检查列是否存在于ALTER TABLE之前-mysql [英] check if column exists before ALTER TABLE -- mysql

问题描述

是否可以在ALTER TABLE ADD coumn_name语句运行之前(或之后)检查mySQL DB中是否存在列? IF column DOES NOT EXIST ALTER TABLE之类的东西.

Is there a way to check if a column exists in a mySQL DB prior to (or as) the ALTER TABLE ADD coumn_name statement runs? Sort of an IF column DOES NOT EXIST ALTER TABLE thing.

我已经尝试过ALTER IGNORE TABLE my_table ADD my_column，但是如果我要添加的列已经存在，这仍然会引发错误.

I've tried ALTER IGNORE TABLE my_table ADD my_column but this still throws the error if the column I'm adding already exists.

用例是升级已经安装的Web应用程序中的表-为了使事情简单，我想确保需要的列存在，如果不存在，请使用ALTER TABLE添加它们

use case is to upgrade a table in an already installed web app-- so to keep things simple, I want to make sure the columns I need exist, and if they don't, add them using ALTER TABLE

推荐答案

由于mysql控制语句(例如"IF")仅在存储过程中起作用，因此可以创建并执行一个临时语句:

Since mysql control statements (e.g. "IF") only work in stored procedures, a temporary one can be created and executed:

DROP PROCEDURE IF EXISTS add_version_to_actor;

DELIMITER $$

CREATE DEFINER=CURRENT_USER PROCEDURE add_version_to_actor ( ) 
BEGIN
DECLARE colName TEXT;
SELECT column_name INTO colName
FROM information_schema.columns 
WHERE table_schema = 'connjur'
    AND table_name = 'actor'
AND column_name = 'version';

IF colName is null THEN 
    ALTER TABLE  actor ADD  version TINYINT NOT NULL DEFAULT  '1' COMMENT  'code version of actor when stored';
END IF; 
END$$

DELIMITER ;

CALL add_version_to_actor;

DROP PROCEDURE add_version_to_actor;

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