项目中数据库字段 创建时间和更新时间 设置

不熟数据库之前，项目数据的更新和创建所涉及到的时间，都是在java程序里面获取系统时间，然后传到dao层执行写入后更新操作。其实这样有点不必要，因为设置数据库时间字段时，可以设置为自动获取；如下：mysql

CURRENT_TIMESTAMP 和 ON UPDATE CURRENT_TIMESTAMP 详解

1.CURRENT_TIMESTAMP : 当要向数据库执行 insert操作时，如果有个 timestamp字段属性设为CURRENT_TIMESTAMP，则无论这个
字段有没有set值都插入当前系统时间

2. ON UPDATE CURRENT_TIMESTAMP : 使用 ON UPDATE CURRENT_TIMESTAMP 放在 TIMESTAMP 类型的字段后
面，在数据发生更新是该字段将自动更新时间

使用一般创建时间和更新时间 字段设置如下：

    `create_time` TIMESTAMP DEFAULT CURRENT_TIMESTAMP COMMENT '创建时间',
    `update_time` TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '更新时间',

一个简单例子：

mysql> CREATE TABLE time(
    -> id INT(22) PRIMARY KEY AUTO_INCREMENT,
    -> name VARCHAR(100),
    -> time1 TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
    -> time2 TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    -> time3 TIMESTAMP DEFAULT '0000-00-00 00:00:00' ON UPDATE CURRENT_TIMESTAMP,
    -> time4 TIMESTAMP DEFAULT '0000-00-00 00:00:00'
    -> ) ENGINE=INNODB DEFAULT CHARSET=utf8;
Query OK, 0 rows affected (0.01 sec)

mysql> INSERT INTO time(name) VALUES('a'),('b'),('c');
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM time;
+----+------+---------------------+---------------------+---------------------+---------------------+
| id | name | time1               | time2               | time3               | time4               |
+----+------+---------------------+---------------------+---------------------+---------------------+
|  1 | a    | 2018-03-08 02:10:44 | 2018-03-08 02:10:44 | 0000-00-00 00:00:00 | 0000-00-00 00:00:00 |
|  2 | b    | 2018-03-08 02:10:44 | 2018-03-08 02:10:44 | 0000-00-00 00:00:00 | 0000-00-00 00:00:00 |
|  3 | c    | 2018-03-08 02:10:44 | 2018-03-08 02:10:44 | 0000-00-00 00:00:00 | 0000-00-00 00:00:00 |
+----+------+---------------------+---------------------+---------------------+---------------------+
3 rows in set (0.00 sec)

mysql> UPDATE time SET name='a1' WHERE id=1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> SELECT * FROM time;
+----+------+---------------------+---------------------+---------------------+---------------------+
| id | name | time1               | time2               | time3               | time4               |
+----+------+---------------------+---------------------+---------------------+---------------------+
|  1 | a1   | 2018-03-08 02:11:03 | 2018-03-08 02:10:44 | 2018-03-08 02:11:03 | 0000-00-00 00:00:00 |
|  2 | b    | 2018-03-08 02:10:44 | 2018-03-08 02:10:44 | 0000-00-00 00:00:00 | 0000-00-00 00:00:00 |
|  3 | c    | 2018-03-08 02:10:44 | 2018-03-08 02:10:44 | 0000-00-00 00:00:00 | 0000-00-00 00:00:00 |
+----+------+---------------------+---------------------+---------------------+---------------------+
3 rows in set (0.00 sec)