Question

What is the best way to insert rows into tables with references 1 to 1 of each other?

I mean, in a MySQL 5.5 and tables InnoDB, I have a database design similar to the following

The problem arises when we try to insert rows in table1 and table2. Since there is no multi-table insert in MySQL, I can not insert a row becouse the foreign keys are NOT NULL fields in both tables and should be inserted simultaneously in both.

Which is the bes way to solve this problem?

I have in mind 3 possible solutions, but I want to know if there are more than these or which is the best and why.

1. Set the foreign key field as NULLABLE and after insert one row in a table, insert the other one and afterwards, update de first one.

2. Just as indicated above but with an special value like -1. First, insert in one table with foreign key = -1 that is equivalent to NULL but avoiding set the field as NULLABLE. Afterwards, we insert the row in the other table and update the first one inserted.

3. Create a relational table between both though it is not really necessary because it is a 1 to 1 ratio

Thanks!!

EDIT
I briefly explain what I need this circular relationship: It is a denormalization from the parent table to one of its childs. It is made in order of high performance to have always the reference of the best ranked child from a parent table.

Answer 1

I’ll make this an answer as I feel this is a design flaw.

First, if the two tables are in true 1:1 relationship, why don’t you just have one table?

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Second, if it’s not a true 1:1 relationship but a supertype-subtype problem, you don’t need this circular foreign keys either. Lets say table1 is Employee and table2 is Customer. Off course most customers are not employees (and vice-versa). But sometimes a customer may be an employee too. This can be solved having 3 tables:

Person
------
id
PRIMARY KEY: id

Employee
--------
personid
lastname
firstname
... other data
PRIMARY KEY: personid
FOREIGN KEY: personid
    REFERENCES Person(id)

Customer
--------
personid
creditCardNumber
... other data
PRIMARY KEY: personid
FOREIGN KEY: personid
    REFERENCES Person(id)

---

In the scenario you describe you have two tables Parent and Child having 1:N relationship. Then, you want to store somehow the best performing (based on a defined calculation) child for every parent.

Would this work?:

Parent
------
id
PRIMARY KEY: id

Child
-----
id
parentid
... other data
PRIMARY KEY: id
FOREIGN KEY: parentid
    REFERENCES Parent(id)
UNIQUE KEY: (id, parentid)             --- needed for the FK below

BestChild
---------
parentid
childid
... other data
PRIMARY KEY: parentid
FOREIGN KEY: (childid, parentid)
    REFERENCES Child(id, parentid)

This way, you enforce the wanted referential integrity (every BestChild is a Child, every Parent has only one BestChild) and there is no circular path in the References. The reference to the best child is stored in the extra table and not in the Parent table.

You can find BestChild for every Parent by joining:

Parent
  JOIN BestChild
    ON Parent.id = BestChild.parentid
  JOIN Child
    ON BestChild.childid = Child.id

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Additionally, if you want to store best children for multiple performance tests (for different types of tests, or tests in various dates), you can add a test field, and alter the Primary Key to (test, parentid):

BestChild
---------
testid
parentid
childid
... other data
PRIMARY KEY: (testid, parentid)
FOREIGN KEY: (childid, parentid)
    REFERENCES Child(id, parentid)
FOREIGN KEY: testid
    REFERENCES Test(id)