以下的文章主要是向大家描述的是MySQL数据库中delimiter的作用是什么?我们一般都认为这个命令和存储过程关系不大,到底是不是这样的呢?以下的文章将会给你相关的知识,望你会有所收获。
其实就是告诉MySQL解释器,该段命令是否已经结束了,MySQL数据库是否可以执行了。默认情况下,delimiter是分号;。在命令行客户端中,如果有一行命令以分号结束,那么回车后,MySQL将会执行该命令。如输入下面的语句
- MySQL> select * from test_table;
然后回车,那么MySQL将立即执行该语句。
但有时候,不希望MySQL这么做。在为可能输入较多的语句,且语句中包含有分号。如试图在命令行客户端中输入如下语句
- MySQL> CREATE FUNCTION `SHORTEN`(S VARCHAR(255), N INT) 
- MySQL> RETURNS varchar(255) 
- MySQL> BEGIN 
- MySQL> IF ISNULL(S) THEN 
- MySQL> RETURN ''; 
- MySQL> ELSEIF N<15 THEN&#160;
- MySQL> RETURN LEFT(S, N);&#160;
- MySQL> ELSE&#160;
- MySQL> IF CHAR_LENGTH(S) <=N THEN&#160;
- MySQL> RETURN S;&#160;
- MySQL> ELSE&#160;
- MySQL> RETURN CONCAT(LEFT(S, N-10), '...', RIGHT(S, 5));&#160;
- MySQL> END IF;&#160;
- MySQL> END IF;&#160;
- MySQL> END;
默认情况下,不可能等到用户把这些语句全部输入完之后,再执行整段语句。因为MySQL一遇到分号,它就要自动执行。即,在语句RETURN '';时,MySQL数据库解释器就要执行了。这种情况下,就需要事先把delimiter换成其它符号,如//或$$。
- MySQL> delimiter //&#160;
- MySQL> CREATE FUNCTION `SHORTEN`(S VARCHAR(255), N INT)&#160;
- MySQL> RETURNS varchar(255)&#160;
- MySQL> BEGIN&#160;
- MySQL> IF ISNULL(S) THEN&#160;
- MySQL> RETURN '';&#160;
- MySQL> ELSEIF N<15 THEN&#160;
- MySQL> RETURN LEFT(S, N);&#160;
- MySQL> ELSE&#160;
- MySQL> IF CHAR_LENGTH(S) <=N THEN&#160;
- MySQL> RETURN S;&#160;
- MySQL> ELSE&#160;
- MySQL> RETURN CONCAT(LEFT(S, N-10), '...', RIGHT(S, 5));&#160;
- MySQL> END IF;&#160;
- MySQL> END IF;&#160;
- MySQL> END;//&#160;
这样只有当//出现之后,MySQL解释器才会执行这段语句
例子:
- MySQL> delimiter //&#160;&#160;
- MySQL> CREATE PROCEDURE simpleproc (OUT param1 INT)&#160;&#160;
- -> BEGIN&#160;&#160;
- -> SELECT COUNT(*) INTO param1 FROM t;&#160;&#160;
- -> END;&#160;&#160;
- -> //&#160;&#160;
- Query OK, 0 rows affected (0.00 sec)&#160;&#160;
- MySQL> delimiter ;&#160;&#160;
- MySQL> CALL simpleproc(@a);&#160;&#160;
- Query OK, 0 rows affected (0.00 sec)&#160;&#160;
- MySQL> SELECT @a;&#160;&#160;
- +------+&#160;&#160;
- | @a |&#160;&#160;
- +------+&#160;&#160;
- | 3 |&#160;&#160;
- +------+&#160;&#160;
- 1 row in set (0.00 sec)&#160;&#160;
本文代码在 MySQL 5.0.41-community-nt 下运行通过。
编写了个统计网站访问情况(user agent)的 MySQL 数据库存储过程。就是下面的这段 SQL 代码。
- drop procedure if exists pr_stat_agent;&#160;
- -- call pr_stat_agent ('2008-07-17', '2008-07-18')&#160;
- create procedure pr_stat_agent&#160;
- (&#160;
- pi_date_from date&#160;
- ,pi_date_to date&#160;
- )&#160;
- begin&#160;
- -- check input&#160;
- if (pi_date_from is null) then&#160;
- set pi_date_from = current_date();&#160;
- end if;&#160;
- if (pi_date_to is null) then&#160;
- set pi_date_to = pi_date_from;&#160;
- end if;&#160;
- set pi_date_to = date_add(pi_date_from, interval 1 day);&#160;
- -- stat&#160;
- select agent, count(*) as cnt&#160;
- from apache_log&#160;
- where request_time >= pi_date_from&#160;
- and request_time group by agent&#160;
- order by cnt desc;&#160;
- end;&#160;
我在 EMS SQL Manager 2005 for MySQL 这个 MySQL 图形客户端下可以顺利运行。但是在 SQLyog MySQL GUI v5.02 这个客户端就会出错。最后找到原因是没有设置好 delimiter 的问题。
默认情况下,delimiter “;” 用于向 MySQL 提交查询语句。在存储过程中每个 SQL 语句的结尾都有个 “;”,如果这时候,每逢 “;” 就向 MySQL 提交的话,当然会出问题了。于是更改 MySQL 的 delimiter,上面 MySQL 存储过程就编程这样子了:
delimiter //; -- 改变 MySQL delimiter 为:“//”
- drop procedure if exists pr_stat_agent //&#160;
- -- call pr_stat_agent ('2008-07-17', '2008-07-18')&#160;
- create procedure pr_stat_agent&#160;
- (&#160;
- pi_date_from date&#160;
- ,pi_date_to date&#160;
- )&#160;
- begin&#160;
- -- check input&#160;
- if (pi_date_from is null) then&#160;
- set pi_date_from = current_date();&#160;
- end if;&#160;
- if (pi_date_to is null) then&#160;
- set pi_date_to = pi_date_from;&#160;
- end if;&#160;
- set pi_date_to = date_add(pi_date_from, interval 1 day);&#160;
- -- stat&#160;
- select agent, count(*) as cnt&#160;
- from apache_log&#160;
- where request_time >= pi_date_from&#160;
- and request_time group by agent&#160;
- order by cnt desc;&#160;
- end; //&#160;
- delimiter ;&#160;
改回默认的 MySQL delimiter:“;”
当然,MySQL delimiter 符号是可以自由设定的,你可以用 “/” 或者“$$” 等。但是 MySQL数据库 存储过程中比较常见的用法是 “//” 和 “$$”。上面的这段在 SQLyog 中的代码搬到 MySQL 命令客户端(MySQL Command Line Client)却不能执行。
MySQL> delimiter //; -- 改变 MySQL delimiter 为:“//”
- MySQL>
- MySQL> drop procedure if exists pr_stat_agent //&#160;
- ->
- -> -- call pr_stat_agent ('2008-07-17', '2008-07-18')&#160;
- ->
- -> create procedure pr_stat_agent&#160;
- -> (&#160;
- -> pi_date_from date&#160;
- -> ,pi_date_to date&#160;
- -> )&#160;
- -> begin&#160;
- -> -- check input&#160;
- -> if (pi_date_from is null) then&#160;
- -> set pi_date_from = current_date();&#160;
- -> end if;&#160;
- ->
- -> if (pi_date_to is null) then&#160;
- -> set pi_date_to = pi_date_from;&#160;
- -> end if;&#160;
- ->
- -> set pi_date_to = date_add(pi_date_from, interval 1 day);&#160;
- ->
- -> -- stat&#160;
- -> select agent, count(*) as cnt&#160;
- -> from apache_log&#160;
- -> where request_time >= pi_date_from&#160;
- -> and request_time -> group by agent&#160;
- -> order by cnt desc;&#160;
- -> end; //&#160;
- ->
- -> delimiter ;
改回默认的 MySQL delimiter:“;”
- -> //&#160;
- -> //&#160;
- -> //&#160;
- -> ;&#160;
- -> ;&#160;
- ->
真是奇怪了!最后终于发现问题了,在 MySQL 命令行下运行 “delimiter //; ” 则 MySQL 的 delimiter 实际上是 “//;”,而不是我们所预想的 “//”。其实只要运行指令 “delimiter //” 就 OK 了。
MySQL> delimiter // -- 末尾不要符号 “;”
- MySQL>
- MySQL> drop procedure if exists pr_stat_agent //&#160;
- Query OK, 0 rows affected (0.00 sec)&#160;
- MySQL>
- MySQL> -- call pr_stat_agent ('2008-07-17', '2008-07-18')&#160;
- MySQL>
- MySQL> create procedure pr_stat_agent&#160;
- -> (&#160;
- -> pi_date_from date&#160;
- -> ,pi_date_to date&#160;
- -> )&#160;
- -> begin&#160;
- -> -- check input&#160;
- -> if (pi_date_from is null) then&#160;
- -> set pi_date_from = current_date();&#160;
- -> end if;&#160;
- ->
- -> if (pi_date_to is null) then&#160;
- -> set pi_date_to = pi_date_from;&#160;
- -> end if;&#160;
- ->
- -> set pi_date_to = date_add(pi_date_from, interval 1 day);&#160;
- ->
- -> -- stat&#160;
- -> select agent, count(*) as cnt&#160;
- -> from apache_log&#160;
- -> where request_time >= pi_date_from&#160;
- -> and request_time -> group by agent&#160;
- -> order by cnt desc;&#160;
- -> end; //&#160;
- Query OK, 0 rows affected (0.00 sec)&#160;
- MySQL>
- MySQL> delimiter ;&#160;&#160;
末尾不要符号 “//”
- MySQL>
顺带一提的是,我们可以在 MySQL 数据库中执行在文件中的 SQL 代码。例如,我把上面存储过程的代码放在文件 d:/pr_stat_agent.sql 中。可以运行下面的代码建立存储过程。
- MySQL> source d:/pr_stat_agent.sql&#160;
- Query OK, 0 rows affected (0.00 sec)&#160;
- Query OK, 0 rows affected (0.00 sec)&#160;
source 指令的缩写形式是:“/.”
- MySQL> /. d:/pr_stat_agent.sql&#160;
- Query OK, 0 rows affected (0.00 sec)&#160;
- Query OK, 0 rows affected (0.00 sec)&#160;
最后,可见 MySQL数据库的客户端工具在有些地方是各自为政,各有各的一套。