SQL多对多关系

Question

Question

Is there a way to have a many-to-many relationship among 3 tables without the use of automatic incrementers (usually ID), or are ID's required for this?

Why I ask

I have 3 relative tables. Since one-to-one relationships seem to can't happen directly, I made a 4th to do one-to-many relationships to the other 3 tables. However, since there's still a primary key to each table, a value can only be used once in a table, which I don't want to happen.

What I have

• Connectors has multiple Pockets which have multiple pins.
• The 4th Table is ConnectorFullInfo Screenshot http://i60.tinypic.com/2exy8og.jpg

Answer 1

There is no requirement that a table have an "automatic incrementer" as a primary key.

But, a familiar pattern is to add a surrogate ID column as primary key on entity tables. The "ideal" primary key will be "anonymous" (carry no meaningful information), "unique" (no duplicate values), "simple" (single column, short simple native datatype), ...

There are a couple of schools of thought on whether it's a good idea to introduce a surrogate key. I will also note that there are those who have been later burned by the decision to use a natural key rather than a surrogate key. And there are those that haven't yet been burned by that decision.

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In the case of "association" tables (tables introduced to resolve many-to-many relationships), the combination of the foreign keys can be used as the primary key. I often do this.

BUT, if the association table is itself turns out to be entity table, with it's own attributes, I will introduce a surrogate ID column. As an example, the association between person and club , a person can be a member of multiple clubs, and a club can have multiple members...

club +--< membership >--+ person

When we start adding attributes to membership (such as status , date_joined , office_held , etc... at that point membership isn't just an association table; it's turning into an entity. When I suspect that an association is actually an entity, so I'll add the surrogate ID column.

The other case where I will add a surrogate ID column to an association table is when we want to allow "duplicates", where we want to allow multiple associations. In that case, I will also introduce a surrogate ID column.

Answer 2

Yes you can but,

It is customary to represent a table row by a unique identifier which is the number, its becomes more efficient.