Using ALTER to drop a column if it exists in MySQL

How can ALTER be used to drop a column in a MySQL table if that column exists?

I know I can use ALTER TABLE my_table DROP COLUMN my_column, but that will throw an error if my_column does not exist. Is there alternative syntax for dropping the column conditionally?

I'm using MySQL version 4.0.18.

Solution 1:

For MySQL, there is none: MySQL Feature Request.

Allowing this is arguably a really bad idea, anyway: IF EXISTS indicates that you're running destructive operations on a database with (to you) unknown structure. There may be situations where this is acceptable for quick-and-dirty local work, but if you're tempted to run such a statement against production data (in a migration etc.), you're playing with fire.

But if you insist, it's not difficult to simply check for existence first in the client, or to catch the error.

MariaDB also supports the following starting with 10.0.2:

DROP [COLUMN] [IF EXISTS] col_name 

i. e.

ALTER TABLE my_table DROP IF EXISTS my_column;

But it's arguably a bad idea to rely on a non-standard feature supported by only one of several forks of MySQL.

Solution 2:

There is no language level support for this in MySQL. Here is a work-around involving MySQL information_schema meta-data in 5.0+, but it won't address your issue in 4.0.18.

drop procedure if exists schema_change;

delimiter ';;'
create procedure schema_change() begin

    /* delete columns if they exist */
    if exists (select * from information_schema.columns where table_schema = schema() and table_name = 'table1' and column_name = 'column1') then
        alter table table1 drop column `column1`;
    end if;
    if exists (select * from information_schema.columns where table_schema = schema() and table_name = 'table1' and column_name = 'column2') then
        alter table table1 drop column `column2`;
    end if;

    /* add columns */
    alter table table1 add column `column1` varchar(255) NULL;
    alter table table1 add column `column2` varchar(255) NULL;

end;;

delimiter ';'
call schema_change();

drop procedure if exists schema_change;

I wrote some more detailed information in a blog post.

Solution 3:

I know this is an old thread, but there is a simple way to handle this requirement without using stored procedures. This may help someone.

set @exist_Check := (
    select count(*) from information_schema.columns 
    where TABLE_NAME='YOUR_TABLE' 
    and COLUMN_NAME='YOUR_COLUMN' 
    and TABLE_SCHEMA=database()
) ;
set @sqlstmt := if(@exist_Check>0,'alter table YOUR_TABLE drop column YOUR_COLUMN', 'select ''''') ;
prepare stmt from @sqlstmt ;
execute stmt ;

Hope this helps someone, as it did me (after a lot of trial and error).