What are Parent-Child relationships?

What is the parent and what is the child in a sql relationship?

In my case, I have a part (Say screw), and product material. For argument's sake, a product material (eg steel) can only belong to one part (but not in the real world). So thi开发者_如何学Gos is 1:n. The part will have its pk as a fk in the ProductMaterial table.

Which is parent and which is child in this case?

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Relational databases such as SQL actually have no concept of parent/child relationships - that is an interpretation that you as a programmer put on the data. There are architectures that explicitly state and use such relationships, such as heirarchical (and to a certain extent OO) databases.

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You can interpret a 1:n relationship in database this way: A child is always that model which holds the foreign key as this indicates where it belongs to.
Of course if you have self referencing models/tables you have to look at it in a different way.

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In this case, Part is parent and ProductMaterial is child.

A parent can have unlimited numbers of children (scary thought - 2 is enough for me!), whereas a child can have only a limited number of parents - and in DB terms, only 1!

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Usually in a one-to-many relationship, it's the "one" record that is the parent, and the "many" records that are the children.

Of course, in some cases it doesn't make any sense to talk about a parent-child relationship. In your example it makes some kind of sense at least. In other examples you may even find the opposite, where a child has many parents, but then it's not very useful to describe it that way.

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Another way to look at this, in addition to what David M. said, is in terms of an ORM implementation (such as Linq to SQL). You have two entities, Part and ProductMaterial. Each part entity has a set of ProductMaterial entities (children entities or an EntitySet). Each ProductMaterial entity has zero or one Part entity (parent entity or an EntityRef).

Randy

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I would say that any table which has a one to many relationship with one or more other tables can be considered a parent to those other tables. Self joining? An ambiguous term which I don't think anyone understands.

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'Product' is parent and 'Product material' is child in Product_ProductMaterial relationship or association.

If you delete a child, parent can live and continue life. If you delete parent, child become orphan or identityless which is not good. If A can not not be deleted before B is deleted, then A is parent and B is child.

If I guess right, in your case in Product_ProductMaterial relationship, if you delete Product, ProductMaterial will be assigned to no one, become orphan, identity crisis. But if you delete ProductMaterial, Product can still be there, no need identity support.

Sorry if my wordings are not good.