#流程控制函数
#if函数

SELECT
    last_name,
    commission_pct,
IF
    ( commission_pct IS NULL, '没奖金，呵呵', '有奖金，嘻嘻' ) 
FROM
    employees

#case函数
#查询员工工资，要求如果部门号=30，工资为1.1倍，如果部门号=40，工资为1.2倍，其他部门号工资为原工资

SELECT salary AS '原工资',department_id,
CASE department_id
    WHEN 30 THEN
        salary*1.1
    WHEN 40 THEN
      salary*1.2
    ELSE
        salary
END AS '新工资' FROM employees

#job grade
#AD_PRES A
#ST_MAN B
#IT_PROG C
#SA_REP D
#ST_CLERK E

SELECT last_name,job_id,
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 Grade
FROM employees;

#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 AS '原工资',salary*1.2 AS '新工资' 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 SUBSTR(last_name,1,1) AS '首字母',LENGTH(last_name) FROM employees ORDER BY 首字母 ASC
#4. 做一个查询，产生下面的结果
/
<last_name> earns <salary> monthly but wants <salary3>
Dream Salary
King earns 24000 monthly but wants 72000
*/

SELECT CONCAT(last_name,' earns ',salary,' monthly but wants ',salary*3)  "Dream Salary"
FROM employees;