常用SQL方法学习笔记

常用SQL方法学习笔记

笔记一：

---截取
SELECT SUBSTR('BAAAAAAAAAAAA',1,5)AS S FROM DUAL;---BAAAA

---為空判斷
SELECT NVL('A','B') AS FLAG  FROM DUAL;---A
SELECT NVL('','B') AS FLAG  FROM DUAL;---B
SELECT NVL(null,'B') AS FLAG  FROM DUAL;---B

---小數四捨五入保留
SELECT ROUND(50.26448,2) AS NM FROM DUAL;---50.26 

---去空格
select trim(' AB CDE ') AS S FROM DUAL;---左右去空格

SELECT ltrim(' ABCD  ')AS S FROM DUAL;---左去空格

SELECT rtrim(' ABCDE  ') AS S FROM DUAL;---右去空格

---DECODE
SELECT DECODE('3', '1', 'UNLOCK', '2', 'LOCK', '3', 'OTHER') AS STATUS FROM DUAL;---OTHER

---TRUNC 不近似保留
SELECT TRUNC(0.123456789, 5) AS STOPTIME_HN FROM DUAL; ---0.12345

笔记二：

1.查询当前年、月、周相关时间

1.1.查询当前年份

SELECT TO_CHAR(SYSDATE,'YYYY') AS YEAR FROM DUAL--查询当前年份
SELECT TO_CHAR(SYSDATE,'YYY') AS YEAR FROM DUAL--查询当前年份后两位
SELECT TO_CHAR(SYSDATE,'YY') AS YEAR FROM DUAL--查询当前年份最后两位
SELECT TO_CHAR(SYSDATE,'Y') AS YEAR FROM DUAL--查询当前年份最后一位

1.2.查询当前年份第几天

SELECT TO_CHAR(SYSDATE,'DDD') AS DAYS FROM DUAL

1.3.查询当前月份第几天

SELECT TO_CHAR(SYSDATE,'DD') AS DAYS FROM DUAL

1.4.查询当前周的星期几

SELECT TO_CHAR(SYSDATE,'dy') AS WEEK FROM DUAL
SELECT TO_CHAR(SYSDATE,'day') AS WEEK FROM DUAL 

1.5.查询当前年的第几天：

SELECT TO_CHAR(SYSDATE,'DDD') AS DAYS FROM DUAL

2.查询当前第几季度

SELECT TO_CHAR(SYSDATE,'Q') AS JD FROM DUAL

3.查询当前在一年内是第几周

SELECT TO_CHAR(SYSDATE,'IW') AS WEEKNUM FROM DUAL
SELECT TO_CHAR(SYSDATE,'WW') AS WEEKNUM FROM DUAL
SELECT TO_CHAR(SYSDATE,'W') AS WEEKNUM FROM DUAL

4.查询当前某年某月有多少天

SELECT TO_CHAR(last_day(to_date('2019/02/01','YYYY/MM/DD')),'DD') AS MONTH_DAYS_NUMBER FROM DUAL

5.查询当前的日期格式

SELECT TO_CHAR(SYSDATE,'YYYY/MM/DD') AS MONTH FROM DUAL--获取当前日期的'YYYY/MM/DD'格式：2020/01/02
SELECT TO_CHAR(SYSDATE,'YYYYMMDD') AS MONTH FROM DUAL--获取当前日期的'YYYYMMDD'格式：20200102
SELECT TO_CHAR(SYSDATE,'YYYYMMDD HH:MI:SS') AS now FROM DUAL--获取当前时间，精确到秒的12小时制格式：20200102 03:18:25
SELECT TO_CHAR(SYSDATE,'YYYYMMDD HH24:MI:SS') AS now FROM DUAL--获取当前时间，精确到秒24小时制的格式：20200102 15:18:13
SELECT TO_CHAR(SYSDATE,'YYYY/MM/DD HH24:MI:SS:PM:DY') AS now FROM DUAL--获取当前时间的格式，显示上、下午和星期几：2020/01/02 15:17:59:下午：星期四

6.查询距离当前时间一段时间间隔的时间，当前时间之前用“-”当前时间之后用“+”

SELECT TO_CHAR(SYSDATE, 'YYYY/MM/DD HH24:MI:SS') AS NOW,TO_CHAR(SYSDATE - INTERVAL '7' second,'YYYY/MM/DD HH24:MI:SS') AS PAST FROM DUAL --查询当前时间的7秒前时间
SELECT TO_CHAR(SYSDATE, 'YYYY/MM/DD HH24:MI:SS') AS NOW,TO_CHAR(SYSDATE - INTERVAL '7' minute,'YYYY/MM/DD HH24:MI:SS') AS PAST FROM DUAL --查询当前时间的7分钟前时间
SELECT TO_CHAR(SYSDATE, 'YYYY/MM/DD HH24:MI:SS') AS NOW,TO_CHAR(SYSDATE - INTERVAL '7'hour ,'YYYY/MM/DD HH24:MI:SS') AS PAST FROM DUAL --查询当前时间的7小时前时间
SELECT TO_CHAR(SYSDATE, 'YYYY/MM/DD HH24:MI:SS') AS NOW,TO_CHAR(SYSDATE - INTERVAL '7'day ,'YYYY/MM/DD HH24:MI:SS') AS PAST FROM DUAL --查询当前时间的7天前时间
SELECT TO_CHAR(SYSDATE, 'YYYY/MM/DD HH24:MI:SS') AS NOW,TO_CHAR(SYSDATE - INTERVAL '7'month ,'YYYY/MM/DD HH24:MI:SS') AS PAST FROM DUAL --查询当前时间的7月前时间
SELECT TO_CHAR(SYSDATE, 'YYYY/MM/DD HH24:MI:SS') AS NOW,TO_CHAR(SYSDATE - INTERVAL '7'year ,'YYYY/MM/DD HH24:MI:SS') AS PAST FROM DUAL --查询当前时间的7年前时间
SELECT TO_CHAR(SYSDATE, 'YYYY/MM/DD HH24:MI:SS') AS NOW,TO_CHAR(SYSDATE - 8*INTERVAL '7'hour ,'YYYY/MM/DD HH24:MI:SS') AS PAST FROM DUAL --查询当前时间参数年、月、日、时、分、秒乘以一个数字倍数之前的时间

后续边学习边整理........