The 'category' field in 'mantis_bug_table' has textual value, but in such cases usually IDs are used.

There is a table 'mantis_project_category_table' with fields:
project_id, category, user_id (with values, e.g. '2', 'sql', '3')

It should be complemented with a field like 'category_id' to look like:
project_id, category_id, category, user_id (with values, e.g. '2', '1', 'sql', '3')

So 'category' field in 'mantis_bug_table' have value not 'sql' but '1'.