Question

From Previous Question I got some idea about having composite primary key in n-m relationships. My condition is very similary yet a little different.

[Person]

    PERSON_ID: int, primary key
    CLIENT_ID: int, primary key
    Name: nvarchar(xx)
    primary_key( PERSON_ID, CLIENT_ID);

[Group]

    GROUP_ID: int
    CLIENT_ID: int
    Name: nvarchar(xx)
    primary_key( GROUP_ID, CLIENT_ID);

Now when I use mysql-workbench tool to generate a relationship table what it does is it creates a table:

option 1:

    [PersonHasGroup]

    PERSON_ID: int 
    CLIENT_ID: int 
    GROUP_ID: int
    CLIENT1_ID: int
primary_key( PERSON_ID, CLIENT_ID, GROUP_ID, CLIENT1_ID);

In my case both client_id would normally have the same value so I edited the table to look like

option 2:

    [PersonHasGroup]

    PERSON_ID: int
    CLIENT_ID: int
    GROUP_ID: int
primary_key( PERSON_ID, CLIENT_ID, GROUP_ID);

Are those a good practices? What my other colleagues prefer is a little different. they use:

Option 3:

   [PersonHasGroup]
    PERSON_HAS_GROUP_ID: int, auto-increment
    CLIENT_ID: int        
    PERSON_ID: int, foreign key
    GROUP_ID: int, foreign key
primary_key( PERSON_HAS_GROUP_ID, CLIENT_ID);

Which of the practise would be suitable when my relationship would be many-to-many explanation with an example would be a lot helpful 🙂

Answer 1

I prefer to have a single id for easy referencing, and another unique key for the unique pair, never used workbench, so here is it first in plain sql:

CREATE TABLE person_has_group (
    person_has_group_id SERIAL,
    client_id BIGINT UNSIGNED NOT NULL REFERENCES clients (client_id),
    person_id BIGINT UNSIGNED NOT NULL,
    group_id BIGINT UNSIGNED NOT NULL,
    FOREIGN KEY (client_id, person_id) REFERENCES persons (client_id, person_id),
    FOREIGN KEY (client_id, group_id) REFERENCES groups (client_id, group_id),
    UNIQUE KEY (client_id, person_id, group_id)
);

and proberly somthing like this in “workbench”:

[PersonHasGroup]
    PERSON_HAS_GROUP_ID: int, auto-increment
    CLIENT_ID: int        
    PERSON_ID: int, foreign key
    GROUP_ID: int, foreign key
    primary_key( PERSON_HAS_GROUP_ID)
    unique_key( CLIENT_ID, PERSON_ID, GROUP_ID);