#第07章_单行函数课后练习

# 1.显示系统时间(注：日期+时间)
SELECT NOW(),SYSDATE(),CURRENT_TIMESTAMP(),LOCALTIME(),LOCALTIMESTAMP() # 大家只需要掌握一个函数就可以了
FROM dual;

# 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 AS '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 last_name,LENGTH(last_name) AS "name_length"
FROM employees
# ORDER BY last_name ASC;
ORDER BY name_length ASC;

# 4.查询员工id,last_name,salary，并作为一个列输出，别名为OUT_PUT
SELECT CONCAT(employee_id,',',last_name,',',salary) AS "OUT_PUT"
FROM employees;

# 5.查询公司各员工工作的年数、工作的天数，并按工作年数的降序排序
SELECT employee_id,DATEDIFF(CURDATE(),hire_date) / 365 AS "worked_years",DATEDIFF(CURDATE(),hire_date) AS "worked_days",
TO_DAYS(CURDATE()) - TO_DAYS(hire_date) AS "worked_days1"
FROM employees
ORDER BY worked_years DESC;

# 6.查询员工姓名，hire_date , department_id，满足以下条件：雇用时间在1997年之后，department_id为80 或 90 或110, commission_pct不为空
SELECT last_name,hire_date,department_id
FROM employees
WHERE department_id IN (80,90,110)
AND commission_pct IS NOT NULL
# AND hire_date >= '1997-01-01';  #存在着隐式转换
# AND DATE_FORMAT(hire_date,'%Y-%m-%d') >= '1997-01-01' # 显示转换操作，格式化：日期 ---> 字符串
# AND DATE_FORMAT(hire_date,'%Y') >= '1997';  # 显示转换操作，格式化
AND hire_date >= STR_TO_DATE('1997-01-01','%Y-%m-%d'); # 显式转换操作，解析：字符串 ---> 日期

# 7.查询公司中入职超过10000天的员工姓名、入职时间
SELECT last_name,hire_date
FROM employees
WHERE DATEDIFF(CURDATE(),hire_date) >= 10000;

# 8.做一个查询，产生下面的结果
# <lastname> earns <salary> monthly but wants <salary*3>
SELECT CONCAT(last_name,' earns ',TRUNCATE(salary,0),' monthly but wants ',TRUNCATE(salary * 3,0)) AS "Dream Salary"
FROM employees;

# 9.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 last_name AS "Last_name",job_id AS "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'
                                                               ELSE 'undefined' END AS "Grade"
FROM employees;