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I just started using MySQL Workbench drawing an entity-relationship diagram that I previously designed in paper.

I want to represent a conceptual one-to-many relationship by means of three tables. For example, I have entity A and entity B with 1-to-N (for every A I have many Bs).

How can I do that with three tables, one of them being a "middle" table?

Like in the diagram that follows

enter image description here

or like in the diagram below

enter image description here

?

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When representing one-to-many (1:n) relationships no "intermediate" table is neccessary. You can simply model that with two tables, one for the 1 cardinality and the other for the n cardinality. The outcome is that one row in the referenced table can be referenced by many rows in the referencing table.

You need an "intermediate" table for representations of many-to-many (n:m) relationships only, because you cannot directly model the situation as described above in both directions.

  • Ok thanks but if i want to do it with three tablets which is the right photo ? I want to do it with three tables cause in the middle i will add one attribute than none of the other two tables have. – user2936672 Apr 3 '15 at 12:49
  • If you insert a 3rd table, regardless how you name it, you essentially have no 1:n relationship anymore between A + B. So at the end you again design a 1:n relationship (say between A + middle) and then add one between one of the participating tables against a third one (B). It's the same process. So, both your approaches are ok. You have to define a 1:n and a 1:1 relationship ultimately. It's up to what you prefer if you do that with A or B (and the 3rd table). – Mike Lischke Apr 3 '15 at 13:00
  • I am still i little bit confused about where i have to put 1:1 , 1-N relationship at middle table. If i want to say that "for every A i have many B, for every B i have only one A ", which is the proper er diagram from above the first one or the second one ? – user2936672 Apr 3 '15 at 13:43

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