语法：SELECT 列名 FROM 表名 WHERE 条件

WHERE 条件：在查询结果中，筛选符合条件的查询结果，条件为布尔表达式

1. 等值判断

   # 查询薪资是10000的员工
   SELECT employee_id, first_name, salary
   FROM t_employees
   WHERE salary = 10000;

2. 逻辑判断（AND OR NOT）

   # 查询新姿是10000并且提成是0.3的员工信息
   SELECT employee_id, first_name, salary
   FROM t_employees
   WHERE salary = 10000 AND commision_pct=0.3;
   

3. 不等值判断（>、<、<=、>=、!=、<>）

!= 和 <> 都表示不等于

# 查询薪资是8000到10000之间的员工
SELECT employee_id, first_name, salary
FROM t_employees
WHERE salary >= 8000 AND salary <= 10000;

1. 区间判断

   # 查询薪资是8000到10000之间的员工
   SELECT employee_id, first_name, salary
   FROM t_employees
   WHERE salary BETWEEN 8000 AND 10000;
   # 从小到大 小值在前 大值在后
   

2. NULL 值判断（IS NULL、IS NOT NULL）

   # 查询没有提成的员工
   SELECT employee_id, first_name, salary, commision_pct
   FROM t_employees
   WHERE commision_pct IS NULL;
   

3. 枚举查询（IN(值1，值2，值3)）

查询效率较低，可通过多条件来查询

# 查询部门编号为60、70、80的员工信息
SELECT employee_id, first_name, salary, commision_pct
FROM t_employees
WHERE department_id IN (60, 70, 80);

1. 模糊查询 ```sql

   查询名字以“L”开头的员工信息

   SELECT employee_id, first_name, salary, commision_pct FROM t_employees WHERE first_name LIKE ‘L%’;

查询名字以“L”开头的、长度为4的员工信息

SELECT employeeid, firstname, salary, commisionpct FROM t_employees WHERE first_name LIKE ‘L‘;

8. 分支结构查询
```sql
# 查询员工信息（编号、名字、薪资、薪资级别<对应条件表达式生成>）
SELECT employee_id, first_name, salary
    CASE
      WHEN salary >= 10000 THEN 'A'
    WHEN salary < 10000 AND salary >= 8000 THEN 'B'
    WHEN salary < 8000 AND salary >= 6000 THEN 'C'
    WHEN salary < 6000 AND salary >= 4000 THEN 'D'
    ELSE 'E'
      END as 'level'
FROM t_employee;