一 創建自定義函數
在使用mysql的過程中，mysql自帶的函數可能不能完成我們的業務需求，這時就需要自定義函數,例如筆者在開發過程中遇到下面這個問題：
mysql表結構如下

復制代碼 代碼如下:

DROP TABLE IF EXISTS `test`;
CREATE TABLE `test` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`pic` varchar(50) NOT NULL,
`hashcode` varchar(16) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of test
-- ----------------------------
INSERT INTO `test` VALUES ('1', '2012120910403250c3fa209bf48.jpg', 'bf8f83818080c0f1');
INSERT INTO `test` VALUES ('2', '2012120620430750c092db26557.JPG', 'ff9880f0f680ceff');
INSERT INTO `test` VALUES ('3', '2012120619582550c08861eb062.jpg', '7f7f004f7f7f7c7f');
INSERT INTO `test` VALUES ('4', '2012112911072650b6d16e7f21f.jpg', '7f7f004f7f7f007f');

建立自定義函數的過程如下：
1.進入mysql命令行
mysql>
2.用delimiter命令來把語句定界符從 ;變為//。這樣就允許在程序體用;定界符傳遞到服務器，而不是被mysql自己來解釋。
mysql> delimiter //
3.創建自定義函數

復制代碼 代碼如下:

mysql>CREATE FUNCTION hashDiff( s1 varchar(16), s2 varchar(16))
->RETURNS INT
->BEGIN
->DECLARE diff, x INT;
->SET diff =0;
->SET x = 0;
->WHILE (x < 16 ) DO
->SET x = x+1;
->if SUBSTRING(s1, x,1)<>SUBSTRING(s2, x,1) then
->set diff=diff+ 1;
->end if;
->END WHILE;
->RETURN diff;
->END
->//
mysql>select * from test t where hashDiff(t.hashcode,'ff9880f0f680ceff') < 5;

復制代碼 代碼如下:

mysql> delimiter //
mysql> CREATE PROCEDURE simpleproc (OUT param1 INT)
-> BEGIN
-> SELECT COUNT(*) INTO param1 FROM t;
-> END
-> //
Query OK, 0 rows affected (0.00 sec)

mysql> delimiter ;
mysql> CALL simpleproc(@a);
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @a;
+------+
| @a |
+------+
| 3 |
+------+
1 row in set (0.00 sec)