REST: Relations between individual resources

Can someone tell me how to "RESTfully" create a relation between two individual resources?
I have figured out several approaches, but I would like to stick to standards.
I'll list several examples to show you guys what I already came up with.
Examples will have a post resource which has to be linked to a category resource.

Note: all examples presume that both resources already exist.

Ex 1:

PATCH /post/1 HTTP/1.1
Host: api-host
Accept: application/json
Content-Type: application/json
Cache-Control: no-cache

{
    "post": {
        "links": {
            "category": "2"
        }
    }
}

Ex 2:

POST /post/1/category/2 HTTP/1.1
Host: api-host
Accept: application/json
Content-Type: application/json
Cache-Control: no-cache

{}

Ex 3:

POST /relations?post=id&category=id
Host: api-host
Accept: application/json
Content-Type: application/json
Cache-Control: no-cache

{}

And what would be the best way to remove the relationship between these 2 resources?

"Solution":
As LINK and UNLINK aren't supported anymore, the best solution imo (hat tip @Bramus) would be the PATCH example.

So to create a link you would call:

PATCH /post/1 HTTP/1.1
Host: api-host
Accept: application/json
Content-Type: application/json
Cache-Control: no-cache

{
    "post": {
        "links": {
            "category": "2"
        }
    }
}

And to remove it:

PATCH /post/1 HTTP/1.1
Host: api-host
Accept: application/json
Content-Type: application/json
Cache-Control: no-cache

{
    "post": {
        "links": {
            "category": ""
        }
    }
}

Solution

To place a Post into a Category I'd use a simple PATCH passing in all those categories as an array (if multiple categories may be selected), or a single category key-value pair.

Multiple Categories:

PATCH /post/1 HTTP/1.1
Host: api-host
Accept: application/json
Content-Type: application/json
Cache-Control: no-cache

{
    "categories": [2, 3]
}

Single Category:

PATCH /post/1 HTTP/1.1
Host: api-host
Accept: application/json
Content-Type: application/json
Cache-Control: no-cache

{
    "category": 2
}

It resembles your first example, yet it differs a little as it passes in the categories as an array / single value. I prefer this method as it mimics what one would do via a normal HTML form: multiple checkboxes for multiple categories (result is an array that gets posted) or a dropdown (result is a single value that gets posted).

On a sidenote I don't see a category as a link, but as a sub-entity/sub-resource:

Sub-entities indicate a relationship between two (mostly different type of) entities.
Links are mostly used for navigation (next, prev, parent, me, ...)

The use of the links key inside your PATCH is not compatible with this ideology.