How will changing the mysql field type from INT to VARCHAR affect data previously stored as an INT

Question

I need to update a field in my production db from INT to VARCHAR.

The information in the current INT field is sensitive and I want to ensure that I don't alter it inadvertently or destroy it while changing the table type. For example, a user may have INT 123456 stored in this column and I want to ensure that is accurately stored once converted to VARCHAR.

What is the recommended process for accomplishing this?

Is there anything I need to worry about when using something like this?

ALTER TABLE table_sample CHANGE col_sample col_sample VARCHAR; 

Thanks in advance for any help.

Answer 1

Get your MySQL server into strict mode before you change the column type and make sure that your varchar(n) column has a large enough n to hold all of the integers when they're converted to strings. If you're not in strict mode then MySQL will silently truncate your data to fit your string size:

If strict SQL mode is not enabled and you assign a value to a CHAR or VARCHAR column that exceeds the column's maximum length, the value is truncated to fit and a warning is generated. For truncation of nonspace characters, you can cause an error to occur (rather than a warning) and suppress insertion of the value by using strict SQL mode.

But if you get into strict mode first:

mysql> set sql_mode = 'STRICT_ALL_TABLES'; mysql> alter table table_sample change col_sample col_sample varchar(6); 

You'll get a nice error message like this:

ERROR 1406 (22001): Data too long for column 'col_sample' at row ... 

if your integers don't all fit in your varchar.

And, of course, you will have a fresh verified backup of your database before you try to change the table. And by verified I mean that you have successfully restored your backup into a test database.