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一、去重和空值

学习运算符之前我们先讨论一下去重和空值的问题：

这是我们要操作的表

select DEPARTMENT_ID from employees order by EMPLOYEE_ID ;

查看部门id，发现很多都是一个重复的

1.distinct

select distinct DEPARTMENT_ID from employees order by DEPARTMENT_ID;

2.null参与运算

select employee_id  , salary "月工资" , salary * ( 1 + commission_pct ) * 12 "年工资" from employees;

运算中有一个null，结果为null

3.用ifnull函数解决问题

select employee_id , salary “月工资” , salary * ( 1 + IFNULL(commission_pct,0) ) * 12 “年工资” , commission_pct from employees;
if null函数本质是将null变为0参与运算。

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二、比较运算符

select employee_id , salary from employees where salary < 2400;

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三、dual伪表和数值运算

1.常规运算

浮点型转化

字符串隐式转化
字符串和null
字符串隐式转化成0（不包含数字）

除法0值的问题

select 100/0.0,100 div 10 ,100 div 0 from dual;

取余0值问题

select 100 % 3,100%0 from dual;

2.比较运算符

select 1=2,1!=2,1='1',1='a' from dual;

select 'a' = 'a','a'='ab','a'='b' from dual;

空值参与比较运算的问题

select last_name,salary from employees where  COMMISSION_PCT =  NULL;

3.<=>安全相等

select last_name,salary from employees where  COMMISSION_PCT <=>  NULL;

除了<=>之外与null相运算都是null
不相等

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四、常用正则相关的比较运算符

1.基本运算符

is null

select last_name,salary from employees where  COMMISSION_PCT is NULL;#

not 取反

select last_name,salary from employees where NOT (COMMISSION_PCT <=> NULL);

等价于is not null

select last_name,salary from employees where  (COMMISSION_PCT IS NOT NULL);

least()和greatest()

select least('a','b','c','e'),greatest('a','b','c','e') from dual;

取两列中最长的字符串和最短的字符串

select least( first_name, last_name ),greatest(first_name, last_name) from employees;

betweent … and 区间
注意区间不能为空，不然输出会有问题

select * from employees where salary between 6000 and 8000 ;

等价于

select * from employees where salary > 6000 and salary < 8000 ;

in和not in

select * from employees where department_id in (10,20,30,40);

等价于

select * from employees where department_id = 10 or department_id = 20 or department_id = 30 or department_id = 40;

not in

 select * from employees where department_id not in (10,20,30,40);

2.模糊查询

%：代表不确定个数的字符
查询包含字符’a’的员工信息

查询字符’a’开头的员工信息

查询包含字符’a’和’e’的员工信息
写法1：

写法2：

select last_name from employees where last_name like '%a%e%' or  last_name like '%e%a%' ;

_：占位符一个不确定的字符

select last_name from employees where last_name like '_a%' ;

转义字符
\
查询JOB_ID的第三个字符是_且第四个是a的JOB_ID：

 select JOB_ID from employees where JOB_ID like "__\_a%" ;

其他字符做转义，需要告诉系统
$做转义字符：

select JOB_ID from employees where JOB_ID like "__$_a%" escape '$' ;

3.正则表达式

regexp 和 rlike

select "shkstart" regexp "^s","shkstart" regexp "t$","shkstart" regexp "hk" , "shkstart" regexp "ht"from dual;

select "guigui" regexp "gu.gu","guigui" regexp "[gu]" from dual;

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五、逻辑运算符

or：

select last_name from employees where last_name like '%a%e%' or  last_name like '%e%a%' ;

and：

select last_name,salary,department_id from employees  where department_id = 50 and salary > 6000;

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六、位运算

select 4<<1,8>>1 from dual;

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总结

常见运算符基本就是这些了，里面有使用案例，希望大家后面多多练习，毕竟mysql除了懂原理外大量的练习才是王道。

可以点个小👍吗