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一、     Mysql 获得当前日期时间

Now() : 获得当前的 日期+ 时间（date + time ）函数：

mysql> select now();

+---------------------+

| now()               |

+---------------------+

| 2011-03-01 10:26:23 |

+---------------------+

1 row in set (0.00 sec)

同等的函数还包括current_timestamp(),localtime() ，但是now() 最容易记忆，所以推荐使用。

Sysdate() : 日期时间函数跟 now() 类似，不同之处在于：now() 在执行开始时值就得到了， sysdate() 在函数执行时动态得到值。

看下面的例子就明白了：

mysql> select now(),sleep(3),now();

+---------------------+----------+---------------------+

| now()               | sleep(3) | now()               |

+---------------------+----------+---------------------+

| 2011-03-01 10:51:43 |        0 | 2011-03-01 10:51:43 |

+---------------------+----------+---------------------+

1 row in set (3.02 sec)

mysql> select sysdate(),sleep(3),sysdate();

+---------------------+----------+---------------------+

| sysdate()           | sleep(3) | sysdate()           |

+---------------------+----------+---------------------+

| 2011-03-01 10:52:09 |        0 | 2011-03-01 10:52:12 |

+---------------------+----------+---------------------+

1 row in set (3.00 sec)

可以看到，虽然中途 sleep 3 秒，但 now() 函数两次的时间值是相同的； sysdate() 函数两次得到的时间值相差 3 秒

也可以只取当前日期或者当前时间

Curdate(): 获得当前日期

mysql> select curdate();

+------------+

| curdate()  |

+------------+

| 2011-03-01 |

+------------+

1 row in set (0.01 sec)

Curtime() : 获得当前时间（time ）函数

mysql> select curtime();

+-----------+

| curtime() |

+-----------+

| 10:55:12  |

+-----------+

1 row in set (0.00 sec)

二、     Mysql 日期时间的抽取extract

通过这一功能，我们可以从一个时间中抽取自己想要的部分，例如

mysql> set @ct='2011-03-01 11:16:14.123456';

Query OK, 0 rows affected (0.01 sec)

设置变量ct 为某一时间值，精确到微妙

获取其日期值

mysql> select date(@ct);

+------------+

| date(@ct)  |

+------------+

| 2011-03-01 |

+------------+

1 row in set (0.00 sec)

查看此日期所属季度

mysql> select quarter(@ct);

+--------------+

| quarter(@ct) |

+--------------+

|            1 |

+--------------+

1 row in set (0.00 sec)

查看此日期属于当年哪一周

mysql> select week(@ct);

+-----------+

| week(@ct) |

+-----------+

|         9 |

+-----------+

1 row in set (0.00 sec)

另外还有year(),day(),hour(),minute(),second() 等，在此不一一赘述。

采用extract() ，也可以实现类似的功能，语法格式为extract(year from @ct) ，

不足之处在于需要多敲几次键盘

Dayof 函数 :

Dayofweek(),dayofmonth(),dayofyear() 分别返回日期再一周、一月以及一年中的位置

mysql> select dayofweek(@ct);

+----------------+

| dayofweek(@ct) |

+----------------+

|              3 |

+----------------+

1 row in set (0.00 sec)

注意：其实3 月1 号是星期二，但是返回数字3 ，因为是从Sunday 开始算起的(1=Sunday,2=Monday, …)

mysql> select dayofmonth(@ct);

+-----------------+

| dayofmonth(@ct) |

+-----------------+

|               1 |

+-----------------+

1 row in set (0.00 sec)

mysql> select dayofyear(@ct);

+----------------+

| dayofyear(@ct) |

+----------------+

|             60 |

+----------------+

1 row in set (0.00 sec)

Week() 函数

查看日期属于当年的第几周

mysql> select weekofyear(@ct);

+-----------------+

| weekofyear(@ct) |

+-----------------+

|               9 |

+-----------------+

1 row in set (0.00 sec)

返回星期名和月份名的函数

Dayname() —计算日期是星期几

mysql> select dayname(@ct);

+--------------+

| dayname(@ct) |

+--------------+

| Tuesday      |

+--------------+

1 row in set (0.02 sec)

Monthname() —计算日期是哪一月

mysql> select monthname(@ct);

+----------------+

| monthname(@ct) |

+----------------+

| March          |

+----------------+

1 row in set (0.00 sec)

Last_day() : 返回月份中最后一天

mysql> select now();

+---------------------+

| now()               |

+---------------------+

| 2011-03-01 13:15:00 |

+---------------------+

1 row in set (0.00 sec)

mysql> select last_day(now());

+-----------------+

| last_day(now()) |

+-----------------+

| 2011-03-31      |

+-----------------+

1 row in set (0.00 sec)

通过该函数，可以计算出当前月份有多少天

mysql> select now(),day(last_day(now())) ;

+---------------------+----------------------+

| now()               | day(last_day(now())) |

+---------------------+----------------------+

| 2011-03-01 13:17:12 |                   31 |

+---------------------+----------------------+

1 row in set (0.00 sec)

三、Mysql 的日期时间计算函数

Date_add() : 为日期增加一个时间间隔

具体语法为date_add(@ct, interval num year/quarter/month/week/day/hour/minute/secont/microsecond);

注意: 此函数并不改变变量@ct 的实际值

mysql> set @ct=now();

Query OK, 0 rows affected (0.00 sec)

mysql> select @ct;

+---------------------+

| @ct                 |

+---------------------+

| 2011-03-01 15:09:16 |

+---------------------+

1 row in set (0.00 sec)

mysql> select date_add(@ct,interval 1 day);

+------------------------------+

| date_add(@ct,interval 1 day) |

+------------------------------+

| 2011-03-02 15:09:16          |

+------------------------------+

1 row in set (0.00 sec)

mysql> select @ct;

+---------------------+

| @ct                 |

+---------------------+

| 2011-03-01 15:09:16 |

+---------------------+

1 row in set (0.00 sec)

mysql> select date_add(@ct,interval 1 week);

+-------------------------------+

| date_add(@ct,interval 1 week) |

+-------------------------------+

| 2011-03-08 15:09:16           |

+-------------------------------+

1 row in set (0.00 sec)

类似功能还有adddate(),addtime() 等函数，与之相对应的是date_sub() ，顾名思义就是日期减法

另类日期函数

Period_add(P,N): 日期加/ 减去N 月，其中P 的格式应为yyyymm 或yymm

Period_diff(P1,P2): 日期p1-p2 ，返回N 个月

mysql> select period_add(201103,2),period_add(201103,-2) ;

+----------------------+-----------------------+

| period_add(201103,2) | period_add(201103,-2) |

+----------------------+-----------------------+

|               201105 |                201101 |

+----------------------+-----------------------+

1 row in set (0.00 sec)

mysql> select period_diff('201103','201101');

+--------------------------------+

| period_diff('201103','201101') |

+--------------------------------+

|                              2 |

+--------------------------------+

1 row in set (0.00 sec)

日期时间相减函数

Datediff(date1,date2) : 两个日期date1-date2

mysql> select datediff('2011-03-09','2011-03-01');

+-------------------------------------+

| datediff('2011-03-09','2011-03-01') |

+-------------------------------------+

|                                   8 |

+-------------------------------------+

1 row in set (0.00 sec)

mysql> select datediff('2011-03-01','2011-03-09');

+-------------------------------------+

| datediff('2011-03-01','2011-03-09') |

+-------------------------------------+

|                                  -8 |

+-------------------------------------+

1 row in set (0.00 sec)

Timediff(time1,time2) : 两个时间相减

mysql> select timediff('2011-03-03 15:33:00','2011-03-02 15:33:59');

+-------------------------------------------------------+

| timediff('2011-03-03 15:33:00','2011-03-02 15:33:59') |

+-------------------------------------------------------+

| 23:59:01                                              |

+-------------------------------------------------------+

1 row in set (0.00 sec)

mysql> select timediff('15:33:00','15:33:59');

+---------------------------------+

| timediff('15:33:00','15:33:59') |

+---------------------------------+

| -00:00:59                       |

+---------------------------------+

1 row in set (0.00 sec)

四  mysql 日期、时间转换函数

Time_to_sec(time) : 时间—> 秒 转换函数

Sec_to_time(num) : 秒--> 时间 转换函数

mysql> select time_to_sec('01:00:00');

+-------------------------+

| time_to_sec('01:00:00') |

+-------------------------+

|                    3600 |

+-------------------------+

1 row in set (0.00 sec)

mysql> select sec_to_time(3600);

+-------------------+

| sec_to_time(3600) |

+-------------------+

| 01:00:00          |

+-------------------+

1 row in set (0.00 sec)

To_days(date) : 日期--> 天 转换函数 起始日期为0000-00-00

From_days(num) : 天--> 日期 将数字转换为具体的日期

mysql> select to_days('2011-03-01');

+-----------------------+

| to_days('2011-03-01') |

+-----------------------+

|                734562 |

+-----------------------+

1 row in set (0.00 sec)

mysql> select from_days(734562);

+-------------------+

| from_days(734562) |

+-------------------+

| 2011-03-01        |

+-------------------+

1 row in set (0.00 sec)

Str_to_date(str,date) : 字符串--> 日期 转换函数

可以将一些杂乱无章的字符转换为日期格式

mysql> select str_to_date('01.03.2011', '%m.%d.%Y');

+---------------------------------------+

| str_to_date('01.03.2011', '%m.%d.%Y') |

+---------------------------------------+

| 2011-01-03                            |

+---------------------------------------+

1 row in set (0.00 sec)

mysql> select str_to_date('01/03/2011', '%m/%d/%Y');

+---------------------------------------+

| str_to_date('01/03/2011', '%m/%d/%Y') |

+---------------------------------------+

| 2011-01-03                            |

+---------------------------------------+

1 row in set (0.00 sec)

小练习：

以表centralmobile_logs 为例，目前该表总共有270 多万条数据

mysql> select count(*) from centralmobile_logs;

+----------+

| count(*) |

+----------+

|  2725403 |

+----------+

1 row in set (0.00 sec)

现在对其做一些统计

查询过去30 天总共有多少数据

mysql> select count(*) from centralmobile_logs where to_days(curdate())- to_days(create_time)<=30;

+----------+

| count(*) |

+----------+

|  2367518 |

+----------+

1 row in set (3.38 sec)

mysql> select count(*) from centralmobile_logs where datediff(curdate(),create_time) <=30;

+----------+

| count(*) |

+----------+

|  2367518 |

+----------+

1 row in set (3.29 sec)

查看每月第一天的数据

mysql> select count(*) from centralmobile_logs where dayofmonth(create_time)=1;

+----------+

| count(*) |

+----------+

|   161293 |

+----------+

1 row in set (3.14 sec)

查看11 年1 月31 日之前的数据

mysql> select count(*) from centralmobile_logs where create_time <='2011-01-31 00:00:00';

+----------+

| count(*) |

+----------+

|   413797 |

+----------+

1 row in set (0.17 sec)

查看11 年整个二月份的数据

mysql> select count(*) from centralmobile_logs where monthname(create_time)='February' and year(create_time)=2011;

+----------+

| count(*) |

+----------+

|  2149284 |

+----------+

1 row in set (3.94 sec)

查看11 年每个周日的累积数据

mysql> select count(*) from centralmobile_logs where dayname(create_time)='Sunday' and year(create_time)=2011;

+----------+

| count(*) |

+----------+

|   479033 |

+----------+

1 row in set (3.88 sec)

查看每天零点时分插入的数据总和

mysql> select count(*) from centralmobile_logs where time(create_time)='00:00:00';

+----------+

| count(*) |

+----------+

|       37 |

+----------+

1 row in set (3.99 sec)