create table main (num int)；insert into main values

create table main (num int);

insert into main values

(3),

(12),

(15),

(25),

(23),

(29),

(34),

(37),

(32);

int num1= 3;

int num2 = 12;

int num3 = 23;

把num值处于[20，29]之间，改为20

num值处于[30,39]之间的，改为30

思路：要把列（字段）当成变量来处理

SELECT FLOOR(num/10)*10 FROM main where num >= 20 and num <= 29;

int num = 23;

System.out.println(Math.floor(num/10.0)*10);

数据库：employee

1.创建一张员工表，表明EMPLOTYEES，有四个字段，EMPLOYEE_ID:员工表（主键）、DEPT_ID：部门号、EMPLOYEE_NAME：员工姓名、EMPLOYEE_SALARY:员工工资。

问题1、写出建表语句

CREATE TABLE EMPLOYEE(employee_ID int primary key,DEPT_ID int,EMPLOYEE_NAME char(40),EMPLOYEE_SALARY decimal(8,2)
);

问题二、检索出员工工资最高的员工的姓名和工资

--方法一
SELLECT * FROM EMPLOYEE WHERE employee_salary = (select max(employee_salary)from EMPLOYEES)
--方法二
SELECT * FROM EMPLOYEE ORDER BY employee_salary DESC limit 1;   --0,1

问题三、检索出部门中员工最多的部门号和此部门的员工数量

SELECT dept_id,count(*) cno from EMPLOYEES BY dept_id order by cno desc limit 1

浮点列与定点列：

float(M，D)：M是精度，D标度，就是小数点后几位。

decimal

float/double:有精度损失

decimal：定点型，精确。用于保存对准确精度有重要要求的值，例如与金钱有关的数据

double d1 = 1234.3435;
double d2 = 1234.3400;
System.out.println(d1-d2);
if ((d1 - d2) == 0.0035) {Sysytem.out.println("==");
} else {System.out.println("!=");
}
// 0.003500000000030923
// !=

Inner join 练习：