主键和外键？

Question

When creating a database what happens if one table only contains 2 primary keys from other tables, i'm assuming they're both foreign keys. Does there have to be a primary key in the table?

Answer 1

A link table with just two foreign keys and nothing else (no surrogate key added to make a simple meaningless primary key) will usually be constrained to be unique (otherwise you will not be able to distinguish duplicates - and this is also a violation of normal form) so you will often just go ahead and have those two keys together as a composite make up the primary key (PKs have to be unique by definition, and they form a natural choice for the PK of such a link table). The order of those columns in the primary key is usually determined by the most frequent order of search - ie personid, accountid might have personid first in a composite primary key on personid, accountid.

Answer 2

No; tables don't have to have primary keys.

They often (usually?) don't when they're linking/mapping tables like this.

Answer 3

When you have an associative entity or table, it is good practice to create a composite primary key consisting of the keys of both parents. In the example, there would be a uniqueness constraint on the Album-Song-Map, on the composite of Album and Song. If you do not make these the primary key and make the combination unique, then you can have duplicates in Album-Song-Map. The composite key of Album-Song-Map is the key of both parents, but individually they are foreign keys back to their parent, Album and Song respectively. In my experience, the associative entity is usually not just for mapping but also contains some business attributes. For example, say the song has a different Duration (playing time) on one album than another. This attribute would have to go in Album-Song-Map. I use a lot of music for dancing, and often the duration of a song is different on different albums.

Answer 4

A Table doesn't necessarily need to have a primary key associated. It i completely valid as follows

Table_Album

pkey | name
1    | name1
2    | name2

Table_Song

pkey | name
1    | song1
2    | song2
3    | song3

You can then have a table which states

Table_Album_Song_Map

id | Album | Song  # Here id is just row number and not primary key
1  | name1 | song1
2  | name1 | song2
3  | name2 | song3

Hope that helps