mysql中timestamp的使用

mysql> CREATE TABLE t1 (

->   id mediumint(9) NOT NULL auto_increment,

->   name char(11) default NULL,

->   rq timestamp default CURRENT_TIMESTAMP on update CURRENT_TIMESTAMP,

->   PRIMARY KEY (id)

-> ) ;

Query OK, 0 rows affected (0.04 sec)

mysql> select * from t1; +----+------+---------------------+

| id | name | rq                  |

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

| 1 | a    | 2008-03-22 10:36:30 |

| 2 | b    | 2008-03-22 10:36:33 |

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

2 rows in set (0.00 sec)

mysql> update t1 set name='f'where id=2;

Query OK, 1 row affected (0.00 sec)

Rows matched: 1 Changed: 1 Warnings: 0

mysql> select * from t1;

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

| id | name | rq                  |

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

| 1 | a    | 2008-03-22 10:36:30 |

| 2 | f    | 2008-03-22 10:37:04 |

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

2 rows in set (0.00 sec)

注意：id=2的字段的日期字段值由“2008-03-22 10:36:33”自动变为了“”2008-03-22 10:37:04!

★思考：rq字段自动变化有什么好处呢?

★解决：现在要根据两个表找出哪些记录变化了,哪些记录是增加的。

演示步骤：

★将t1表备份为t2表,注意备份表t2与t1表中的rq字段值也是完全相同的：

mysql> create table t2 as select * from t1

Query OK, 2 rows affected (0.08 sec)

Records: 2 Duplicates: 0 Warnings: 0

mysql> select * from t2;

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

| id | name | rq                  |

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

| 1 | a    | 2008-03-22 10:36:30 |

| 2 | f    | 2008-03-22 10:37:04 |

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

2 rows in set (0.00 sec)

★修改id=2的字段：

mysql> update t1 set name='g'where id=2;

Query OK, 1 row affected (0.01 sec)

Rows matched: 1 Changed: 1 Warnings: 0

★增加新记录：

mysql> insert into t1(name) values('h');

Query OK, 1 row affected (0.01 sec)

mysql> select * from t1;

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

| id | name | rq                  |

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

| 1 | a    | 2008-03-22 10:36:30 |

| 2 | g    | 2008-03-22 10:43:38 |

| 3 | h    | 2008-03-22 10:44:46 |

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

3 rows in set (0.00 sec)

★查出新增加的记录(ID不在t2表中的)：

mysql> select a.*

-> from t1 a

-> where not exists

-> (

-> select b.*

-> from t2 b

-> where a.id=b.id

-> );

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

| id | name | rq                  |

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

| 3 | h    | 2008-03-22 10:44:46 |

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

1 row in set (0.00 sec)

★说明：查询哪些记录是被修改过的(ID相同而RQ不同)

mysql> select a.*

-> from t1 a

-> where exists

-> (

-> select b.*

-> from t2 b

-> where a.id=b.id

-> and   a.rq!=b.rq

-> )

-> ;

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

| id | name | rq                  |

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

| 2 | g    | 2008-03-22 10:43:38 |

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

1 row in set (0.00 sec)

★同理,也可以删除哪些记录是被删除的(id在t2中却不在t1中的)：

mysql> delete from t1 where id=2;

Query OK, 1 row affected (0.00 sec)

mysql> select a.*

-> from t2 a

-> where not exists

-> (

-> select b.*

-> from t1 b

-> where a.id=b.id

-> );

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

| id | name | rq                  |

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

| 2 | f    | 2008-03-22 10:37:04 |

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

1 row in set (0.00 sec)

说明：

1、在数据仓库系统的设计中,经常会将系统1的定期送到系统2中中,但是系统1中的数据有增加的、删除的和修改的。其中,哪些记录修改的,一直是比较难于解决的问题。

2、在Oracle、SQLServer都有类似的字段,所以上面的方法是通用的。

来源：hi.baidu.com/it培训网

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2010-03-15 14:51

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分类:数据库

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