Question

I have a table say ITEM that should contain :

ID_SERVICE , // foreign key to table SERVICE
ID_FILE , // foreign key to table FILE
ID_FUNCTION  // foreign key to table FUNCTION

and it must be identified by those 3 columns (compound primary key) , however my problem is that sometimes i might have an item that doesnt have a function (ID_FUNCTION could be null).

How can i redesign this table so that it can handle these constraintes ?

Thanks.

Answer 1

You create a unique index not a primary key constraint. A unique index allows for any key to be null, but still enforces uniqueness over every column. The syntax is the same as index but you add unique in front:

create unique index i_table_name
    on table_name ( column_names )
       tablespace
       storage ( ... )
       < etc >

A unique index will automatically create a unique constraint underlying it. If you don’t want the index you could just create the constraint on it’s own:

alter table table_name
add constraint constraint_name unique (column_names)

Here is more information on the different types of constraints.