MySQL返回更新值（RETURNING）
在寫SQL中，經常會有諸如更新了一行記錄，之後要獲取更新過的這一行。 本身從程序來說，沒啥難度，大不了把這行緩存起來，完了直接訪問。 但是從數據庫的角度出發，怎麼能快速的拿出來，而又不對原表進行二次掃描？ 比如其他數據庫提供了如下的語法來實現：
t_girl=# update t1 set log_time = now() where id in (1,2,3) returning *;
id | log_time
----+----------------------------
1 | 2014-11-26 11:06:53.555217
2 | 2014-11-26 11:06:53.555217
3 | 2014-11-26 11:06:53.555217
(3 rows)
UPDATE 3
Time: 6.991 ms
返回刪除掉的行：
t_girl=# delete from t1 where id < 2 returning *;
id | log_time
----+----------------------------
1 | 2014-11-26 11:06:53.555217
(1 row)
DELETE 1
Time: 6.042 ms
返回插入後的行：
t_girl=# insert into t1 select 1,now() returning *;
id | log_time
----+----------------------------
1 | 2014-11-26 11:07:40.431766
(1 row)
INSERT 0 1
Time: 6.107 ms
t_girl=#
那在MySQL裡如何實現呢？
我可以創建幾張內存表來來保存這些返回值，如下：
CREATE TABLE t1_insert ENGINE MEMORY SELECT * FROM t1 WHERE FALSE;
CREATE TABLE t1_update ENGINE MEMORY SELECT * FROM t1 WHERE FALSE;
CREATE TABLE t1_delete ENGINE MEMORY SELECT * FROM t1 WHERE FALSE;
ALTER TABLE t1_insert ADD PRIMARY KEY (id);
ALTER TABLE t1_update ADD PRIMARY KEY (id);
ALTER TABLE t1_delete ADD PRIMARY KEY (id);
以上建立了三張表來存放對應的操作。 t1_insert 保存插入；t1_update 保存更新；t1_delete 保存刪除。
那這樣的話，我來創建對應的觸發器完成。
DELIMITER $$
USE `t_girl`$$
DROP TRIGGER /*!50032 IF EXISTS */ `tr_t1_insert_after`$$
CREATE
/*!50017 DEFINER = 'root'@'localhost' */
TRIGGER `tr_t1_insert_after` AFTER INSERT ON `t1`
FOR EACH ROW BEGIN
REPLACE INTO t1_insert VALUES (new.id,new.log_time);
END;
$$
DELIMITER ;
DELIMITER $$
USE `t_girl`$$
DROP TRIGGER /*!50032 IF EXISTS */ `tr_t1_update_after`$$
CREATE
/*!50017 DEFINER = 'root'@'localhost' */
TRIGGER `tr_t1_update_after` AFTER UPDATE ON `t1`
FOR EACH ROW BEGIN
REPLACE INTO t1_update VALUES (new.id,new.log_time);
END;
$$
DELIMITER ;
DELIMITER $$
USE `t_girl`$$
DROP TRIGGER /*!50032 IF EXISTS */ `tr_t1_delete_after`$$
CREATE
/*!50017 DEFINER = 'root'@'localhost' */
TRIGGER `tr_t1_delete_after` AFTER DELETE ON `t1`
FOR EACH ROW BEGIN
REPLACE INTO t1_delete VALUES (old.id,old.log_time);；
END;
$$
DELIMITER ;
創建好了以上的表和觸發器後， 拿到返回值就非常容易了， 我直接從以上幾張表來查詢就是。
我現在來演示：
更新：
mysql> truncate table t1_update;
Query OK, 0 rows affected (0.00 sec)
mysql> UPDATE t1 SET log_time = NOW() WHERE id < 15;
Query OK, 3 rows affected (0.01 sec)
Rows matched: 3 Changed: 3 Warnings: 0
獲取更新記錄：
mysql> select * from t1_update;
+----+----------------------------+
| id | log_time |
+----+----------------------------+
| 12 | 2014-11-26 13:38:06.000000 |
| 13 | 2014-11-26 13:38:06.000000 |
| 14 | 2014-11-26 13:38:06.000000 |
+----+----------------------------+
3 rows in set (0.00 sec)
插入：
mysql> truncate table t1_insert;
Query OK, 0 rows affected (0.00 sec)
mysql> INSERT INTO t1 VALUES (1,NOW());
Query OK, 1 row affected (0.08 sec)
獲取插入記錄：
mysql> select * from t1_insert;
+----+----------------------------+
| id | log_time |
+----+----------------------------+
| 1 | 2014-11-26 13:38:06.000000 |
+----+----------------------------+
1 row in set (0.00 sec)
刪除：
mysql> truncate table t1_delete;
Query OK, 0 rows affected (0.00 sec)
mysql> DELETE FROM t1 WHERE id < 15;
Query OK, 4 rows affected (0.01 sec)
獲取刪除記錄：
mysql> select * from t1_delete;
+----+----------------------------+
| id | log_time |
+----+----------------------------+
| 1 | 2014-11-26 13:38:06.000000 |
| 12 | 2014-11-26 13:38:06.000000 |
| 13 | 2014-11-26 13:38:06.000000 |
| 14 | 2014-11-26 13:38:06.000000 |
+----+----------------------------+
4 rows in set (0.00 sec)
返回刪除掉的行：
t_girl=# delete from t1 where id < 2 returning *;
id | log_time
----+----------------------------
1 | 2014-11-26 11:06:53.555217
(1 row)
DELETE 1
Time: 6.042 ms
mysql> select * from t1_delete;
+----+----------------------------+
| id | log_time |
+----+----------------------------+
| 1 | 2014-11-26 13:38:06.000000 |
| 12 | 2014-11-26 13:38:06.000000 |
| 13 | 2014-11-26 13:38:06.000000 |
| 14 | 2014-11-26 13:38:06.000000 |
+----+----------------------------+
4 rows in set (0.00 sec)
