考查 case when 语句
需要注意的问题，Case函数只返回第一个符合条件的值，剩下的Case部分将会被自动忽略

SELECT e.emp_no, e.first_name, e.last_name, eb.btype, s.salary,
CASE WHEN eb.btype = 1 THEN s.salary * 0.1
WHEN eb.btype = 2 THEN s.salary * 0.2
ELSE s.salary * 0.3
END as bonus
FROM emp_bonus AS eb, employees AS e, salaries AS s
WHERE eb.emp_no = e.emp_no
AND s.emp_no = e.emp_no
AND s.to_date='9999-01-01';