mysql数据库标识符的使用

ddl

CREATE TABLE `student` (
  `id` int(11) NOT NULL AUTO_INCREMENT COMMENT '学号',
  `createDate` datetime DEFAULT NULL,
  `userName` varchar(20) DEFAULT NULL,
  `pwd` varchar(36) DEFAULT NULL,
  `phone` varchar(11) DEFAULT NULL,
  `age` tinyint(3) unsigned DEFAULT NULL,
  `sex` char(2) DEFAULT '男',
  `introduce` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
 
CREATE TABLE `result` (
  `rid` int(11) NOT NULL AUTO_INCREMENT COMMENT '成绩编号',
  `testName` varchar(255) DEFAULT NULL COMMENT '测试名称',
  `score` double(4,2) DEFAULT NULL COMMENT '成绩',
  `studentId` int(11) DEFAULT NULL COMMENT '学生id',
  PRIMARY KEY (`rid`),
  KEY `studentId` (`studentId`),
  CONSTRAINT `result_ibfk_1` FOREIGN KEY (`studentId`) REFERENCES `student` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
insert into student values(0,'2024-02-25 10:10:10','赵灵儿','123',
'15612345678',16,'女','逍遥哥哥，你终于找到我了。');
insert into student values(0,'2024-02-25 10:10:10','王语嫣','123',
'15612345678',17,'女','慕容复，我和你不共戴天。');
insert into student values(0,'2024-02-25 10:10:10','龙姑娘','123',
'15612345678',22,'女','我想过过过儿过过的日子。');
insert into student values(0,'2024-02-25 10:10:10','杨过','123',
'15612345678',18,'男','一遇杨过误终身。');
insert into student values(0,'2024-02-25 10:10:10','杨逍','123',
'15612345678',27,'男','杨过跟程英的大儿子。');
insert into student (userName,age,introduce)values('黄衣女子',26,'杨过与龙姑娘的大女儿。');
 
insert into result values(0,'政治',96,1);
insert into result values(0,'地理',86,5);
insert into result values(0,'政治',77,3);
insert into result values(0,'英语',99,3);
insert into result values(0,'历史',22,6);
insert into result values(0,'化学',69,2);
insert into result values(0,'Java',79,5);
insert into result values(0,'Java',67,6);
insert into result values(0,'政治',97,2);
insert into result values(0,'物理',76,6);
insert into result values(0,'物理',82,2);
insert into result values(0,'化学',56,4);

dml

# MySQL变量的声明与使用
#1、标识符不能以数字作为开头。
#2、只能使用 或$符号，不允许使用其它符号
#3、不允许使用系统关键字
 set @userName = '刘德华';
 select @userName:= '刘青云';# 将赋值与查询结合
 # 查询变量/使用变量·匿名的时候建议加上as
select @userName as '读取到的userName变量值';
#整数类型与浮点数类型测试
set @x=10,@y=0;
select @x / @y as '10/0的结果';
set @dx=0.55,@dy=2;
select @dx + @dy;# 浮点数计算异常显示无限000\
#去零操作·重新赋值既可以做到
set @result=(select @dx + @dy)
select @result;
#通过修改变量的方式进行精准查询
set @cityName='kabul';
set @cityName='Qandahar';
set @cityName='Maastricht';
select * from city where `Name`=@cityName;
select * from city where `Name` in(@cityName1,@cityName2,@cityName3,);
select @cityName;

效果图