#1.	显示系统时间(注：日期+时间)
SELECT NOW();

#2.	查询员工号，姓名，工资，以及工资提高百分之20%后的结果（new salary）

SELECT employee_id,last_name,salary,salary*1.2 "new salary"
FROM employees;
#3.	将员工的姓名按首字母排序，并写出姓名的长度（length）
SELECT LENGTH(last_name) 长度,SUBSTR(last_name,1,1) 首字符,last_name
FROM employees
ORDER BY 首字符;

#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,'earn',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
Last_name	Job_id	Grade
king	AD_PRES	A

SELECT last_name,job_id AS job,
CASE job_id
WHEN 'AD_PRES' THEN 'A'
WHEN 'ST_MAN' THEN 'B' 
WHEN 'IT_PROG' THEN 'C' 
WHEN 'SA_PRE' THEN 'D'
WHEN 'ST_CLERK' THEN 'E'
END AS Grade
FROM employees
WHERE job_id = 'AD_PRES';