Summary of Mysql operators and functions

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 between 4500 and 5000;

select * from employee where sal not between 4500 and 5000;
  --null不为包括进去

  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 * from employee where sal >= 4500;

  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 greatest(54,76,4,65,76,87,87,56,65,654,45,23,1,76);

  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 sum(sal) from employee where sal >= 6000;

 select min(sal) from employee;

 select max(sal) from employee;

  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 curdate(); 
  --只有年月日

select curtime(); 
  --只有时分秒

  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_sub(history, interval '1-1' year_month) from employee;

 select history, sysdate(), datediff(sysdate(), history) from employee;     --以天数来表示

  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;

select history, dayname(history) 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'));

select left('abcdefg', 5);

select length('abcdefg');

 select lower('HELLO');

select substring('helloworld',2,3);

 select concat(emp_name, '员工') from employee;

select replace(emp_name, '李', '老') from employee where emp_name = '李四';