第07章_单行函数 练习

#第07章_单行函数 练习

DESC employees;

# 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)"length"
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 hire_date,DATEDIFF(NOW(),hire_date)/365 "years",
DATEDIFF(NOW(),hire_date) "days"
FROM employees
ORDER BY years DESC;

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 department_id IN (80,90,110)
AND commission_pct IS NOT  NULL
AND EXTRACT(YEAR FROM hire_date)>=1997;

SELECT last_name, hire_date, department_id
FROM employees
#WHERE hire_date >= '1997-01-01'
#WHERE hire_date >= STR_TO_DATE('1997-01-01', '%Y-%m-%d')
WHERE DATE_FORMAT(hire_date,'%Y') >= '1997'
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,hire_date
FROM employees
WHERE DATEDIFF(NOW(),hire_date)>10000;

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

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,job_id,
CASE 
WHEN job_id='AD_PRES' THEN 'A'
WHEN job_id='ST_MAN' THEN 'B'
WHEN job_id='IT_PROG' THEN 'C'
WHEN job_id='AD_PRES' THEN 'D'
WHEN job_id='ST_CLERK' THEN 'E'
ELSE "F"
END "grade"
FROM employees;

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;