排序查询

案例一：
查询员工信息，要求工资从高到低排序
SELECT * FROM employees ORDER BY salary DESC;
SELECT * FROM employees ORDER BY salary ;
案例二：查询部门编号>=90员工信息，按照入职的先后排序
SELECT *
FROM employees
WHERE department_id>=90
ORDER BY hiredate ASC
案例三：按照年薪的高低显示员工的信息和年薪【按表达式排序】
SELECT ,salary12*(1+IFNULL(commission_pct,0))
FROM employees
ORDER BY salary12(1+IFNULL(commission_pct,0)）DESC；
案例四：按照年薪的高低显示员工的信息和年薪【按照别名排序】
SELECT ,salary12*(1+IFNULL(commission_pct,0)) AS 年薪
FROM employees
ORDER BY 年薪 DESC；
案例5：
按照姓名的长度显示员工的名字和工资（按函数排序）
SELECT LENGTH（last_name）字节长度，last_name,salary
FROM employees
ORDER BY LENGTH(last name) DESC;
案例6：查询员工信息，要求先按照工资升序，再按照员工编号降序【按多个字段排序】
SELECT*
FROM employees
ORDER BY salary ASC,employee_id DESC;