MySQL函数查询练习

1.显示系统时间(注：日期+时间) 

SELECT NOW()
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 '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 DESC;

 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 DATEDIFF(SYSDATE(),hire_date)/365 worked_years,DATEDIFF(SYSDATE(),hire_date) worked_days
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 hire_date>'1997-12-31'
AND department_id IN(80,90,110)
AND commission_pct IS NOT NULL;

 7.查询公司中入职超过10000天的员工姓名、入职时间 

SELECT last_name,start_date
FROM employees e JOIN job_history j
AND e.`department_id`=j.`department_id`
WHERE DATEDIFF(end_date,start_date);

 8.做一个查询，产生下面的结果

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

9. 

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 'F'
                             END'Grade'
FROM employees;