第七章-单行函数-课后练习

#1.显示系统时间(注：日期+时间)
SELECT  NOW(),SYSDATE(),LOCALTIME(),CURRENT_TIMESTAMP(),LOCALTIMESTAMP()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*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 last_name,LENGTH(last_name)
FROM employees
ORDER BY last_name 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) "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,ROUND(DATEDIFF(CURDATE(),hire_date)/365) "work_years",DATEDIFF(CURDATE(),hire_date)"work_days"
FROM employees
ORDER BY work_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 not  commission_pct <=> null;
AND commission_pct IS NOT NULL
#and hire_date >= '1997-01-01';  
#存在一个隐式转换
#and date_format(hire_date,'%Y-%m-%d') >= '1997-01-01'; 
#显示转换，格式化：日期 –> 字符串
AND hire_date >= STR_TO_DATE('1997-01-01','%Y-%m-%d');

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

#8.做一个查询，产生下面的结果#<last_name> earns <salary> monthly but wants <salary*3>
SELECT CONCAT(last_name,'earns',TRUNCATE(salary,0),'monthly but wants',TRUNCATE((salary*3),0)) "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
SELECT last_name "Last_name" ,job_id "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 'underined'                             
                                    END "Geade"
FROM employees;