Using ALTER to drop a column if it exists in MySQL

Question

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 wi

Answer 1

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.