MySQL 中的分隔符

2021-11-20 00:00:00 delimiter mysql

我经常看到人们使用分隔符.我试图自己找出什么是分隔符以及它们的目的是什么.经过 20 分钟的谷歌搜索，我找不到让我满意的答案.所以，我现在的问题是:什么是分隔符，我应该在什么时候使用它们?

I often see people are using Delimiters. I tried myself to find out what are delimiters and what is their purpose. After 20 minutes of googling, I was not able to find an answer which satisfies me. So, my question is now: What are delimiters and when should I use them?

推荐答案

除默认 ; 之外的定界符通常用于定义函数、存储过程和触发器，其中必须定义多个语句.您定义了一个不同的分隔符，如 $$，它用于定义整个过程的结束，但在其中，各个语句都以 ; 终止.这样，当代码在mysql客户端中运行时，客户端可以知道整个过程在哪里结束并作为一个单元执行，而不是执行内部的单个语句.

Delimiters other than the default ; are typically used when defining functions, stored procedures, and triggers wherein you must define multiple statements. You define a different delimiter like $$ which is used to define the end of the entire procedure, but inside it, individual statements are each terminated by ;. That way, when the code is run in the mysql client, the client can tell where the entire procedure ends and execute it as a unit rather than executing the individual statements inside.

请注意，DELIMITER 关键字只是命令行 mysql 客户端(和一些其他客户端)的函数，而不是常规 MySQL 语言功能.如果您尝试通过编程语言 API 将其传递给 MySQL，它将无法工作.其他一些客户端(如 PHPMyAdmin)有其他方法来指定非默认分隔符.

Note that the DELIMITER keyword is a function of the command line mysql client (and some other clients) only and not a regular MySQL language feature. It won't work if you tried to pass it through a programming language API to MySQL. Some other clients like PHPMyAdmin have other methods to specify a non-default delimiter.

DELIMITER $$
/* This is a complete statement, not part of the procedure, so use the custom delimiter $$ */
DROP PROCEDURE my_procedure$$

/* Now start the procedure code */
CREATE PROCEDURE my_procedure ()
BEGIN    
  /* Inside the procedure, individual statements terminate with ; */
  CREATE TABLE tablea (
     col1 INT,
     col2 INT
  );

  INSERT INTO tablea
    SELECT * FROM table1;

  CREATE TABLE tableb (
     col1 INT,
     col2 INT
  );
  INSERT INTO tableb
    SELECT * FROM table2;
  
/* whole procedure ends with the custom delimiter */
END$$

/* Finally, reset the delimiter to the default ; */
DELIMITER ;

尝试在不支持它的客户端上使用 DELIMITER 会导致它被发送到服务器，服务器会报告语法错误.例如，使用 PHP 和 MySQLi:

Attempting to use DELIMITER with a client that doesn't support it will cause it to be sent to the server, which will report a syntax error. For example, using PHP and MySQLi:

$mysqli = new mysqli('localhost', 'user', 'pass', 'test');
$result = $mysqli->query('DELIMITER $$');
echo $mysqli->error;

错误:

您的 SQL 语法有错误；检查对应的手册到您的 MySQL 服务器版本，以便在第 1 行的DELIMITER $$"附近使用正确的语法

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'DELIMITER $$' at line 1

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