Question

The ‘thread’ column in an old Drupal 4.7 Comments table is varchar(255), and I need to convert it to bigint(20) to make it work for the ‘comment_parent’ column in a WordPress wp_comments table.

I’ve tried various Cast and Convert commands I’ve seen and always get syntax errors.

Answer 1

This works in SQL Server:

create table Comments 
(
    [thread] nvarchar(255)
) 

insert comments 
select '1' 
union select '2' 
union select '3' 
union select '4' 
union select '5' 
union select 'x' 

select 
    case 
        when ISNUMERIC([thread]) > 0 
            then CAST([thread] as bigint) 
        else 
            null 
    end colAsBigInt 
    , [thread] colAsNvarChar 
from comments

http://sqlfiddle.com/#!6/337eb/1

For MySQL try:

create table if not exists Comments 
(
    thread varchar(255) character set UTF8 not null
);

insert comments(thread) values ('1');
insert comments(thread) values ('2');
insert comments(thread) values ('3');
insert comments(thread) values ('4');
insert comments(thread) values ('5');
insert comments(thread) values ('6.1');
insert comments(thread) values ('x');

select 
    case 
        when thread  REGEXP ('^(-|\\+)?([0-9]+\\.[0-9]*|[0-9]*\\.[0-9]+|[0-9]+)$') 
            then cast(thread as signed)
        else 
            null 
    end colAsBigInt 
    , thread colAsVarChar 
from comments

--regex trick from here: http://forums.mysql.com/read.php?60,1907,241284#msg-241284
--without the regex you'll get 0s instead of nulls for invalid values
--MySQL's cast only works on certain data types, given here http://www.roseindia.net/sql/mysql-example/mysql-cast.shtml

Runnable MySQL sample here: http://sqlfiddle.com/#!2/6d848/9