mysql--SQL编程(关于mysql中的日期) 学习笔记2

一.mysql数据库中的date
1.DATETIME和DATE:
DATETIME占用8个字节,日期范围为"1000-01-01 00:00:00"到"9999-12-31 23:59:59"
DATE占用3个字节,日期范围为"1000-01-01"到"9999-12-31"

mysql的CAST函数在强制转换到DATETIME时会保留到微秒数,不过在插入后同样会截断.
e.g.

select cast('2013-02-11 00:22:33' as datetime) DG

*************************** 1. row ***************************
D: 2013-02-11 00:22:33
1 row in set (0.00 sec)

mysql> insert into date 
-> select cast('2013-12-06 21:18:33' as datetime);

Query OK, 1 row affected (0.04 sec)
Records: 1 Duplicates: 0 Warnings: 0


type_name(fsp) TIME.DATETIME.TIMESTAMP
select CURTIME(4) AS TIMEG
*************************** 1. row ***************************
TIME: 22:02:38
1 row in set (0.01 sec)

2.TIMESTAMP
UTC:世界统一时间,由英文(COordinated Universal Time)和法文(Temps Universal Cordonne')而来
TIMESTAMP和DATETIME格式是一样的,均为"YYYY-MM-DD HH:MM:SS";
区别是:

1).TIMESTAMP占用４个字节,显示为"1970-01-01 00:00:00" UTC到"2038-01-19 03:14:07" UTC,而DATETIME占用8个字节,日期范围为"1000-01-01 00:00:00"到"9999-12-31 23:59:59"
2).在建表进可以为TIMESTAMP建一个默认值,而DATETIME不行.
3).在更新表时,可以设置TIMESTAMP类型的列自动更新时间为当前时间.

eg.1:

mysql> create table s_timestamp
-> ( a INT,
-> b timestamp default current_timestamp
-> )engine=innodb
-> ;
Query OK, 0 rows affected (0.16 sec)

mysql> desc s_timestamp;
+-------+-----------+------+-----+-------------------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-----------+------+-----+-------------------+-------+
| a | int(11) | YES | | NULL | |
| b | timestamp | NO | | CURRENT_TIMESTAMP | |
+-------+-----------+------+-----+-------------------+-------+
2 rows in set (0.00 sec)

mysql> insert into s_timestamp (a) values (1);
Query OK, 1 row affected (0.04 sec)

mysql> select * from s_timestamp;
+------+---------------------+
| a | b |
+------+---------------------+
| 1 | 2013-12-07 22:23:35 |
+------+---------------------+
1 row in set (0.00 sec)

再来新建一个时间随着更改值时自动更新的:

create table s_timestamp2(a INT, b timestamp default on update current_timestamp) Engine=InnoDB;
mysql> insert into s_timestamp2 (a) values (1);

Query OK, 1 row affected (0.05 sec)

mysql> select * from s_timestamp2;
+------+---------------------+
| a | b |
+------+---------------------+
| 1 | 2013-12-07 22:30:13 |
+------+---------------------+
1 row in set (0.00 sec)

mysql> update s_timestamp2 set a=2;
Query OK, 1 row affected (0.07 sec)
Rows matched: 1 Changed: 1 Warnings: 0

mysql> select * from s_timestamp2;
+------+---------------------+
| a | b |
+------+---------------------+
| 2 | 2013-12-07 22:31:39 |
+------+---------------------+
1 row in set (0.00 sec)

3.YEAR和TIME
1)YEAR类型战胜1个字节,并且在定义时可以指定显示的宽度为YEAR(4)或YEAR(2);

mysql> create table s_year(
-> a year(2)) engine=innodb;

mysql> desc s_year;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| a | year(2) | YES | | NULL | |
+-------+---------+------+-----+---------+-------+
1 row in set (0.01 sec)

mysql> select * from s_year;
+------+
| a |
+------+
| 13 |
+------+
1 row in set (0.00 sec)

对于year(4)日期范围为1970-2070,year(2)中00-69代表2000-2069年.
2)TIME占用3个字节,显示的范围为-838:59:59 ---- 838:59:59因为,TIME类型不仅可以用来保存一天中的时间,也可以用来保存时间间隔,所以time可以为负值和最大值大于23作出解释.TIME类型同样可以显示微秒时间,但是在插入时,数据库同样会进行截断.
eg:

mysql> create table s_time (a TIME)ENGINE=InnoDB;
Query OK, 0 rows affected (0.12 sec)

mysql> desc s_time;
+-------+------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------+------+-----+---------+-------+
| a | time | YES | | NULL | |
+-------+------+------+-----+---------+-------+
1 row in set (0.00 sec)
mysql> insert into s_time select '22:49:33'
mysql> select * from s_time;
+----------+
| a |
+----------+
| 22:49:33 |
+----------+
1 row in set (0.00 sec)

二.与日期和时间相关的函数
1.now(),current_timestamp(),sysdate(),表示当前时间的函数

mysql> select now(),current_timestamp(),sysdate() G
*************************** 1. row ***************************
now(): 2013-12-07 22:53:21
current_timestamp(): 2013-12-07 22:53:21
sysdate(): 2013-12-07 22:53:21
1 row in set (0.00 sec)

mysql> select now(), current_timestamp(), sysdate(), sleep(3), now(), current_timestamp(), sysdate() G
*************************** 1. row ***************************
now(): 2013-12-07 23:02:33
current_timestamp(): 2013-12-07 23:02:33
sysdate(): 2013-12-07 23:02:33
sleep(3): 0
now(): 2013-12-07 23:02:33
current_timestamp(): 2013-12-07 23:02:33
sysdate(): 2013-12-07 23:02:36
1 row in set (3.00 sec)

由上述例子可以看出,now()和current_timestamp()函数是同义的,即作用是相同的,都是sql语句开始执行时的时间,而sysdate()函数则表示sql语句执行到当前函数的时间.
2.时间加减函数
DATE_ADD(date,INTERVAL expr unit),DATE_SUB(date,INTERVAL expr unit),其中expr的值可以为负值，所以可以用date_add()函数来执行时间相减。

mysql> select now() as now ,date_add(now(),interval 1 day) as tomrrow, date_sub(now(),interval 1 day ) as yseterday ;
+---------------------+---------------------+---------------------+
| now | tomrrow | yseterday |
+---------------------+---------------------+---------------------+
| 2013-12-07 23:09:52 | 2013-12-08 23:09:52 | 2013-12-06 23:09:52 |
+---------------------+---------------------+---------------------+
1 row in set (0.00 sec)

mysql> select date_add(now(),interval -1 day) as yesterday G
*************************** 1. row ***************************
yesterday: 2013-12-06 23:13:30
1 row in set (0.00 sec)

如果是闰月mysql会自动判别月份日期，闰月，那么就有2.29，否则就没有，看下面例子

mysql> select date_add('2000-2-29',interval 4 year) as 4_year G
*************************** 1. row ***************************
4_year: 2004-02-29
1 row in set (0.00 sec)

mysql> select date_add('2000-2-29',interval 3 year) as 4_year G
*************************** 1. row ***************************
4_year: 2003-02-28
1 row in set (0.00 sec)
INTERVAL expr unit也可以是hour,second,minute,week,month,microsecond等时间单位
mysql> select date_add(now(),interval 10 minute) as min G
*************************** 1. row ***************************
min: 2013-12-07 23:32:56
1 row in set (0.00 sec)

3.Date_Format函数
按照用户的需求来打印日期

mysql> select date_format(now(),'%Y-%m%d') as datetime;
+-----------+
| datetime |
+-----------+
| 2013-1207 |
+-----------+
1 row in set (0.00 sec)