # 指定查询的记录从哪条至哪条 */
注意：【】代表可选的，{ }代表必须的
2.实例演示在创建的school数据库中进行：
创建数据库的SQL语句如下：
1 SQLyog Ultimate v10.00 Beta1 2 MySQL - 5.7.19 : Database - school 3 ********************************************************************* 4 */ 5 6 7 /*!40101 SET NAMES utf8 */; 8 9 /*!40101 SET SQL_MODE=''*/; 10 11 /*!40014 SET @OLD_UNIQUE_CHECKS=@@UNIQUE_CHECKS, UNIQUE_CHECKS=0 */; 12 /*!40014 SET @OLD_FOREIGN_KEY_CHECKS=@@FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0 */; 13 /*!40101 SET @OLD_SQL_MODE=@@SQL_MODE, SQL_MODE='NO_AUTO_VALUE_ON_ZERO' */; 14 /*!40111 SET @OLD_SQL_NOTES=@@SQL_NOTES, SQL_NOTES=0 */; 15 CREATE DATABASE /*!32312 IF NOT EXISTS*/`school` /*!40100 DEFAULT CHARACTER SET utf8 */; 16 17 USE `school`; 18 19 /*Table structure for table `grade` */ 20 21 DROP TABLE IF EXISTS `grade`; 22 23 CREATE TABLE `grade` ( 24 `GradeID` INT(11) NOT NULL AUTO_INCREMENT COMMENT '年级编号', 25 `GradeName` VARCHAR(50) NOT NULL COMMENT '年级名称', 26 PRIMARY KEY (`GradeID`) 27 ) ENGINE=INNODB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8; 28 29 /*Data for the table `grade` */ 30 31 INSERT INTO `grade`(`GradeID`,`GradeName`) VALUES (1,'大一'),(2,'大二'),(3,'大三'),(4,'大四'),(5,'预科班'); 32 33 /*Table structure for table `result` */ 34 35 DROP TABLE IF EXISTS `result`; 36 37 CREATE TABLE `result` ( 38 `StudentNo` INT(4) NOT NULL COMMENT '学号', 39 `SubjectNo` INT(4) NOT NULL COMMENT '课程编号', 40 `ExamDate` DATETIME NOT NULL COMMENT '考试日期', 41 `StudentResult` INT(4) NOT NULL COMMENT '考试成绩', 42 KEY `SubjectNo` (`SubjectNo`) 43 ) ENGINE=INNODB DEFAULT CHARSET=utf8; 44 45 /*Data for the table `result` */ 46 47 INSERT INTO `result`(`StudentNo`,`SubjectNo`,`ExamDate`,`StudentResult`) VALUES (1000,1,'2013-11-11 16:00:00',94),(1000,2,'2012-11-10 10:00:00',75),(1000,3,'2011-12-19 10:00:00',76),(1000,4,'2010-11-18 11:00:00',93),(1000,5,'2013-11-11 14:00:00',97),(1000,6,'2012-09-13 15:00:00',87),(1000,7,'2011-10-16 16:00:00',79),(1000,8,'2010-11-11 16:00:00',74),(1000,9,'2013-11-21 10:00:00',69),(1000,10,'2012-11-11 12:00:00',78),(1000,11,'2011-11-11 14:00:00',66),(1000,12,'2010-11-11 15:00:00',82),(1000,13,'2013-11-11 14:00:00',94),(1000,14,'2012-11-11 15:00:00',98),(1000,15,'2011-12-11 10:00:00',70),(1000,16,'2010-09-11 10:00:00',74),(1001,1,'2013-11-11 16:00:00',76),(1001,2,'2012-11-10 10:00:00',93),(1001,3,'2011-12-19 10:00:00',65),(1001,4,'2010-11-18 11:00:00',71),(1001,5,'2013-11-11 14:00:00',98),(1001,6,'2012-09-13 15:00:00',74),(1001,7,'2011-10-16 16:00:00',85),(1001,8,'2010-11-11 16:00:00',69),(1001,9,'2013-11-21 10:00:00',63),(1001,10,'2012-11-11 12:00:00',70),(1001,11,'2011-11-11 14:00:00',62),(1001,12,'2010-11-11 15:00:00',90),(1001,13,'2013-11-11 14:00:00',97),(1001,14,'2012-11-11 15:00:00',89),(1001,15,'2011-12-11 10:00:00',72),(1001,16,'2010-09-11 10:00:00',90),(1002,1,'2013-11-11 16:00:00',61),(1002,2,'2012-11-10 10:00:00',80),(1002,3,'2011-12-19 10:00:00',89),(1002,4,'2010-11-18 11:00:00',88),(1002,5,'2013-11-11 14:00:00',82),(1002,6,'2012-09-13 15:00:00',91),(1002,7,'2011-10-16 16:00:00',63),(1002,8,'2010-11-11 16:00:00',84),(1002,9,'2013-11-21 10:00:00',60),(1002,10,'2012-11-11 12:00:00',71),(1002,11,'2011-11-11 14:00:00',93),(1002,12,'2010-11-11 15:00:00',96),(1002,13,'2013-11-11 14:00:00',83),(1002,14,'2012-11-11 15:00:00',69),(1002,15,'2011-12-11 10:00:00',89),(1002,16,'2010-09-11 10:00:00',83),(1003,1,'2013-11-11 16:00:00',91),(1003,2,'2012-11-10 10:00:00',75),(1003,3,'2011-12-19 10:00:00',65),(1003,4,'2010-11-18 11:00:00',63),(1003,5,'2013-11-11 14:00:00',90),(1003,6,'2012-09-13 15:00:00',96),(1003,7,'2011-10-16 16:00:00',97),(1003,8,'2010-11-11 16:00:00',77),(1003,9,'2013-11-21 10:00:00',62),(1003,10,'2012-11-11 12:00:00',81),(1003,11,'2011-11-11 14:00:00',76),(1003,12,'2010-11-11 15:00:00',61),(1003,13,'2013-11-11 14:00:00',93),(1003,14,'2012-11-11 15:00:00',79),(1003,15,'2011-12-11 10:00:00',78),(1003,16,'2010-09-11 10:00:00',96),(1004,1,'2013-11-11 16:00:00',84),(1004,2,'2012-11-10 10:00:00',79),(1004,3,'2011-12-19 10:00:00',76),(1004,4,'2010-11-18 11:00:00',78),(1004,5,'2013-11-11 14:00:00',81),(1004,6,'2012-09-13 15:00:00',90),(1004,7,'2011-10-16 16:00:00',63),(1004,8,'2010-11-11 16:00:00',89),(1004,9,'2013-11-21 10:00:00',67),(1004,10,'2012-11-11 12:00:00',100),(1004,11,'2011-11-11 14:00:00',94),(1004,12,'2010-11-11 15:00:00',65),(1004,13,'2013-11-11 14:00:00',86),(1004,14,'2012-11-11 15:00:00',77),(1004,15,'2011-12-11 10:00:00',82),(1004,16,'2010-09-11 10:00:00',87),(1005,1,'2013-11-11 16:00:00',82),(1005,2,'2012-11-10 10:00:00',92),(1005,3,'2011-12-19 10:00:00',80),(1005,4,'2010-11-18 11:00:00',92),(1005,5,'2013-11-11 14:00:00',97),(1005,6,'2012-09-13 15:00:00',72),(1005,7,'2011-10-16 16:00:00',84),(1005,8,'2010-11-11 16:00:00',79),(1005,9,'2013-11-21 10:00:00',76),(1005,10,'2012-11-11 12:00:00',87),(1005,11,'2011-11-11 14:00:00',65),(1005,12,'2010-11-11 15:00:00',67),(1005,13,'2013-11-11 14:00:00',63),(1005,14,'2012-11-11 15:00:00',64),(1005,15,'2011-12-11 10:00:00',99),(1005,16,'2010-09-11 10:00:00',97),(1006,1,'2013-11-11 16:00:00',82),(1006,2,'2012-11-10 10:00:00',73),(1006,3,'2011-12-19 10:00:00',79),(1006,4,'2010-11-18 11:00:00',63),(1006,5,'2013-11-11 14:00:00',97),(1006,6,'2012-09-13 15:00:00',83),(1006,7,'2011-10-16 16:00:00',78),(1006,8,'2010-11-11 16:00:00',88),(1006,9,'2013-11-21 10:00:00',89),(1006,10,'2012-11-11 12:00:00',82),(1006,11,'2011-11-11 14:00:00',70),(1006,12,'2010-11-11 15:00:00',69),(1006,13,'2013-11-11 14:00:00',64),(1006,14,'2012-11-11 15:00:00',80),(1006,15,'2011-12-11 10:00:00',90),(1006,16,'2010-09-11 10:00:00',85),(1007,1,'2013-11-11 16:00:00',87),(1007,2,'2012-11-10 10:00:00',63),(1007,3,'2011-12-19 10:00:00',70),(1007,4,'2010-11-18 11:00:00',74),(1007,5,'2013-11-11 14:00:00',79),(1007,6,'2012-09-13 15:00:00',83),(1007,7,'2011-10-16 16:00:00',86),(1007,8,'2010-11-11 16:00:00',76),(1007,9,'2013-11-21 10:00:00',65),(1007,10,'2012-11-11 12:00:00',87),(1007,11,'2011-11-11 14:00:00',69),(1007,12,'2010-11-11 15:00:00',69),(1007,13,'2013-11-11 14:00:00',90),(1007,14,'2012-11-11 15:00:00',84),(1007,15,'2011-12-11 10:00:00',95),(1007,16,'2010-09-11 10:00:00',92),(1008,1,'2013-11-11 16:00:00',96),(1008,2,'2012-11-10 10:00:00',62),(1008,3,'2011-12-19 10:00:00',97),(1008,4,'2010-11-18 11:00:00',84),(1008,5,'2013-11-11 14:00:00',86),(1008,6,'2012-09-13 15:00:00',72),(1008,7,'2011-10-16 16:00:00',67),(1008,8,'2010-11-11 16:00:00',83),(1008,9,'2013-11-21 10:00:00',86),(1008,10,'2012-11-11 12:00:00',60),(1008,11,'2011-11-11 14:00:00',61),(1008,12,'2010-11-11 15:00:00',68),(1008,13,'2013-11-11 14:00:00',99),(1008,14,'2012-11-11 15:00:00',77),(1008,15,'2011-12-11 10:00:00',73),(1008,16,'2010-09-11 10:00:00',78),(1009,1,'2013-11-11 16:00:00',67),(1009,2,'2012-11-10 10:00:00',70),(1009,3,'2011-12-19 10:00:00',75),(1009,4,'2010-11-18 11:00:00',92),(1009,5,'2013-11-11 14:00:00',76),(1009,6,'2012-09-13 15:00:00',90),(1009,7,'2011-10-16 16:00:00',62),(1009,8,'2010-11-11 16:00:00',68),(1009,9,'2013-11-21 10:00:00',70),(1009,10,'2012-11-11 12:00:00',83),(1009,11,'2011-11-11 14:00:00',88),(1009,12,'2010-11-11 15:00:00',65),(1009,13,'2013-11-11 14:00:00',91),(1009,14,'2012-11-11 15:00:00',99),(1009,15,'2011-12-11 10:00:00',65),(1009,16,'2010-09-11 10:00:00',83),(1010,1,'2013-11-11 16:00:00',83),(1010,2,'2012-11-10 10:00:00',87),(1010,3,'2011-12-19 10:00:00',89),(1010,4,'2010-11-18 11:00:00',99),(1010,5,'2013-11-11 14:00:00',91),(1010,6,'2012-09-13 15:00:00',96),(1010,7,'2011-10-16 16:00:00',72),(1010,8,'2010-11-11 16:00:00',72),(1010,9,'2013-11-21 10:00:00',98),(1010,10,'2012-11-11 12:00:00',73),(1010,11,'2011-11-11 14:00:00',68),(1010,12,'2010-11-11 15:00:00',62),(1010,13,'2013-11-11 14:00:00',67),(1010,14,'2012-11-11 15:00:00',69),(1010,15,'2011-12-11 10:00:00',71),(1010,16,'2010-09-11 10:00:00',66),(1011,1,'2013-11-11 16:00:00',62),(1011,2,'2012-11-10 10:00:00',72),(1011,3,'2011-12-19 10:00:00',96),(1011,4,'2010-11-18 11:00:00',64),(1011,5,'2013-11-11 14:00:00',89),(1011,6,'2012-09-13 15:00:00',91),(1011,7,'2011-10-16 16:00:00',95),(1011,8,'2010-11-11 16:00:00',96),(1011,9,'2013-11-21 10:00:00',89),(1011,10,'2012-11-11 12:00:00',73),(1011,11,'2011-11-11 14:00:00',82),(1011,12,'2010-11-11 15:00:00',98),(1011,13,'2013-11-11 14:00:00',66),(1011,14,'2012-11-11 15:00:00',69),(1011,15,'2011-12-11 10:00:00',91),(1011,16,'2010-09-11 10:00:00',69),(1012,1,'2013-11-11 16:00:00',86),(1012,2,'2012-11-10 10:00:00',66),(1012,3,'2011-12-19 10:00:00',97),(1012,4,'2010-11-18 11:00:00',69),(1012,5,'2013-11-11 14:00:00',70),(1012,6,'2012-09-13 15:00:00',74),(1012,7,'2011-10-16 16:00:00',91),(1012,8,'2010-11-11 16:00:00',97),(1012,9,'2013-11-21 10:00:00',84),(1012,10,'2012-11-11 12:00:00',82),(1012,11,'2011-11-11 14:00:00',90),(1012,12,'2010-11-11 15:00:00',91),(1012,13,'2013-11-11 14:00:00',91),(1012,14,'2012-11-11 15:00:00',97),(1012,15,'2011-12-11 10:00:00',85),(1012,16,'2010-09-11 10:00:00',90),(1013,1,'2013-11-11 16:00:00',73),(1013,2,'2012-11-10 10:00:00',69),(1013,3,'2011-12-19 10:00:00',91),(1013,4,'2010-11-18 11:00:00',72),(1013,5,'2013-11-11 14:00:00',76),(1013,6,'2012-09-13 15:00:00',87),(1013,7,'2011-10-16 16:00:00',61),(1013,8,'2010-11-11 16:00:00',77),(1013,9,'2013-11-21 10:00:00',83),(1013,10,'2012-11-11 12:00:00',99),(1013,11,'2011-11-11 14:00:00',91),(1013,12,'2010-11-11 15:00:00',84),(1013,13,'2013-11-11 14:00:00',98),(1013,14,'2012-11-11 15:00:00',74),(1013,15,'2011-12-11 10:00:00',92),(1013,16,'2010-09-11 10:00:00',90),(1014,1,'2013-11-11 16:00:00',64),(1014,2,'2012-11-10 10:00:00',81),(1014,3,'2011-12-19 10:00:00',79),(1014,4,'2010-11-18 11:00:00',74),(1014,5,'2013-11-11 14:00:00',65),(1014,6,'2012-09-13 15:00:00',88),(1014,7,'2011-10-16 16:00:00',86),(1014,8,'2010-11-11 16:00:00',77),(1014,9,'2013-11-21 10:00:00',86),(1014,10,'2012-11-11 12:00:00',85),(1014,11,'2011-11-11 14:00:00',86),(1014,12,'2010-11-11 15:00:00',75),(1014,13,'2013-11-11 14:00:00',89),(1014,14,'2012-11-11 15:00:00',79),(1014,15,'2011-12-11 10:00:00',73),(1014,16,'2010-09-11 10:00:00',68),(1015,1,'2013-11-11 16:00:00',99),(1015,2,'2012-11-10 10:00:00',60),(1015,3,'2011-12-19 10:00:00',60),(1015,4,'2010-11-18 11:00:00',75),(1015,5,'2013-11-11 14:00:00',78),(1015,6,'2012-09-13 15:00:00',78),(1015,7,'2011-10-16 16:00:00',84),(1015,8,'2010-11-11 16:00:00',95),(1015,9,'2013-11-21 10:00:00',93),(1015,10,'2012-11-11 12:00:00',79),(1015,11,'2011-11-11 14:00:00',74),(1015,12,'2010-11-11 15:00:00',65),(1015,13,'2013-11-11 14:00:00',63),(1015,14,'2012-11-11 15:00:00',74),(1015,15,'2011-12-11 10:00:00',67),(1015,16,'2010-09-11 10:00:00',65),(1016,1,'2013-11-11 16:00:00',97),(1016,2,'2012-11-10 10:00:00',90),(1016,3,'2011-12-19 10:00:00',77),(1016,4,'2010-11-18 11:00:00',75),(1016,5,'2013-11-11 14:00:00',75),(1016,6,'2012-09-13 15:00:00',97),(1016,7,'2011-10-16 16:00:00',96),(1016,8,'2010-11-11 16:00:00',92),(1016,9,'2013-11-21 10:00:00',62),(1016,10,'2012-11-11 12:00:00',83),(1016,11,'2011-11-11 14:00:00',98),(1016,12,'2010-11-11 15:00:00',94),(1016,13,'2013-11-11 14:00:00',62),(1016,14,'2012-11-11 15:00:00',97),(1016,15,'2011-12-11 10:00:00',76),(1016,16,'2010-09-11 10:00:00',82),(1017,1,'2013-11-11 16:00:00',100),(1017,2,'2012-11-10 10:00:00',88),(1017,3,'2011-12-19 10:00:00',86),(1017,4,'2010-11-18 11:00:00',73),(1017,5,'2013-11-11 14:00:00',96),(1017,6,'2012-09-13 15:00:00',64),(1017,7,'2011-10-16 16:00:00',81),(1017,8,'2010-11-11 16:00:00',66),(1017,9,'2013-11-21 10:00:00',76),(1017,10,'2012-11-11 12:00:00',95),(1017,11,'2011-11-11 14:00:00',73),(1017,12,'2010-11-11 15:00:00',82),(1017,13,'2013-11-11 14:00:00',85),(1017,14,'2012-11-11 15:00:00',68),(1017,15,'2011-12-11 10:00:00',99),(1017,16,'2010-09-11 10:00:00',76); 48 49 /*Table structure for table `student` */ 50 51 DROP TABLE IF EXISTS `student`; 52 53 CREATE TABLE `student` ( 54 `StudentNo` INT(4) NOT NULL COMMENT '学号', 55 `LoginPwd` VARCHAR(20) DEFAULT NULL, 56 `StudentName` VARCHAR(20) DEFAULT NULL COMMENT '学生姓名', 57 `Sex` TINYINT(1) DEFAULT NULL COMMENT '性别，取值0或1', 58 `GradeId` INT(11) DEFAULT NULL COMMENT '年级编号', 59 `Phone` VARCHAR(50) NOT NULL COMMENT '联系电话，允许为空，即可选输入', 60 `Address` VARCHAR(255) NOT NULL COMMENT '地址，允许为空，即可选输入', 61 `BornDate` DATETIME DEFAULT NULL COMMENT '出生时间', 62 `Email` VARCHAR(50) NOT NULL COMMENT '邮箱账号，允许为空，即可选输入', 63 `IdentityCard` VARCHAR(18) DEFAULT NULL COMMENT '身份证号', 64 PRIMARY KEY (`StudentNo`), 65 UNIQUE KEY `IdentityCard` (`IdentityCard`), 66 KEY `Email` (`Email`) 67 ) ENGINE=MYISAM DEFAULT CHARSET=utf8; 68 69 /*Data for the table `student` */ 70 71 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','test1@bdqn.cn','450323198612111234'),(1001,'123456','李文才',1,2,'13500000002','河南洛阳','1981-12-31 00:00:00','test1@bdqn.cn','450323198112311234'),(1002,'111111','李斯文',1,1,'13500000003','天津市和平区','1986-11-30 00:00:00','test1@bdqn.cn','450323198611301234'),(1003,'123456','武松',1,3,'13500000004','上海卢湾区','1986-12-31 00:00:00','test1@bdqn.cn','450323198612314234'),(1004,'123456','张三',1,4,'13500000005','北京市通州','1989-12-31 00:00:00','test1@bdqn.cn','450323198612311244'),(1005,'123456','张秋丽 ',2,1,'13500000006','广西桂林市灵川','1986-12-31 00:00:00','test1@bdqn.cn','450323198612311214'),(1006,'123456','肖梅',2,4,'13500000007','地址不详','1986-12-31 00:00:00','test1@bdqn.cn','450323198612311134'),(1007,'111111','欧阳峻峰',1,1,'13500000008','北京东城区','1986-12-31 00:00:00','test1@bdqn.cn','450323198612311133'),(1008,'111111','梅超风',1,1,'13500000009','河南洛阳','1986-12-31 00:00:00','test1@bdqn.cn','450323198612311221'),(1009,'123456','刘毅',1,2,'13500000011','安徽','1986-12-31 00:00:00','test1@bdqn.cn','450323198612311231'),(1010,'111111','大凡',1,1,'13500000012','河南洛阳','1986-12-31 00:00:00','test1@bdqn.cn','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','test1@bdqn.cn','450323198412311234'),(1012,'123456','多伦',2,3,'13500000014','广西南宁中央大街','1986-12-31 00:00:00','test1@bdqn.cn','450323198612311334'),(1013,'123456','李梅',2,1,'13500000015','上海卢湾区','1986-12-31 00:00:00','test1@bdqn.cn','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','test1@bdqn.cn','450323198612311264'),(1015,'123456','李东方',1,4,'13500000017','广西桂林市灵川','1976-12-31 00:00:00','test1@bdqn.cn','450323197612311234'),(1016,'111111','刘奋斗',1,1,'13500000018','上海卢湾区','1986-12-31 00:00:00','test1@bdqn.cn','450323198612311251'),(1017,'123456','可可',2,3,'13500000019','北京长安街1号','1981-09-10 00:00:00','test1@bdqn.cn','450323198109108311'),(10066,'','Tom',1,1,'13500000000','','0000-00-00 00:00:00','email@22.com','33123123123123123'); 72 73 /*Table structure for table `subject` */ 74 75 DROP TABLE IF EXISTS `subject`; 76 77 CREATE TABLE `subject` ( 78 `SubjectNo` INT(11) NOT NULL AUTO_INCREMENT COMMENT '课程编号', 79 `SubjectName` VARCHAR(50) DEFAULT NULL COMMENT '课程名称', 80 `ClassHour` INT(4) DEFAULT NULL COMMENT '学时', 81 `GradeID` INT(4) DEFAULT NULL COMMENT '年级编号', 82 PRIMARY KEY (`SubjectNo`) 83 ) ENGINE=INNODB AUTO_INCREMENT=18 DEFAULT CHARSET=utf8; 84 85 /*Data for the table `subject` */ 86 87 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);
3.查询语句：
3.1 查询指定字段
select * from student; #查询student表中的所有数据
select `studentNo`, `studentName` from `student; #查询student表中的studentno，studentname的指定列
3.2 AS 取别名（可省略不写）
可以给字段取别名，表取别名，查询结果取别名
字段取别名： select `studentname` as '姓名' from `student`;
表取别名： select `studentname` from `student` as s;
select concat('姓名：',studentname) as '新名字' from student; #concat函数拼接字符串
3.3 distinct 去重复项
select distinct studentid from result;
3.4 对使用表达式的列进行查询
select version()； #查询mysql版本
select 100/2 as '结果'； #计算结果
select studentid,studentresult+1 from result; #给所有的成绩加1
3.5 select的where条件句
搜索记录中满足条件的，可同时要求满足多个条件
# 满足条件的查询(where) SELECT Studentno,StudentResult FROM result; # 查询考试成绩在95-100之间的 SELECT Studentno,StudentResult FROM result WHERE StudentResult>=95 AND StudentResult<=100; # AND也可以写成 && SELECT Studentno,StudentResult FROM result WHERE StudentResult>=95 && StudentResult<=100; # 模糊查询(对应的词:精确查询) SELECT Studentno,StudentResult FROM result WHERE StudentResult BETWEEN 95 AND 100; # 除了1000号同学,要其他同学的成绩 SELECT studentno,studentresult FROM result WHERE studentno!=1000; # 使用NOT SELECT studentno,studentresult FROM result WHERE NOT studentno=1000;
3.6模糊查询：
#模糊查询 between and \ like \ in \ null #============================================= # LIKE #============================================= #查询姓李的同学的学号及姓名 #like结合使用的通配符 : % (代表0到任意个字符) _ (一个字符) SELECT studentno,studentname FROM student WHERE studentname LIKE '李%'; #查询姓李的同学,后面只有一个字的 SELECT studentno,studentname FROM student WHERE studentname LIKE '李_'; #查询姓李的同学,后面只有两个字的 SELECT studentno,studentname FROM student WHERE studentname LIKE '李__'; #查询姓名中含有 文 字的 SELECT studentno,studentname FROM student WHERE studentname LIKE '%文%'; #查询姓名中含有特殊字符的需要使用转义符号 '\' #自定义转义符关键字: ESCAPE ':' #============================================= # IN #============================================= #查询学号为1000,1001,1002的学生姓名 SELECT studentno,studentname FROM student WHERE studentno IN (1000,1001,1002); #查询地址在北京,南京,河南洛阳的学生 SELECT studentno,studentname,address FROM student WHERE address IN ('北京','南京','河南洛阳'); #============================================= # NULL 空 #============================================= #查询出生日期没有填写的同学 #不能直接写=NULL , 这是代表错误的 , 用 is null SELECT studentname FROM student WHERE BornDate IS NULL; #查询出生日期填写的同学 SELECT studentname FROM student WHERE BornDate IS NOT NULL; #查询没有写家庭住址的同学(空字符串不等于null) SELECT studentname FROM student WHERE Address='' OR Address IS NULL;
3.7连接查询
JOIN对比
3.7.1 inner join（内连接）：
和自然连接不同之处：内连接可以指定两张表不同的字段，但是不会进行字段的去重。
语句：select * from a_table a inner join b_table b on a.a_id=b.b_id;
说明：在两个表中，返回关联字段相符的部分，即取交集。
3.7.2left join（左连接）
语句：select * from a_table a left join b_table b on a.a_id=b.b_id;
说明：左连接会显示左表的全部内容，仅显示右表满足条件的部分，不足为null
查询结果如下：
3.7.3右连接（right join）
语句：select * from a_table right join b_table on a.a_id=b.b_id;
说明：右连接正好和左连接相反，会全部显示右表的部分，左表只会显示符合条件的，不足为null；
3.7.4自然连接：不用选择列，默认比较两张表里相同的列，也不用on语句,会去重（比如重复的列）。
语句：select * from student natural join result;
示例：
结果：
3.7.5等值连接：可以进行两张表的不同字段的值相同的连接，但不会去掉重复的字段，从结果集来看等值连接是内连接的子集。
/* 实例演示： 单词 ambiguous 暧昧的------》数据库中的错误，两张表的相同字段的别名问题 连接查询 如需要多张数据表的数据进行查询,则可通过连接运算符实现多个查询 内连接 inner join 查询两个表中的结果集中的交集 外连接 outer join 左外连接 left join (以左表作为基准,右边表来一一匹配,匹配不上的,返回左表的记录,右表以NULL填充) 右外连接 right join (以右表作为基准,左边表来一一匹配,匹配不上的,返回右表的记录,左表以NULL填充) 等值连接和非等值连接 自连接 */ #查询参加了考试的同学信息(学号,学生姓名,科目编号,分数) SELECT * FROM student; SELECT * FROM result; /*思路: (1):分析需求,确定查询的列来源于两个类,student result,连接查询 (2):确定使用哪种连接查询?(内连接) */ SELECT s.studentno,studentname,subjectno,StudentResult FROM student s INNER JOIN result r ON r.studentno = s.studentno # 右连接(也可实现) SELECT s.studentno,studentname,subjectno,StudentResult FROM student s RIGHT JOIN result r ON r.studentno = s.studentno # 等值连接 SELECT s.studentno,studentname,subjectno,StudentResult FROM student s , result r WHERE r.studentno = s.studentno # 左连接 (查询了所有同学,不考试的也会查出来) SELECT s.studentno,studentname,subjectno,StudentResult FROM student s LEFT JOIN result r ON r.studentno = s.studentno #查一下缺考的同学(左连接应用场景) SELECT s.studentno,studentname,subjectno,StudentResult FROM student s LEFT JOIN result r ON r.studentno = s.studentno WHERE StudentResult IS NULL #思考题:查询参加了考试的同学信息(学号,学生姓名,科目名,分数) SELECT s.studentno,studentname,subjectname,StudentResult FROM student s INNER JOIN result r ON r.studentno = s.studentno INNER JOIN `subject` sub ON sub.subjectno = r.subjectno
3.8自连接：
假想以下场景：某一电商网站想要对站内产品做层级分类，一个类别下面有若干子类，子类下面也会有别的子类。例如数码产品这个类别下面有笔记本，台式机，智能手机等；笔记本，台式机，智能手机又可以按照品牌分类；品牌又可以按照价格分类，等等。也许这些分类会达到一个很深的层次，呈现一种树状的结构。那么这些数据要怎么在数据库中表示呢？我们可以在数据库中创建两个字段来存储id和类别名称，使用第三个字段存储类别的子类或者父类的id，最后通过自连接去查询想要的结果。
自连接查询其实也是连接查询，只不过左表（父表）和右表（子表）都是它自己，自连接查询是自己和自己进行连接，分别给父表和子表（还是自己本身）取个别名，然后附上查询条件进行查询。
1.建表 create table tdb_cates( id smallint primary key auto_increment, cate_name varchar(20) not null, parent_id smallint not null ); 2.插入数据 insert into tdb_cates(cate_name, parent_id) values('数码产品', 0); insert into tdb_cates(cate_name, parent_id) values('家用产品', 0); insert into tdb_cates(cate_name, parent_id) values('笔记本', 1); insert into tdb_cates(cate_name, parent_id) values('智能手机', 1); insert into tdb_cates(cate_name, parent_id) values('电器', 2); insert into tdb_cates(cate_name, parent_id) values('家具', 2); insert into tdb_cates(cate_name, parent_id) values('冰箱', 5); insert into tdb_cates(cate_name, parent_id) values('洗衣机', 5); insert into tdb_cates(cate_name, parent_id) values('汽车品牌', 0); insert into tdb_cates(cate_name, parent_id) values('别克', 9); insert into tdb_cates(cate_name, parent_id) values('宝马', 9); insert into tdb_cates(cate_name, parent_id) values('雪佛兰', 9); insert into tdb_cates(cate_name, parent_id) values('家纺', 0); 3.1 查询所有分类以及分类的父类：假想有左右两张表（都是tdb_cates），左表是子表，右表是父表；查询子表的id，子表的cate_name，父表的cate_name；连接条件是子表的parent_id等于父表的id。 #查询所有分类以及分类的父类 SELECT s.id,s.cate_name,p.cate_name AS '父名' FROM tdb_cates s LEFT JOIN tdb_cates p ON s.`parent_id`=p.`id`; 3.2 #查询所有分类以及分类的子类 SELECT s.id,s.cate_name,p.cate_name AS '子名' FROM tdb_cates s LEFT JOIN tdb_cates p ON s.`id`=p.`parent_id`;
结果视图：
数据表：
3.1查询结果：
3.2查询结果：
一个实例代码测试：
/* 自连接 数据表与自身进行连接 需求:从一个包含栏目ID , 栏目名称和父栏目ID的表中 查询父栏目名称和其他子栏目名称 */ # 创建一个表 CREATE TABLE `category` ( `categoryid` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT COMMENT '主题id', `pid` INT(10) NOT NULL COMMENT '父id', `categoryName` VARCHAR(50) NOT NULL COMMENT '主题名字', PRIMARY KEY (`categoryid`) ) ENGINE=INNODB AUTO_INCREMENT=9 DEFAULT CHARSET=utf8 # 插入数据 INSERT INTO `category` (`categoryid`, `pid`, `categoryName`) VALUES('2','1','信息技术'), ('3','1','软件开发'), ('4','3','数据库'), ('5','1','美术设计'), ('6','3','web开发'), ('7','5','ps技术'), ('8','2','办公信息'); # 编写SQL语句,将栏目的父子关系呈现出来 (父栏目名称,子栏目名称) # 核心思想:把一张表看成两张一模一样的表,然后将这两张表连接查询(自连接) SELECT a.categoryName AS '父栏目',b.categoryName AS '子栏目' FROM category AS a,category AS b WHERE a.`categoryid`=b.`pid` #思考题:查询参加了考试的同学信息(学号,学生姓名,科目名,分数) SELECT s.studentno,studentname,subjectname,StudentResult FROM student s INNER JOIN result r ON r.studentno = s.studentno INNER JOIN `subject` sub ON sub.subjectno = r.subjectno # 查询学员及所属的年级(学号,学生姓名,年级名) SELECT studentno AS 学号,studentname AS 学生姓名,gradename AS 年级名称 FROM student s INNER JOIN grade g ON s.`GradeId` = g.`GradeID` # 查询科目及所属的年级(科目名称,年级名称) SELECT subjectname AS 科目名称,gradename AS 年级名称 FROM SUBJECT sub INNER JOIN grade g ON sub.gradeid = g.gradeid # 查询 数据库结构-1 的所有考试结果(学号 学生姓名 科目名称 成绩) SELECT s.studentno,studentname,subjectname,StudentResult FROM student s INNER JOIN result r ON r.studentno = s.studentno INNER JOIN `subject` sub ON r.subjectno = sub.subjectno WHERE subjectname='数据库结构-1'
3.9排序和分页
排序： order by （ASC）/（DESC）
分页：limit 0,5；
/*============== 排序 ================ 语法 : ORDER BY ORDER BY 语句用于根据指定的列对结果集进行排序。
默认是升序（ASC）排列，降序 order by DESC； */ # 查询 数据库结构-1 的所有考试结果(学号 学生姓名 科目名称 成绩) # 按成绩降序排序 SELECT s.studentno,studentname,subjectname,StudentResult FROM student s INNER JOIN result r ON r.studentno = s.studentno INNER JOIN `subject` sub ON r.subjectno = sub.subjectno WHERE subjectname='数据库结构-1' ORDER BY StudentResult DESC , studentno /*============== 分页 ================ 语法 : SELECT * FROM table LIMIT [offset,] rows | rows OFFSET offset 好处 : (用户体验,网络传输,查询压力) 推导: 第一页 : limit 0,5 第二页 : limit 5,5 第三页 : limit 10,5 ...... 第N页 : limit (pageNo-1)*pageSzie,pageSzie [pageNo:页码,pageSize:单页面显示条数] */ # 每页显示5条数据 SELECT s.studentno,studentname,subjectname,StudentResult FROM student s INNER JOIN result r ON r.studentno = s.studentno INNER JOIN `subject` sub ON r.subjectno = sub.subjectno WHERE subjectname='数据库结构-1' ORDER BY StudentResult DESC , studentno LIMIT 0,5 #查询 JAVA第一学年 课程成绩前10名并且分数大于80的学生信息(学号,姓名,课程名,分数) SELECT s.studentno,studentname,subjectname,StudentResult FROM student s INNER JOIN result r ON r.studentno = s.studentno INNER JOIN `subject` sub ON r.subjectno = sub.subjectno WHERE subjectname='JAVA第一学年' ORDER BY StudentResult DESC LIMIT 0,10
3.10 子查询：
在where的查询语句中又嵌套了另一个查询，嵌套查询可由多个子查询构成顺序是由里及外。
子查询返回的结果一般都是集合，故而用关键字 in。
/*============== 子查询 ================ 什么是子查询? 在查询语句中的WHERE条件子句中,又嵌套了另一个查询语句 嵌套查询可由多个子查询组成,求解的方式是由里及外; 子查询返回的结果一般都是集合,故而建议使用IN关键字; */ # 查询 数据库结构-1 的所有考试结果(学号,科目编号,成绩),并且成绩降序排列
#子查询
# 查询 数据库结构-1 的所有考试结果(学号,科目编号,成绩),并且成绩降序排列
#方法1 连接查询
SELECT r.StudentNo,sb.SubjectNo,r.StudentResult
FROM result r
INNER JOIN
`subject` sb
ON
r.SubjectNo=sb.SubjectNo
WHERE sb.SubjectName='数据库结构-1'
ORDER BY r.StudentResult DESC;
#方法2 子查询
SELECT StudentNo,SubjectNo,StudentResult
FROM result
WHERE SubjectNo = (
SELECT SubjectNo FROM `subject`
WHERE SubjectName='数据库结构-1'
)
ORDER BY StudentResult DESC;
#查询课程为 高等数学-2 且分数不小于80分的学生的学号和姓名 #方法一:使用连接查询 SELECT s.studentno,studentname FROM student s INNER JOIN result r ON s.`StudentNo` = r.`StudentNo` INNER JOIN `subject` sub ON sub.`SubjectNo` = r.`SubjectNo` WHERE subjectname = '高等数学-2' AND StudentResult>=80 #方法二:使用连接查询+子查询 #分数不小于80分的学生的学号和姓名 SELECT r.studentno,studentname FROM student s INNER JOIN result r ON s.`StudentNo`=r.`StudentNo` WHERE StudentResult>=80 #在上面SQL基础上,添加需求:课程为 高等数学-2 SELECT r.studentno,studentname FROM student s INNER JOIN result r ON s.`StudentNo`=r.`StudentNo` WHERE StudentResult>=80 AND subjectno=( SELECT subjectno FROM `subject` WHERE subjectname = '高等数学-2' ) #方法三:使用子查询 #分步写简单sql语句,然后将其嵌套起来 SELECT studentno,studentname FROM student WHERE studentno IN( SELECT studentno FROM result WHERE StudentResult>=80 AND subjectno=( SELECT subjectno FROM `subject` WHERE subjectname = '高等数学-2' ) ) /* 练习题目: 查 C语言-1 的前5名学生的成绩信息(学号,姓名,分数) 使用子查询,查询郭靖同学所在的年级名称 */
