1、显示系统时间（注：日期+时间）
SELECT NOW();
2、查询员工号，姓名，工资，以及工资提高20%之后的结果（new salary）
SELECT employee_id,last_name,salary*1.2 "new salary"
FROM employees;
3、将员工的名字按首字母排序，并写出名字的长度（length)
SELECT LENGTH(last_name) 长度,SUBSTR(last_name,1,1) 首字符,last_name
FROM employees
ORDER BY last_name;
4、做一个查询，产生下面的结果
<last_name> earns<salary>monthly but wants <salary*3>
Dream salary
King earns 24000 monthly but wants 72000
SELECT CONCAT(last_name,'earns',salary,'monthly but wants',salary*3)AS "Dream salary"
FROM employees
WHERE salary=24000;
5、使用case-when，按照下面的条件：
job grade
AD_PRES A
ST_MAN B
IT_PROG C
SA_REP D
ST_CLERK E
产生下面的结果
SELECT job_id AS job,
CASE job_id
WHEN 'AD_PRES' THEN 'A'
WHEN 'ST_MAN ' THEN 'B'
WHEN 'IT_PROG' THEN 'C'
WHEN 'SA_REP' THEN 'D'
WHEN 'ST_CLERK' THEN 'E'
END AS Grade
FROM employees;
