Databases
StackOverflow2013
Postsdbo
POWhy is the foreign key part of the primary key in an identifying relationship?
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POWhy is the foreign key part of the primary key in an identifying relationship?
    primarykey
    data
    text
    <p>OK, I hope this is an appropriate question for stackoverflow as I'm trying to understand a concept rather than fixing a piece of code that won't work.</p> <p>I'll take a general example of a <strong>form</strong> (parent table) and a <strong>form field</strong> (child table). <strong>Logically</strong>, this would be an identifying relationship, since a form field cannot exist without a form.</p> <p><img src="https://i.stack.imgur.com/CaC1h.png" alt="form and form_field tables"></p> <p>This would make me think that in order to translate the <strong>logical</strong> relationship into the <strong>technical</strong> relationship, a simple <code>NOT NULL</code> for the form_id field in the form_field table would suffice. (see the left part of above screenshot)</p> <p>However, when I add an identifying relationship using MySQL Workbench, form_id is not only <code>NOT NULL</code> but also part of the primary key. (see the right part of above screenshot)<br> And when I add a non-identifying relationship, <code>NOT NULL</code> is still applied so logically it would actually be an identifying relationship as well.</p> <p>I guess this confuses me a little, as well as the fact that until now I always simply used the id field as primary key.</p> <p>So I understand the <strong>logical</strong> concept of identifying vs. non-identifying relationships, but I don't understand the <strong>technical</strong> part. Why is it, as <a href="https://stackoverflow.com/a/762994/1001110">this answer</a> states, 'the "right" way to make the foreign key part of the child's primary key'? What is the benefit of these composite primary keys?</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USCommunity
    1. USNic Wortel
    plurals
    1. PL
      singulars
      1. LTLinked
    1. PL
      singulars
      1. LTLinked
    2. PL
      singulars
      1. LTLinked
    3. PL
      singulars
      1. LTLinked
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POWhy is the foreign key part of the primary key in an identifying relationship?
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. POWhy is the foreign key part of the primary key in an identifying relationship?
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. POWhy is the foreign key part of the primary key in an identifying relationship?
      1. This table or related slice is empty.
      1. VTUpMod
    1. COUnless the field ids are numbered per form, I don't see any point in including them in the PK.
      singulars
      1. POWhy is the foreign key part of the primary key in an identifying relationship?
      1. USOlivier Jacot-Descombes
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload