I have a user who may have 0 or more foo's and bar's. Here is a diagram:

enter image description here

Each foo has a unique integer ord.

If a user has a bar, then this implies that he has a foo with ord=1. A user may not have foo with ord=1 without a bar.

My thought how to avoid logical inconsistencies is to simple never insert any foo_user relationship with ord=1. However when I want to query the foo of a user with smallest ord I need to do this a bit complicated like this:

    1. Query: Check if bar relationship exists. If it does, get foo with ord=1
    1. Query: If no bar bar relationship exists, get foo with smallest ord column.

Is there maybe a more convenient database structure for this scenario?

  • 3
    If a user has a bar, is that associated with a specific foo? Can a user have 10 bars but only 3 foos? Jan 23, 2019 at 13:49
  • @ypercubeᵀᴹ well they are associated in the sense, that if a user has a bar he must have the unqiue foo with ord=1. Its possible that a user has 10 bars but only 3 foos.
    – Adam
    Jan 23, 2019 at 14:18

2 Answers 2



CREATE TABLE users (id, 
                    PK (id));

CREATE TABLE user_foo (user_id,
                       PK (user_id, foo_id),
                       FK (user_id) REF users (id),
                       FK (foo_id) REF foo (id));

CREATE TABLE foo (id, 
                  PK (id),  
                  KEY (id, ord));

CREATE TABLE foo_bar (foo_id,  
                      ord AS (1) VIRTUAL,  
                      PK (bar_id, foo_id),
                      FK (foo_id, ord) REF foo (id, ord),
                      FK (bar_id) REF bar (id));

CREATE TABLE bar (id,  
                  PK (id));

One foo may also have many users so the pivot tables are necessary.

Taken into account.

foo and bar are ManyToMany relations

Taken into account.

  • Thank you for this proposal. However its possible that 2 users have the same foo but different bars. Foo and bar are in general independet. They only have the logical implication described in the OP.
    – Adam
    Jan 23, 2019 at 14:40
  • @Adam Foo and bar are in general independet. It is critical. Add this info into a question text. Additionally: does a user can have multiple bars? if so, does each bar must have its own foo with ord=1, or it's enough to have one such foo for to accept any count of bars?
    – Akina
    Jan 24, 2019 at 4:52
  • A user can have multiple bars. Sorry for not being clear enough, this is what I meant with I have a user who may have 0 or more foo's and bar's.. The condition is that the user should have one foo with ord=1 to accept any count of bars. It would also not be possible that a user has multiple foo's with ord=1 because ord is unique.
    – Adam
    Jan 24, 2019 at 7:44
  • @Adam Oh... it's bidirectional check... but MySQL ignores CHECK constraints... so the only way is to use trigger logic. Q1: If you want to delete user reference to a foo with ord=1, you will delete all references to all bars for this user, or you will eject this foo reference deletion until all bars references deleted explicitely? Q2: Maybe better way is to realize all logic as a server one (as stored procs) restricting direct (by query) edition of these tables?
    – Akina
    Jan 24, 2019 at 9:32
  • to Q1,Q2: at the moment I do not insert any foo references for a user with ord=1. I handle it with server logic. So if I want all foo's from the user, I get all foo's that are stored as related in the DB and then I check if the user has any bar's, and if he has any, I add the foo with ord= 1 to the foo collection of the user. Its acceptable, but I was wondering if there is maybe a more elegant solution to this.
    – Adam
    Jan 24, 2019 at 12:21

Assuming you aren't missing any relationships between foo and bar, then I think you may be mixing your entity-relationship model with your business logic's restrictions.

An user has many foos, many bars, and vice versa. So far, so good. Checking wether a foo with ord=1 exists or not is your db engine's responsibility, not your e-r model's

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