1. 基本类型
Oracle支持不同的日期格式模型，其中包括：
- ISO 8601:
YYYY-MM-DDTHH:MI:SS，例如2024-06-20T14:30:00 - Oracle内部格式:
DD-MON-YYYY HH:MI:SS AM，例如20-JUN-2024 02:30:00 PM
DATE： 存储日期和时间，精确到秒
CREATE TABLE test_date (col DATE);
INSERT INTO test_date (col) VALUES (TO_DATE('2024-06-20 12:34:56', 'YYYY-MM-DD HH24:MI:SS'));
TIMESTAMP：比DATE类型更精确，可以精确到小数秒
CREATE TABLE test_timestamp (col TIMESTAMP);
INSERT INTO test_timestamp (col) VALUES (TO_TIMESTAMP('2024-06-20 12:34:56.789', 'YYYY-MM-DD HH24:MI:SS.FF3'));
INTERVAL YEAR TO MONTH：存储年份和月份的时间间隔
CREATE TABLE test_interval_ym (col INTERVAL YEAR TO MONTH); INSERT INTO test_interval_ym (col) VALUES (INTERVAL '2-3' YEAR TO MONTH);
INTERVAL DAY TO SECOND：存储天、小时、分钟、秒以及小数秒的时间间隔
CREATE TABLE test_interval_ds (col INTERVAL DAY TO SECOND); INSERT INTO test_interval_ds (col) VALUES (INTERVAL '5 12:34:56.789' DAY TO SECOND);
2. 常用函数
SYSDATE：返回当前系统日期和时间。SELECT SYSDATE FROM dual;
CURRENT_TIMESTAMP：返回当前系统时间戳：SELECT CURRENT_TIMESTAMP FROM dual;
EXTRACT: 从日期或时间戳中提取特定的部分（如年、月、日、小时等）
SELECT EXTRACT(YEAR FROM SYSDATE) AS year FROM dual; # 2024 SELECT EXTRACT(MONTH FROM SYSDATE) AS month FROM dual; # 6 SELECT EXTRACT(DAY FROM SYSDATE) AS day FROM dual; # 20
TO_DATE: 将字符串转换为DATE类型：SELECT TO_DATE('2024-06-20', 'YYYY-MM-DD') FROM dual;
TO_TIMESTAMP: 将字符串转换为TIMESTAMP类型：SELECT TO_TIMESTAMP('2024-06-20 12:34:56.789', 'YYYY-MM-DD HH24:MI:SS.FF3') FROM dual;
TO_CHAR: 将日期或时间戳转换为字符串，可以指定格式：SELECT TO_CHAR(SYSDATE, 'YYYY-MM-DD HH24:MI:SS') FROM dual;
ADD_MONTHS: 给日期加上指定的月份数：SELECT ADD_MONTHS(SYSDATE, 6) AS new_date FROM dual;
MONTHS_BETWEEN: 计算两个日期之间的月份数：SELECT MONTHS_BETWEEN(TO_DATE('2024-12-20', 'YYYY-MM-DD'), SYSDATE) AS months_between FROM dual;
NEXT_DAY: 返回指定日期之后的第一个指定星期几：SELECT NEXT_DAY(SYSDATE, 'FRIDAY') AS next_friday FROM dual;
LAST_DAY: 返回指定月份的最后一天：SELECT LAST_DAY(SYSDATE) AS last_day_of_month FROM dual;
3. Demo
示例的查询有如下：
当前系统日期和时间
SELECT SYSDATE, CURRENT_TIMESTAMP FROM dual;
将字符串转换为日期并进行加减操作
SELECT TO_DATE('2024-06-20', 'YYYY-MM-DD') + 10 AS new_date FROM dual;
提取日期的不同部分
SELECT EXTRACT(YEAR FROM SYSDATE) AS year,
EXTRACT(MONTH FROM SYSDATE) AS month,
EXTRACT(DAY FROM SYSDATE) AS day
FROM dual;
