单表查询操作

//创建数据库（这个可以在命令行下用）
CREATE TABLE t_studentType(
id int primary key auto_increment,
studentTypeName varchar(20),
studentTypeDesc varchar(200)
);
CREATE TABLE T_student(
id int primary key auto_increment,
studentName varchar(20),
teacher varchar(200),
price decimal(6,2),
studentTypeID int,
constraint `fk` foreign key (`studentTypeID`) references `t_studentType`(`id`)
);
//查看数据库
desc t_studentType;
//插入数据
insert into t_studentType (`id`, `studentTypeName`, `studentTypeDesc`) value (`1`, `张三`, `软件工程`);
//查询命令
SELECT id,studentTypeName,studentTypeDesc FROM t_studenttype;
SELECT * FROM t_studenttype;
SELECT * FROM t_studenttype WHERE studentTypeName = 5;
SELECT * FROM t_studenttype WHERE studentTypeName > 5;
SELECT * FROM t_studenttype WHERE studentTypeName IN (5, 10);

//模糊查询
SELECT * FROM t_studenttype WHERE id LIKE 1;
SELECT * FROM t_studenttype WHERE studentTypeName LIKE '1%';//只要前边第一个是1的都可以查出来， '' 这个是英文状态下的分号
//值为空的项
SELECT * FROM t_studenttype WHERE studentTypeDesc IS NULL ;
//多项查询
SELECT * FROM t_studenttype WHERE studentTypeName='以为' AND studentTypeDesc IS NULL;
//显示某一项的所有信息
SELECT studentTypeDesc FROM t_studenttype;
SELECT DISTINCT studentTypeDesc FROM t_studenttype;//去除重复
//对查询结果进行排
SELECT * FROM t_studenttype ORDER BY id ASC;//升序
SELECT * FROM t_studenttype ORDER BY id DESC;//降序
//按组分数据
//选出studentTypeDesc，分组中显示信息studentTypeName，以//studentTypeDesc作为分组标准
SELECT studentTypeDesc,GROUP_CONCAT(studentTypeName) FROM
t_studenttype GROUP BY studentTypeDesc;
//计算每一个分组有多少人
SELECT studentTypeDesc,COUNT(studentTypeName) FROM t_studenttype GROUP BY studentTypeDesc;
//筛选出人数多于某一个值得信息
SELECT studentTypeDesc,COUNT(studentTypeName) FROM t_studenttype GROUP BY studentTypeDesc HAVING COUNT(studentTypeName)>3;