addAttribute() vs addColumn()?

Question

While I am aware of the similar question. My question is more generic, that what would happen to both calls whether I actually create a database column or not.

EDIT

I know addColumn() will add a real table column, but when I'm diving into some core modules, some of them are calling both addColumn() and addAttribute() to the same field.

What is the difference between them?

EDIT 2

Because addAttribute() looks very much EAV related to me, after some research I also find some similar names like addAttributeSet() and addAttributeSetNames().

How are they differ to each other?

Answer 1

Let's take it step by step.

• addColumn adds a column to a table.
• addAttribute adds a new attribute for EAV entities. So a new record will be created in the eav_attribute table.
• The exception: For the sales module, addColumn and addAttribute do the same thing. They add a column to a table. The reason is backwards compatibility. Before version 1.4, the sales entities (orders, invoices, shipments, items, ...) were EAV so you had to use addAttribute. Starting with 1.4 the sales entities are flat so you have to use addColumn. But for backwards compatibility, starting version 1.4, addAttribute is just a wrapper for addColumn.
• addAttributeSet is used for EAV entities and it does what it says. It creates a new attribute set for that entity. I think it is used for products only, because all the other EAV entities (category, customer, customer_address) use a single attribute set.
• addAttributeSetNames - I didn't find any occurrence of this anywhere in the core code.