Should my intermediate table contain a composite PK or a unique index

Let's say I have a Products table, ProductsCategory table and a Category table.

The ProductsCategory table has two columns: Produc开发者_运维百科tID and CategoryID. Should I be using a composite primary key or a unique index on the two columns?

Additionally if I use the index, I can make it a unique index or a key.

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Might as well use a composite key - no need to add a unique index when you already have the uniqueness semantics of a composite primary key.

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You must create two way foreign key both table with on delete cascade option. Because if you delete one of categories then it must remove relational rows on ProductCategory.

i mean you can use like this :

alter table ProductsCategory add constraint ForeignKey1 foreign key (ProductId) references Products (ID) ON DELETE CASCADE;
alter table ProductsCategory add constraint ForeignKey2 foreign key (CategoryId) references Category (ID) ON DELETE CASCADE;