MySQL之日期函数

【Date and time】：

（补充说明）DATE/TIME 格式：

NOW()、CURRENT_TIMESTAMP()与CURRENT_TIMESTAMP、LOCALTIME()与LOCALTIME、LOCALTIMESTAMP()与LOCALTIMESTAMP：（均为同义词）
返回当前的日期和时间，基于Satement（DATETIME格式）

mysql> SELECT NOW(),CURRENT_TIMESTAMP();
+---------------------+---------------------+
| NOW()               | CURRENT_TIMESTAMP() |
+---------------------+---------------------+
| 2015-09-25 14:14:15 | 2015-09-25 14:14:15 |
+---------------------+---------------------+
1 row in set (0.00 sec)

mysql> SELECT NOW(),LOCALTIME(),LOCALTIME,LOCALTIMESTAMP(),LOCALTIMESTAMP;
+---------------------+---------------------+---------------------+---------------------+---------------------+
| NOW()               | LOCALTIME()         | LOCALTIME           | LOCALTIMESTAMP()    | LOCALTIMESTAMP      |
+---------------------+---------------------+---------------------+---------------------+---------------------+
| 2015-09-25 14:21:23 | 2015-09-25 14:21:23 | 2015-09-25 14:21:23 | 2015-09-25 14:21:23 | 2015-09-25 14:21:23 | 
+---------------------+---------------------+---------------------+---------------------+---------------------+
1 row in set (0.00 sec)

SYSDATE()：返回当前的日期和时间，基于系统时间（DATETIME格式）

mysql> mysql> SELECT NOW(),SYSDATE(),SLEEP(1),NOW(),SYSDATE();
+---------------------+---------------------+----------+---------------------+---------------------+
| NOW()               | SYSDATE()           | SLEEP(1) | NOW()               | SYSDATE()           |
+---------------------+---------------------+----------+---------------------+---------------------+
| 2015-09-28 10:49:00 | 2015-09-28 10:49:00 | 0        | 2015-09-28 10:49:00 | 2015-09-28 10:49:01 |
+---------------------+---------------------+----------+---------------------+---------------------+
1 row in set (1.00 sec)

CURDATE()与CURRENT_DATE()与CURRENT_DATE：返回当前的日期，基于服务器主机（DATE格式）

mysql> SELECT CURDATE(),CURRENT_DATE();
+------------+----------------+
| CURDATE()  | CURRENT_DATE() |
+------------+----------------+
| 2015-09-25 | 2015-09-25     |
+------------+----------------+
1 row in set (0.00 sec)

CURTIME()与CURRENT_TIME()与CURRENT_TIME：返回当前的时间，基于服务器主机（TIME格式）

mysql> SELECT CURTIME(),CURRENT_TIME();
+-----------+----------------+
| CURTIME() | CURRENT_TIME() |
+-----------+----------------+
| 14:15:19  | 14:15:19       |
+-----------+----------------+
1 row in set (0.00 sec)

YEAR()：返回日期的年份（YEAR格式）
MONTH()：返回日期的月份
DAY()与DAYOFMONTH()：返回日期的天数
DAYNAME()：返回星期几

mysql> SELECT YEAR(NOW()),
    -> MONTH(NOW()),
    -> DAY(NOW()),
    -> DAYOFMONTH(NOW()),
    -> DAYNAME(NOW());
+-------------+--------------+------------+-------------------+----------------+
| YEAR(NOW()) | MONTH(NOW()) | DAY(NOW()) | DAYOFMONTH(NOW()) | DAYNAME(NOW()) |
+-------------+--------------+------------+-------------------+----------------+
| 2015        | 9            | 25         | 25                | Friday         |
+-------------+--------------+------------+-------------------+----------------+
1 row in set (0.00 sec)

DAYOFWEEK()：返回日期对应的星期，1--周日 2--周一 3--周二 4--周三 5--周四 6--周五 7--周六
DAYOFYEAR()：返回日期为该年的第多少天

mysql> \! cal
   September 2015 
Su Mo Tu We Th Fr Sa
       1 2 3 4 5
 6 7 8 9 10 11 12
13 14 15 16 17 18 19
20 21 22 23 24 25 26
27 28 29 30

mysql> SELECT DAYOFWEEK(NOW()),
    -> DAYOFYEAR(NOW());
+------------------+------------------+
| DAYOFWEEK(NOW()) | DAYOFYEAR(NOW()) |
+------------------+------------------+
| 2                | 271              |
+------------------+------------------+
1 row in set (0.00 sec)

HOUR()：提取时间的小时
MINUTE()：提取时间的分钟
SECOND()：提取时间的秒数
MICROSECOND()：提取时间的微秒

mysql> SELECT HOUR('11:11:12.000123') h,
    -> MINUTE('11:11:12.000123') m,
    -> SECOND('11:11:12.000123') s,
    -> MICROSECOND('11:11:12.000123') ms;
+------+------+------+------+
| h    | m    | s    | ms   |
+------+------+------+------+
| 11   | 11   | 12   | 123  |
+------+------+------+------+
1 row in set (0.00 sec)

DATE_FORMAT()：将日期时间格式化

mysql> SELECT DATE_FORMAT('2015-10-31 08:30:59','%H:%i:%s');    
+-----------------------------------------------+
| DATE_FORMAT('2015-10-31 08:30:59','%H:%i:%s') |
+-----------------------------------------------+
| 08:30:59                                      |
+-----------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT DATE_FORMAT(NOW(),'%y/%m/%d');    -- 也可以这样用
+-------------------------------+
| DATE_FORMAT(NOW(),'%y/%m/%d') |
+-------------------------------+
| 15/09/25                      |
+-------------------------------+
1 row in set (0.00 sec)

一般情况下，多数场景可用DATE_FORMAT()代替STR_TO_DATE()。

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