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
浏览 1552
分类:数据库
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