I have to design a table(s) with a parent and child relationship (category and sub category) Is it better to have two tables with the parent_id as a foreign key in the child table or having one table with parent_id as on column.
IF the main category and the subcategory have the same schema, one table.
IF they have different schemas, two tables. An example of "doing this wrong" was a forum (or email) schema that tried to put Threads and Messages into the same table.
The 'top' category can have its parent_id equal to 0 or NULL. The subcategories should have parent_id set to the id of the immediate parent. Note that this provides arbitrary nesting of categories (in a tree, no loops, etc).