在ALTER TABLE - mysql之前检查列是否存在

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.

有没有办法检查ALTER TABLE ADD coumn_name语句运行之前（或作为）ALSQL DB中是否存在列？ IF列的排序不存在ALTER TABLE事物。

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.

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

EDIT: 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

编辑：用例是升级已安装的Web应用程序中的表 - 为了简单起见，我想确保我需要的列存在，如果不存在，请使用ALTER TABLE添加它们

9 个解决方案

#1

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

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临时控制语句：

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;

#2

Do you think you can try this?:

你认为你可以尝试这个吗？：

SELECT IFNULL(column_name, '') INTO @colName
FROM information_schema.columns 
WHERE table_name = 'my_table'
AND column_name = 'my_column';

IF @colName = '' THEN 
    -- ALTER COMMAND GOES HERE --
END IF;

It's no one-liner, but can you at least see if it will work for you? At least while waiting for a better solution..

这不是单行，但你至少可以看看它是否适合你？至少在等待更好的解决方案时......

#3

Utility functions and procedures

First, I have a set of utility functions and procedures that I use to do things like drop foreign keys, normal keys and columns. I just leave them in the database so I can use them as needed.

首先，我有一组实用程序函数和过程，用于执行诸如删除外键，普通键和列之类的操作。我只是将它们留在数据库中，以便我可以根据需要使用它们。

Here they are.

他们来了。

delimiter $$

create function column_exists(ptable text, pcolumn text)
  returns bool
  reads sql data
begin
  declare result bool;
  select
    count(*)
  into
    result
  from
    information_schema.columns
  where
    `table_schema` = 'my_database' and
    `table_name` = ptable and
    `column_name` = pcolumn;
  return result;
end $$

create function constraint_exists(ptable text, pconstraint text)
  returns bool
  reads sql data
begin
  declare result bool;
  select
    count(*)
  into
    result
  from
    information_schema.table_constraints
  where
    `constraint_schema` = 'my_database' and
    `table_schema` = 'my_database' and
    `table_name` = ptable and
    `constraint_name` = pconstraint;
  return result;
end $$

create procedure drop_fk_if_exists(ptable text, pconstraint text)
begin
  if constraint_exists(ptable, pconstraint) then
    set @stat = concat('alter table ', ptable, ' drop foreign key ', pconstraint);
    prepare pstat from @stat;
    execute pstat;
  end if;
end $$

create procedure drop_key_if_exists(ptable text, pconstraint text)
begin
  if constraint_exists(ptable, pconstraint) then
    set @stat = concat('alter table ', ptable, ' drop key ', pconstraint);
    prepare pstat from @stat;
    execute pstat;
  end if;
end $$

create procedure drop_column_if_exists(ptable text, pcolumn text)
begin
  if column_exists(ptable, pcolumn) then
    set @stat = concat('alter table ', ptable, ' drop column ', pcolumn);
    prepare pstat from @stat;
    execute pstat;
  end if;
end $$

delimiter ;

Dropping constraints and columns using the utilities above

With those in place, it is pretty easy to use them to check columns and constraints for existence:

有了这些，就可以很容易地使用它们来检查存在的列和约束：

-- Drop service.component_id
call drop_fk_if_exists('service', 'fk_service_1');
call drop_key_if_exists('service', 'component_id');
call drop_column_if_exists('service', 'component_id');

-- Drop commit.component_id
call drop_fk_if_exists('commit', 'commit_ibfk_1');
call drop_key_if_exists('commit', 'commit_idx1');
call drop_column_if_exists('commit', 'component_id');

-- Drop component.application_id
call drop_fk_if_exists('component', 'fk_component_1');
call drop_key_if_exists('component', 'application_id');
call drop_column_if_exists('component', 'application_id');

#4

Make a count sentence with the example below by John Watson.

用John Watson的例子制作一个计数句子。

 SELECT count(*) FROM information_schema.COLUMNS
     WHERE COLUMN_NAME = '...'
     and TABLE_NAME = '...'
     and TABLE_SCHEMA = '...'

Save that result in an integer and then make it a condition to apply the ADD COLUMN sentence.

将结果保存为整数，然后使其成为应用ADD COLUMN语句的条件。

#5

Although its quite an old post but still i feel good about sharing my solution to this issue. If column doesn't exist then an exception would occur definitely and then i am creating the column in table.

虽然它是一个相当古老的帖子，但我仍然感觉很好分享我对这个问题的解决方案。如果列不存在，那么肯定会发生异常，然后我在表中创建列。

I just used the code below:

我刚刚使用下面的代码：

 try
   {
         DATABASE_QUERY="SELECT gender from USER;";
         db.rawQuery(DATABASE_QUERY, null);
   }
   catch (Exception e)
   {
    e.printStackTrace();

        DATABASE_UPGRADE="alter table USER ADD COLUMN gender VARCHAR(10) DEFAULT 0;";
                db.execSQL(DATABASE_UPGRADE);
   } 

#6

You can create a procedure with a CONTINUE handler in case the column exists (please note this code doesn't work in PHPMyAdmin):

如果列存在，您可以使用CONTINUE处理程序创建过程（请注意此代码在PHPMyAdmin中不起作用）：

DROP PROCEDURE IF EXISTS foo;
CREATE PROCEDURE foo() BEGIN
    DECLARE CONTINUE HANDLER FOR 1060 BEGIN END;
    ALTER TABLE `tableName` ADD `columnName` int(10) NULL AFTER `otherColumn`;
END;
CALL foo();
DROP PROCEDURE foo;

This code should not raise any error in case the column already exists. It will just do nothing and carry on executing the rest of the SQL.

如果列已存在，此代码不应引发任何错误。它将什么都不做，继续执行SQL的其余部分。

#7

DELIMITER $$

DROP PROCEDURE IF EXISTS `addcol` $$
CREATE DEFINER=`admin`@`localhost` PROCEDURE `addcol`(tbn varchar(45), cn varchar(45), ct varchar(45))
BEGIN
#tbn: table name, cn: column name, ct: column type
DECLARE CONTINUE HANDLER FOR 1060 BEGIN END;
set cn = REPLACE(cn, ' ','_');
set @a = '';
set @a = CONCAT("ALTER TABLE `", tbn ,"` ADD column `", cn ,"` ", ct);
PREPARE stmt FROM @a;
EXECUTE stmt;

END $$

DELIMITER ;

#8

You can test if a column exists with:

您可以测试列是否存在：

IF EXISTS (
     SELECT * FROM information_schema.COLUMNS
     WHERE COLUMN_NAME = '...'
     and TABLE_NAME = '...'
     and TABLE_SCHEMA = '...')

...

Just fill in your column name, table name, and database name.

只需填写您的列名，表名和数据库名称即可。

#9

As per MYSQL Community:

根据MYSQL社区：

IGNORE is a MySQL extension to standard SQL. It controls how ALTER TABLE works if there are duplicates on unique keys in the new table or if warnings occur when strict mode is enabled. If IGNORE is not specified, the copy is aborted and rolled back if duplicate-key errors occur. If IGNORE is specified, only one row is used of rows with duplicates on a unique key. The other conflicting rows are deleted. Incorrect values are truncated to the closest matching acceptable value.

IGNORE是标准SQL的MySQL扩展。如果新表中的唯一键上存在重复项，或者启用了严格模式时出现警告，它将控制ALTER TABLE的工作方式。如果未指定IGNORE，则复制将中止并在发生重复键错误时回滚。如果指定了IGNORE，则只使用一行在唯一键上具有重复项的行。其他冲突的行将被删除。不正确的值将被截断为最接近的匹配可接受值。

So a working Code is: ALTER IGNORE TABLE CLIENTS ADD CLIENT_NOTES TEXT DEFAULT NULL;

所以一个正常工作的代码是：ALTER IGNORE TABLE CLIENTS ADD CLIENT_NOTES TEXT DEFAULT NULL;

Data posted here: http://dev.mysql.com/doc/refman/5.1/en/alter-table.html

此处发布的数据：http：//dev.mysql.com/doc/refman/5.1/en/alter-table.html