Mysql之运算符与函数的总结

我们先来看个示例

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');

常用的运算符:
1: 等于( = )

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

2: 等于( <=> )

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

3: is判断(null)

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

4: null值判断还可以使用isnull();

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

5: 在区间(between)内 between min and max ps:这里是一个闭区间

select * from employee where sal between 4500 and 5000;

6: 不在区间内

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

7: and 和 or

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

8: 小于(<), 大于(>), 小于等于(<=), 大于等于(>=)

select * from employee where sal >= 4500;

***************************************************************************************************************


数学函数
1: rand();

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

2: least(value1, value2, ...) 返回最小值

  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

3: greatest(value1, value2, ...) 返回最大值

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

4: round(M, D); 返回M的四舍五入的值, D表示要保留几们小数,默认值是0

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

5: abs() 绝对值

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

***************************************************************************************************************

汇总函数

1: avg();

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

2: count()

  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;

3: sum()

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

4: min()

 select min(sal) from employee;

5: max()

 select max(sal) from employee;

***************************************************************************************************************

日期函数

1: 获取当前的日期时间

  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

2: 获取当前日期

select curdate(); 
  --只有年月日

3: 获取当前时间

select curtime(); 
  --只有时分秒

4: 日期的加运算date_add

  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)

5: 日期的减运算data_sub

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

6: 计算日期差

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

7: 获取日期的指定部分(把日期转换为指定的格式) date_format()

  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;

8: 计算出一个日期是星期几

select history, dayname(history) from employee;

9: 中文日期字符串转换日期str_to_date()

  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'));

insert into employee values(12, '二哥', '男', 22, 3000, str_to_date('2017年02月01号 23时02分02秒', '%Y年%m月%d号 %H时%i分%s秒'));
insert into employee values(12, '二哥', '男', 22, 3000, str_to_date('2017年02月01号 11时02分02秒', '%Y年%m月%d号 %h时%i分%s秒'));
ps: 如果是h则表示12小制, 如果是大H则表示24小明制;

字符串函数

1: left(str, len) 返回字符串str的左端len个字符

select left('abcdefg', 5);

2: length()

select length('abcdefg');

3: lower(str) 返回小写的字符串str

 select lower('HELLO');

4: substring() 取子字符串, 第二个参数是截取的起始位置, 第三个参数是要截取的长度

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

5: concat() 字符串拼接

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

6: replace(替换

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

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