MySQL中同時存在建立和上次更新時間戳記欄位解決方案淺析,mysql欄位
在寫這篇文章之前,明確我的MySQL版本。
mysql> SELECT VERSION();+------------+| VERSION() |+------------+| 5.5.29-log |+------------+1 row in set (0.00 sec)
現在有這樣的需求,一張表中有一個欄位created_at記錄建立該條記錄的時間戳記,另一個欄位updated_at記錄更新該條記錄的時間戳記。
我們嘗試以下幾個語句。
第一個,測試通過。
CREATE TABLE temp(id INT(11) PRIMARY KEY AUTO_INCREMENT,name VARCHAR(10),updated_at timestamp NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP);
第二個,測試不通過。報ERROR 1293 (HY000)錯誤。(完整錯誤資訊:ERROR 1293 (HY000): Incorrect table definition; there can be only one TIMESTAMP column with CURRENT_TIMESTAMP in DEFAULT or ON UPDATE clause)
CREATE TABLE temp(id INT(11) PRIMARY KEY AUTO_INCREMENT,name VARCHAR(10),created_at timestamp NULL DEFAULT CURRENT_TIMESTAMP,updated_at timestamp NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP);
MySQL 5.5.29中有這樣的奇葩限制,不明白為什麼。既然有這樣的限制,那麼只有繞道而行,現在嘗試給出如下幾種解決辦法。
第一種,created_at使用DEFAULT CURRENT_TIMESTAMP或者DEFAULT now(),updated_at使用觸發器。
具體解決方案如下:
1.temp表結構如下:
CREATE TABLE temp(id INT(11) PRIMARY KEY AUTO_INCREMENT,name VARCHAR(10),created_at timestamp NULL DEFAULT CURRENT_TIMESTAMP,updated_at timestamp NULL);
2.插入測試資料:
mysql> INSERT INTO temp(name,created_at,updated_at) VALUES('robin',now(),now());Query OK, 1 row affected (0.03 sec)mysql> INSERT INTO temp(name,created_at,updated_at) VALUES('wentasy',now(),now());Query OK, 1 row affected (0.01 sec)mysql> SELECT * FROM temp;+----+---------+---------------------+---------------------+| id | name | created_at | updated_at |+----+---------+---------------------+---------------------+| 1 | robin | 2014-09-01 14:00:39 | 2014-09-01 14:00:39 || 2 | wentasy | 2014-09-01 14:01:11 | 2014-09-01 14:01:11 |+----+---------+---------------------+---------------------+2 rows in set (0.00 sec)
3.在temp上建立觸發器,實現更新時記錄更新時間;
delimiter |DROP TRIGGER IF EXISTS tri_temp_updated_at;CREATE TRIGGER tri_temp_updated_at BEFORE UPDATE ON tempFOR EACH ROWBEGINSET NEW.updated_at = now();END;|delimiter ;
4.測試。
mysql> UPDATE temp SET name='robinwen' WHERE id=1;Query OK, 1 row affected (0.01 sec)Rows matched: 1 Changed: 1 Warnings: 0#可以看到已經記錄了第一條資料的更新時間mysql> SELECT * FROM temp;+----+----------+---------------------+---------------------+| id | name | created_at | updated_at |+----+----------+---------------------+---------------------+| 1 | robinwen | 2014-09-01 14:00:39 | 2014-09-01 14:03:05 || 2 | wentasy | 2014-09-01 14:01:11 | 2014-09-01 14:01:11 |+----+----------+---------------------+---------------------+2 rows in set (0.00 sec)
第二種,created_at使用觸發器,updated_at使用DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP或者DEFAULT now() ON UPDATE now();
具體解決方案如下:
1.temp表結構如下:
CREATE TABLE temp(id INT(11) PRIMARY KEY AUTO_INCREMENT,name VARCHAR(10),created_at timestamp NULL,updated_at timestamp NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP);
2.在temp上建立觸發器,實現插入資料記錄建立時間;
delimiter |DROP TRIGGER IF EXISTS tri_temp_created_at;CREATE TRIGGER tri_temp_created_at BEFORE INSERT ON tempFOR EACH ROWBEGINIF new.created_at IS NULLTHENSET new.created_at=now();END IF;END;|delimiter ;
3.插入測試資料:
mysql> INSERT INTO temp(name,created_at,updated_at) VALUES('robin',now(),now());Query OK, 1 row affected (0.01 sec)mysql> INSERT INTO temp(name,created_at,updated_at) VALUES('wentasy',now(),now());Query OK, 1 row affected (0.01 sec)mysql> SELECT * FROM temp;+----+---------+---------------------+---------------------+| id | name | created_at | updated_at |+----+---------+---------------------+---------------------+| 1 | robin | 2014-09-01 14:08:36 | 2014-09-01 14:08:36 || 2 | wentasy | 2014-09-01 14:08:44 | 2014-09-01 14:08:44 |+----+---------+---------------------+---------------------+2 rows in set (0.00 sec)
4.測試。
mysql> UPDATE temp SET name='robinwen' WHERE id=1;Query OK, 1 row affected (0.01 sec)Rows matched: 1 Changed: 1 Warnings: 0#可以看到已經記錄了第一條資料的更新時間mysql> SELECT * FROM temp;+----+----------+---------------------+---------------------+| id | name | created_at | updated_at |+----+----------+---------------------+---------------------+| 1 | robinwen | 2014-09-01 14:08:36 | 2014-09-01 14:09:09 || 2 | wentasy | 2014-09-01 14:08:44 | 2014-09-01 14:08:44 |+----+----------+---------------------+---------------------+2 rows in set (0.00 sec)
第三種,created_at指定timestamp DEFAULT '0000-00-00 00:00:00',updated_at指定DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP或者timestamp DEFAULT now() ON UPDATE now();
具體解決方案如下:
1.temp表結構如下:
CREATE TABLE temp(id INT(11) PRIMARY KEY AUTO_INCREMENT,name VARCHAR(10),created_at timestamp NULL DEFAULT '0000-00-00 00:00:00',updated_at timestamp NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP);
2.插入測試資料:
mysql> INSERT INTO temp(name,created_at,updated_at) VALUES('robin',now(),now());Query OK, 1 row affected (0.01 sec)mysql> INSERT INTO temp(name,created_at,updated_at) VALUES('wentasy',now(),now());Query OK, 1 row affected (0.01 sec)mysql> SELECT * FROM temp;+----+---------+---------------------+---------------------+| id | name | created_at | updated_at |+----+---------+---------------------+---------------------+| 1 | robin | 2014-09-01 14:10:43 | 2014-09-01 14:10:43 || 2 | wentasy | 2014-09-01 14:10:57 | 2014-09-01 14:10:57 |+----+---------+---------------------+---------------------+2 rows in set (0.00 sec)
3.測試。
mysql> UPDATE temp SET name='robinwen' WHERE id=1;Query OK, 1 row affected (0.01 sec)Rows matched: 1 Changed: 1 Warnings: 0#可以看到已經記錄了第一條資料的更新時間mysql> SELECT * FROM temp;+----+----------+---------------------+---------------------+| id | name | created_at | updated_at |+----+----------+---------------------+---------------------+| 1 | robinwen | 2014-09-01 14:10:43 | 2014-09-01 14:11:24 || 2 | wentasy | 2014-09-01 14:10:57 | 2014-09-01 14:10:57 |+----+----------+---------------------+---------------------+2 rows in set (0.00 sec)
第四種,更換MySQL版本,MySQL 5.6已經去除了此限制。
我們可以看下MySQL 5.5和5.6協助文檔對於這個問題的解釋。
From the MySQL 5.5 documentation:
One TIMESTAMP column in a table can have the current timestamp as the default value for initializing the column, as the auto-update value, or both. It is not possible to have the current timestamp be the default value for one column and the auto-update value for another column.
Changes in MySQL 5.6.5:
Previously, at most one TIMESTAMP column per table could be automatically initialized or updated to the current date and time. This restriction has been lifted. Any TIMESTAMP column definition can have any combination of DEFAULT CURRENT_TIMESTAMP and ON UPDATE CURRENT_TIMESTAMP clauses. In addition, these clauses now can be used with DATETIME column definitions. For more information, see Automatic Initialization and Updating for TIMESTAMP and DATETIME.
我們確定下MySQL的版本。
mysql> SELECT VERSION();+---------------------------------------+| VERSION() |+---------------------------------------+| 5.6.20-enterprise-commercial-advanced |+---------------------------------------+1 row in set (0.00 sec)
我們把文首測試不通過的SQL語句在MySQL 5.6下執行,可以看到沒有任何錯誤。
CREATE TABLE temp(id INT(11) PRIMARY KEY AUTO_INCREMENT,name VARCHAR(10),created_at timestamp NULL DEFAULT CURRENT_TIMESTAMP,updated_at timestamp NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP);Query OK, 0 rows affected (0.28 sec)
接著我們插入測試語句,並作測試。
mysql> INSERT INTO temp(name) VALUES('robin');Query OK, 1 row affected (0.07 sec)mysql> INSERT INTO temp(name) VALUES('wentasy');Query OK, 1 row affected (0.00 sec)mysql> SELECT * FROM temp;+----+---------+---------------------+---------------------+| id | name | created_at | updated_at |+----+---------+---------------------+---------------------+| 1 | robin | 2014-09-01 15:05:57 | 2014-09-01 15:05:57 || 2 | wentasy | 2014-09-01 15:06:02 | 2014-09-01 15:06:02 |+----+---------+---------------------+---------------------+2 rows in set (0.01 sec)mysql> UPDATE temp SET name='robinwen' WHERE id=1;Query OK, 1 row affected (0.02 sec)Rows matched: 1 Changed: 1 Warnings: 0#可以看到已經記錄了第一條資料的更新時間mysql> SELECT * FROM temp;+----+----------+---------------------+---------------------+| id | name | created_at | updated_at |+----+----------+---------------------+---------------------+| 1 | robinwen | 2014-09-01 15:05:57 | 2014-09-01 15:06:45 || 2 | wentasy | 2014-09-01 15:06:02 | 2014-09-01 15:06:02 |+----+----------+---------------------+---------------------+2 rows in set (0.00 sec)
總結
本文介紹的方法歸根結底,就兩條,一是建表語句指定預設值和更新動作,二是使用觸發器插入預設值和更新時間。面對當前無法更改的事實,只能採取折中的辦法或者犧牲更多來彌補。還有一條值得注意的是,遇到問題多想想不同的解決辦法,儘可能地列出所有可能或者可行的方案,這樣一來讓自己學到更多,二來可以鍛煉思維的廣度,三來多種方案可以彌補某種方案在特定環境下不可行的不足。
Good Luck!
Robin
2014年9月1日
mysql更新欄位問題
有一個解決方案是使用mysql 的 case when 條件判斷。假定你的整型欄位名為field_1,表名為table_x,則以下語句可以解決你的問題。
update table_x set field_1= (case when field_1 =0 then 0 else field_1-1 end) where field_x='xxxx';
即是你要的效果。注意執行的時候的where條件不要漏掉哦。
但我個人不是很贊成這種用法,一是商務邏輯封裝在sql中很不清楚,當業務變化的時候改動可能影響其他功能,二是降低了mysql的效能,如果資料量很大,影響可能會明顯。
視你的情況來使用吧。
我想在MySQL中建立一個觸發器,使用update更新同一個表中的不同欄位,幫忙
。。。
一個表你幹嘛要設定觸發器啊?
你插入或更新這條記錄的時候 不能同時 設定 或 更新 FeedBack_State 嗎
幹嘛
多此一舉呢