条件查询
语法：
select 查询列表
from 表名
where 筛选条件;
select last_name,first_name from employees where salary>20000;
-
按关系表达式筛选
> < >= <= = <>（不等于最好使用<>不使用!=） -
按逻辑表达式筛选
and or not（也可以使用&& || !） -
模糊查询
like in between and is null
1. 按关系表达式筛选
案例1：查询部门编号不是100的员工信息
SELECT *
FROM employees
WHERE department_id <> 100;
案例2：查询工资<15000的姓名、工资
SELECT last_name,salary
FROM employees
WHERE salary<15000;
2. 按逻辑表达式筛选
案例1：查询部门编号不是 50-100之间员工姓名、部门编号、邮箱
方式1：
SELECT last_name,department_id,email
FROM employees
WHERE department_id <50 OR department_id>100;
方式2：
SELECT last_name,department_id,email
FROM employees
WHERE NOT(department_id>=50 AND department_id<=100);
案例2：查询奖金率>0.03 或者 员工编号在60-110之间的员工信息
SELECT *
FROM employees
WHERE commission_pct>0.03 OR (employee_id >=60 AND employee_id<=110);
3. 模糊查询
3.1 like
功能：一般和通配符搭配使用，对字符型数据进行部分匹配查询
常见的通配符：
_ 任意单个字符
% 任意多个字符,支持0-多个
like/not like
案例1：查询姓名中包含字符a的员工信息
SELECT *
FROM employees
WHERE last_name LIKE '%a%';
案例2：查询姓名中包含最后一个字符为e的员工信息
SELECT *
FROM employees
WHERE last_name LIKE '%e';
案例3：查询姓名中包含第一个字符为e的员工信息
SELECT *
FROM employees
WHERE last_name LIKE 'e%';
案例4：查询姓名中包含第三个字符为x的员工信息
SELECT *
FROM employees
WHERE last_name LIKE '__x%';
案例5：查询姓名中包含第二个字符为_的员工信息
SELECT *
FROM employees
WHERE last_name LIKE '_\_%';
SELECT *
FROM employees
WHERE last_name LIKE '_$_%' ESCAPE '$';
3.2 in
功能：查询某字段的值是否属于指定的列表之内
a in(常量值1,常量值2,常量值3,...)
a not in(常量值1,常量值2,常量值3,...)
in/not in
案例1：查询部门编号是30/50/90的员工名、部门编号
方式1：
SELECT last_name,department_id
FROM employees
WHERE department_id IN(30,50,90);
方式2：
SELECT last_name,department_id
FROM employees
WHERE department_id = 30
OR department_id = 50
OR department_id = 90;
案例2：查询工种编号不是SH_CLERK或IT_PROG的员工信息
方式1：
SELECT *
FROM employees
WHERE job_id NOT IN('SH_CLERK','IT_PROG');
方式2：
SELECT *
FROM employees
WHERE NOT(job_id ='SH_CLERK'
OR job_id = 'IT_PROG');
3.3 between X and X
功能：判断某个字段的值是否介于xx之间
between and/not between and
案例1：查询部门编号是30-90之间的部门编号、员工姓名
方式1：
SELECT department_id,last_name
FROM employees
WHERE department_id BETWEEN 30 AND 90;
方式2：
SELECT department_id,last_name
FROM employees
WHERE department_id>=30 AND department_id<=90;
案例2：查询年薪不是100000-200000之间的员工姓名、工资、年薪
SELECT last_name,salary,salary*12*(1+IFNULL(commission_pct,0)) 年薪
FROM employees
WHERE salary*12*(1+IFNULL(commission_pct,0))<100000 OR salary*12*(1+IFNULL(commission_pct,0))>200000;
SELECT last_name,salary,salary*12*(1+IFNULL(commission_pct,0)) 年薪
FROM employees
WHERE salary*12*(1+IFNULL(commission_pct,0)) NOT BETWEEN 100000 AND 200000;
3.4 is null/is not null
案例1：查询没有奖金的员工信息
SELECT *
FROM employees
WHERE commission_pct IS NULL;
案例2：查询有奖金的员工信息
SELECT *
FROM employees
WHERE commission_pct IS NOT NULL;
对比
= 只能判断普通的内容
IS 只能判断NULL值
<=> 安全等于，既能判断普通内容，又能判断NULL值
