use test;
create table `employee`(
 emp_no int unsigned,
 emp_name varchar(30),
 emp_sex varchar(3),
 emp_age tinyint unsigned,
 sal double,
 history datetime
);
insert into employee values(1, '张三', '男', 18, 5000, '2012-04-23'),
(2, '李四', '男', 27, 4500, '2013-05-23'),
(3, '王五', '男', 23, 4700, '2012-04-21'),
(4, '子龙', '男', 19, 3800, '2011-03-04'),
(5, '李白', '男', 15, 6200, '2015-09-09'),
(6, '刘备', '男', 28, 2500, '2016-02-11'),
(7, '吕布', '男', 21, 6000, '2010-10-18'),
(8, '尚香', '女', 16, 4500, '2011-09-26'),
(9, '小乔', '女', 15, null, '2013-07-05'),
(10, '大乔', '女', 16, 5000, '2017-09-01');

 select * from employee where sal = 3800;
 select * from employee where sal = null;  --这里查询不到为null的数据

 select * from employee where sal <=> 3800;
 select * from employee where sal <=> null; --这里可以查询到为null的数据

 select * from employee where sal is null;
 select * from employee where sal is not null;

 select * from employee where isnull(sal);
 select * from employee where !isnull(sal);

 select * from employee where sal not between 4500 and 5000 or sal is null;
 select * from employee where sal = 4500 and emp_sex = '女';

 select rand() from dual; --dual是一个伪表
 select 1+1 from dual;
 select rand(); --可以简写

 select least(54,76,4,65,76,87,87,56,65,654,45,23,1,76);
 select least(54,76,4,65,76,87,87,56,65,654,45,23,1,76) as min_value; --列名可以起一个别名

 select round(1.69);
 select round(1.69, 1);

 select 5-10;
 select abs(5-10);

 select * from employee where sal >= 6000;
 select avg(sal) from employee where sal >= 6000;

 select count(*) from employee;
 select count(emp_name) from employee;
 select count(sal) from employee;  --打印9 这里会忽略null值
 select count(*) from employee where sal >= 4000;
 select count(*) from employee where sal <= 4000 or sal is null;

 select now(), sysdate(), current_timestamp();
 select now(6), sysdate(6), current_timestamp(6);
 ps: now(), current_timestamp();没有区别, 表示sql开始执行时的时间
  sysdate()表示这个函数开始时间

 select history, date_add(history, interval '1 12:10' day_minute) from employee; --date_add(history, interval '1 12:10' day_minute)
 select history, date_add(history, interval '1-1' year_month) from employee;  --date_add(history, interval '1-1' year_month)
 select history, date_add(history, interval '1' second) from employee;    --date_add(history, interval '1' second)

 select history, date_format(history, '%Y年%m月%d号') from employee;
 select history, date_format(history, '%d号') from employee;
 select history, date_format(history, '%Y年%m月%d号 %H时%i分%s秒') from employee;

 insert into employee values(11, '张飞', '男', 22, 3000, '2017年02月01号'); --报错
 insert into employee values(11, '张飞', '男', 22, 3000, str_to_date('2017年02月01号', '%Y年%m月%d号 %H时%i分%s秒'));