如果不存在，则将列添加到 mysql 表

2021-11-20 00:00:00 mysql ddl alter-table

我的研究和实验还没有得出答案，所以我希望得到一些帮助.

My research and experiments haven't yielded an answer yet, so I am hoping for some help.

我正在修改一个应用程序的安装文件，该文件在以前的版本中没有我现在要添加的列.我不想手动添加列，而是在安装文件中并且仅当表中不存在新列时才添加.

I am modifying the install file of an application which in previous versions did not have a column which I want to add now. I do not want to add the column manually, but in the installation file and only if the new column does not already exist in the table.

表格创建如下:

CREATE TABLE IF NOT EXISTS `#__comm_subscribers` (
      `subscriber_id` int(11) NOT NULL auto_increment,
      `user_id` int(11) NOT NULL default '0',
      `subscriber_name` varchar(64) NOT NULL default '',
      `subscriber_surname` varchar(64) NOT NULL default '',
      `subscriber_email` varchar(64) NOT NULL default '',
      `confirmed` tinyint(1) NOT NULL default '0',
      `subscribe_date` datetime NOT NULL default '0000-00-00 00:00:00',
      PRIMARY KEY  (`subscriber_id`),
      UNIQUE KEY `subscriber_email` (`subscriber_email`)
    ) ENGINE=MyISAM CHARACTER SET 'utf8' COLLATE 'utf8_general_ci' COMMENT='Subscribers for Comm are stored here.';

如果我在 create table 语句下方添加以下内容，那么我不确定如果该列已经存在(并且可能已填充)会发生什么:

If I add the following, below the create table statement, then I am not sure what happens if the column already exists (and perhaps is populated):

ALTER TABLE `#__comm_subscribers` ADD `subscriber_surname`;
ALTER TABLE `#__comm_subscribers` MODIFY `subscriber_surname` varchar(64) NOT NULL default '';

所以，我尝试了以下在某处找到的方法.这似乎不起作用，但我不完全确定我是否正确使用它.

So, I tried the following which I found somewhere. This does not seem to work but I am not entirely sure I used it properly.

/*delimiter '//'
CREATE PROCEDURE addcol() BEGIN
IF NOT EXISTS(
SELECT * FROM information_schema.COLUMNS
WHERE COLUMN_NAME='subscriber_surname' AND TABLE_NAME='#__comm_subscribers'
)
THEN
    ALTER TABLE `#__comm_subscribers`
    ADD COLUMN `subscriber_surname` varchar(64) NOT NULL default '';
END IF;
END;
//
delimiter ';'
CALL addcol();
DROP PROCEDURE addcol;*/

有没有人有好的方法来做到这一点?

Does anyone have a good way to do this?

推荐答案

请注意，5.0 之前的 MySQL 不支持 INFORMATION_SCHEMA.5.0之前也不支持存储过程，所以如果你需要支持MySQL 4.1，这个方案就不好说了.

Note that INFORMATION_SCHEMA isn't supported in MySQL prior to 5.0. Nor are stored procedures supported prior to 5.0, so if you need to support MySQL 4.1, this solution isn't good.

使用数据库迁移的框架使用的一种解决方案是在您的数据库中记录架构的修订号.只是一个单列单行的表格，用一个整数表示当前有效的修订版本.更新架构时，请增加数字.

One solution used by frameworks that use database migrations is to record in your database a revision number for the schema. Just a table with a single column and single row, with an integer indicating which revision is current in effect. When you update the schema, increment the number.

另一种解决方案是尝试ALTER TABLE ADD COLUMN 命令.如果列已经存在，它应该抛出一个错误.

Another solution would be to just try the ALTER TABLE ADD COLUMN command. It should throw an error if the column already exists.

ERROR 1060 (42S21): Duplicate column name 'newcolumnname'

捕获错误并在升级脚本中忽略它.

Catch the error and disregard it in your upgrade script.

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