#查询符合条件的数据
SELECT employee_id,first_name,salary
FROM t_employees
WHERE salary = 10000;

# 查询员工的工资再10000~40000之间的员工信息（编号，名字，薪资）
SELECT employee_id,first_name,salary
FROM t_employees
WHERE salary >= 10000 AND salary 

#查询员工的薪资在10000~50000之间的员工信息（编号，名字，薪资）
SELECT employee_id,first_name,salary
FROM t_employees
WHERE salary BETWEEN 10000 AND 50000;

#查询没有提成的员工信息（编号，名字，薪资,提成）
SELECT employee_id,first_name,salary,commission_pct
FROM t_employees
WHERE commission_pct IS NULL;

#查询部门编号为90、30的员工信息（编号，名字，薪资，部门编号）
SELECT employee_id,first_name,salary,department_id
FROM t_employees
WHERE department_id IN(90,30);

LIKE_ (单个任意字符)
列名 LIKE ‘张_'
LIKE % (任意长度的任意字符）
列名 LIKE ‘张%'

#查询名字已“L”开头的员工信息（编号，名字，薪资，部门编号）
SELECT employee_id,first_name,salary,department_id
FROM t_employees
WHERE first_name LIKE 'L%';

#查询名字已“L”开头并且长度为3的员工信息（编号，名字，薪资，部门编号）
SELECT employee_id,first_name,salary,department_id
FROM t_employees
WHERE first_name LIKE 'L__';

CASE
WHEN 条件1 THEN 结果1
WHEN 条件2 THEN 结果2
WHEN 条件3 THEN 结果3
ELSE 结果4
END

#查询员工信息（编号，名字，薪资，薪资级别)
SELECT employee_id,first_name,salary,
CASE
	WHEN salary>40000 THEN 'A'
	WHEN salary>30000 AND salary 20000 AND salary 10000 AND salary