mysql 加法空值 mysql加减法怎么用

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# 运算符
#1. 算术运算符： +  -  *  /  div  % mod

SELECT 100, 100 + 0, 100 - 0, 100 + 50, 100 + 50 * 30, 100 + 35.5, 100 - 35.5 
FROM DUAL;

# 在SQL中，+没有连接的作用，就表示加法运算。此时，会将字符串转换为数值（隐式转换）
SELECT 100 + '1'  # 在Java语言中，结果是：1001。 
FROM DUAL;

SELECT 100 + 'a' #此时将'a'看做0处理
FROM DUAL;

SELECT 100 + NULL  # null值参与运算，结果为null
FROM DUAL;

SELECT 100, 100 * 1, 100 * 1.0, 100 / 1.0, 100 / 2,
100 + 2 * 5 / 2,100 / 3, 100 DIV 0  # 分母如果为0，则结果为null
FROM DUAL;

# 取模运算： % mod
SELECT 12 % 3,12 % 5, 12 MOD -5,-12 % 5,-12 % -5
FROM DUAL;

#练习：查询员工id为偶数的员工信息
SELECT employee_id,last_name,salary
FROM employees
WHERE employee_id % 2 = 0;

#2. 比较运算符
#2.1 =  <=>  <> !=  <  <=  >  >= 

# = 的使用
SELECT 1 = 2,1 != 2,1 = '1',1 = 'a',0 = 'a' #字符串存在隐式转换。如果转换数值不成功，则看做0
FROM DUAL;

SELECT 'a' = 'a','ab' = 'ab','a' = 'b' #两边都是字符串的话，则按照ANSI的比较规则进行比较。
FROM DUAL;

SELECT 1 = NULL,NULL = NULL # 只要有null参与判断，结果就为null
FROM DUAL;

SELECT last_name,salary,commission_pct
FROM employees
#where salary = 6000;
WHERE commission_pct = NULL;  #此时执行，不会有任何的结果

# <=> ：安全等于。 记忆技巧：为NULL而生。

SELECT 1 <=> 2,1 <=> '1',1 <=> 'a',0 <=> 'a'
FROM DUAL;

SELECT 1 <=> NULL, NULL <=> NULL
FROM DUAL;

#练习：查询表中commission_pct为null的数据有哪些
SELECT last_name,salary,commission_pct
FROM employees
WHERE commission_pct <=> NULL;

SELECT 3 <> 2,'4' <> NULL, '' != NULL,NULL != NULL
FROM DUAL;

#2.2 
#① IS NULL \ IS NOT NULL \ ISNULL
#练习：查询表中commission_pct为null的数据有哪些
SELECT last_name,salary,commission_pct
FROM employees
WHERE commission_pct IS NULL;
#或
SELECT last_name,salary,commission_pct
FROM employees
WHERE ISNULL(commission_pct);

#练习：查询表中commission_pct不为null的数据有哪些
SELECT last_name,salary,commission_pct
FROM employees
WHERE commission_pct IS NOT NULL;
#或
SELECT last_name,salary,commission_pct
FROM employees
WHERE NOT commission_pct <=> NULL;

#② LEAST() \ GREATEST 

SELECT LEAST('g','b','t','m'),GREATEST('g','b','t','m')
FROM DUAL;

SELECT LEAST(first_name,last_name),LEAST(LENGTH(first_name),LENGTH(last_name))
FROM employees;

#③ BETWEEN 条件下界1 AND 条件上界2  （查询条件1和条件2范围内的数据，包含边界）
#查询工资在6000 到 8000的员工信息
SELECT employee_id,last_name,salary
FROM employees
#where salary between 6000 and 8000;
WHERE salary >= 6000 && salary <= 8000;

#交换6000 和 8000之后，查询不到数据
SELECT employee_id,last_name,salary
FROM employees
WHERE salary BETWEEN 8000 AND 6000;

#查询工资不在6000 到 8000的员工信息
SELECT employee_id,last_name,salary
FROM employees
WHERE salary NOT BETWEEN 6000 AND 8000;
#where salary < 6000 or salary > 8000;

#④ in (set)\ not in (set)

#练习：查询部门为10,20,30部门的员工信息
SELECT last_name,salary,department_id
FROM employees
#where department_id = 10 or department_id = 20 or department_id = 30;
WHERE department_id IN (10,20,30);

#练习：查询工资不是6000,7000,8000的员工信息
SELECT last_name,salary,department_id
FROM employees
WHERE salary NOT IN (6000,7000,8000);

#⑤ LIKE :模糊查询
# % : 代表不确定个数的字符 （0个，1个，或多个）

#练习：查询last_name中包含字符'a'的员工信息
SELECT last_name
FROM employees
WHERE last_name LIKE '%a%';

#练习：查询last_name中以字符'a'开头的员工信息
SELECT last_name
FROM employees
WHERE last_name LIKE 'a%';

#练习：查询last_name中包含字符'a'且包含字符'e'的员工信息
#写法1：
SELECT last_name
FROM employees
WHERE last_name LIKE '%a%' AND last_name LIKE '%e%';
#写法2：
SELECT last_name
FROM employees
WHERE last_name LIKE '%a%e%' OR last_name LIKE '%e%a%';

# _ ：代表一个不确定的字符

#练习：查询第3个字符是'a'的员工信息
SELECT last_name
FROM employees
WHERE last_name LIKE '__a%';

#练习：查询第2个字符是_且第3个字符是'a'的员工信息
#需要使用转义字符: \ 
SELECT last_name
FROM employees
WHERE last_name LIKE '_\_a%';

#或者  (了解)
SELECT last_name
FROM employees
WHERE last_name LIKE '_$_a%' ESCAPE '$';

#⑥ REGEXP \ RLIKE :正则表达式

SELECT 'shkstart' REGEXP '^shk', 'shkstart' REGEXP 't$', 'shkstart' REGEXP 'hk'
FROM DUAL;

SELECT 'atguigu' REGEXP 'gu.gu','atguigu' REGEXP '[ab]'
FROM DUAL;

#3. 逻辑运算符： OR ||  AND && NOT ! XOR

# or  and 
SELECT last_name,salary,department_id
FROM employees
#where department_id = 10 or department_id = 20;
#where department_id = 10 and department_id = 20;
WHERE department_id = 50 AND salary > 6000;

# not 
SELECT last_name,salary,department_id
FROM employees
#where salary not between 6000 and 8000;
#where commission_pct is not null;
WHERE NOT commission_pct <=> NULL;

# XOR :追求的"异"
SELECT last_name,salary,department_id
FROM employees
WHERE department_id = 50 XOR salary > 6000;

#注意：AND的优先级高于OR

#4. 位运算符： & |  ^  ~  >>   <<

SELECT 12 & 5, 12 | 5,12 ^ 5 
FROM DUAL;

SELECT 10 & ~1 FROM DUAL;

#在一定范围内满足：每向左移动1位，相当于乘以2；每向右移动一位，相当于除以2。
SELECT 4 << 1 , 8 >> 1
FROM DUAL;

# 运算符课后练习

# 1.选择工资不在5000到12000的员工的姓名和工资

SELECT last_name,salary
FROM employees
#where salary not between 5000 and 12000;
WHERE salary < 5000 OR salary > 12000;

# 2.选择在20或50号部门工作的员工姓名和部门号
SELECT last_name,department_id
FROM employees
# where department_id in (20,50);
WHERE department_id = 20 OR department_id = 50;

# 3.选择公司中没有管理者的员工姓名及job_id

SELECT last_name,job_id,manager_id
FROM employees
WHERE manager_id IS NULL;

SELECT last_name,job_id,manager_id
FROM employees
WHERE manager_id <=> NULL;

# 4.选择公司中有奖金的员工姓名，工资和奖金级别
SELECT last_name,salary,commission_pct
FROM employees
WHERE commission_pct IS NOT NULL;


SELECT last_name,salary,commission_pct
FROM employees
WHERE NOT commission_pct <=> NULL;


# 5.选择员工姓名的第三个字母是a的员工姓名

SELECT last_name
FROM employees
WHERE last_name LIKE '__a%';


# 6.选择姓名中有字母a和k的员工姓名

SELECT last_name
FROM employees
WHERE last_name LIKE '%a%k%' OR last_name LIKE '%k%a%';
#where last_name like '%a%' and last_name LIKE '%k%';

# 7.显示出表 employees 表中 first_name 以 'e'结尾的员工信息

SELECT first_name,last_name
FROM employees
WHERE first_name LIKE '%e';

SELECT first_name,last_name
FROM employees
WHERE first_name REGEXP 'e$'; # 以e开头的写法：'^e'

# 8.显示出表 employees 部门编号在 80-100 之间的姓名、工种
SELECT last_name,job_id
FROM employees
#方式1：推荐
WHERE department_id BETWEEN 80 AND 100;
#方式2：推荐，与方式1相同
#where department_id >= 80 and department_id <= 100;
#方式3：仅适用于本题的方式。
#where department_id in (80,90,100);

SELECT * FROM departments;

# 9.显示出表 employees 的 manager_id 是 100,101,110 的员工姓名、工资、管理者id

SELECT last_name,salary,manager_id
FROM employees
WHERE manager_id IN (100,101,110);