mysql学习笔记-日期

-- 1.获取当前日期时间
SELECT SYSDATE() FROM DUAL;
SELECT CURRENT_DATE() FROM DUAL;

-- 获取时间戳
SELECT CURRENT_TIMESTAMP() FROM DUAL;

-- 2,日期的截取
-- 获取到当前日期时间所在的年份

select extract(year FROM SYSDATE()) FROM DUAL;

select extract(month FROM SYSDATE()) FROM DUAL;

select extract(DAY FROM SYSDATE()) FROM DUAL;

-- 3.日期的截断,这个四舍五入没用

select round(SYSDATE(),'year') FROM DUAL;

select TRUNCATE(SYSDATE(),'year')FROM DUAL;

 --
select ename,empno months_between(sysdate(),hiredate) months FROM t_emptest;

SELECT * FROM t_emptest;

SELECT SYSDATE() FROM DUAL;
 

SELECT * FROM t_emptest WHERE SAL = '3000';
SELECT * FROM t_emptest WHERE SAL = 3000;

SELECT 100 + 2000 FROM DUAL;

SELECT '100' + 2000 FROM DUAL;

SELECT 100 + CAST('200' AS INT) FROM DUAL;

SELECT CAST(100 AS VARCHAR)+100 FROM DUAL;
SELECT CAST(100 AS char) + 100 FROM DUAL;

select cast(date_cloumn as signed) as date from t_date ;

select cast(timstamp_cloumn as time) as date from t_date ;

-- 获取到最大的
select greatest(10,'101',9) from dual;
-- 找到最小的，按照accii码值来比较
select least('中','ABC','ab',1000) from dual

select 1 + null from dual;