Question

So, I’m learning (and using) a database, and so far I think I’m understanding it

Today, a question arise on the following example:

I have a table with identities (for instance, objects, which have a name and a physical description and naturally a primary auto-increment integer index). And I have identities (e.g. persons) which interact with then).

Is there any way of categorizing these identities in my a database? For instance, assume a mechanic only interacts with a very specific set of those objects (tools, car pieces, etc). So, it would make sense to restrict any search by a mechanic on the objects to that set.

Any hints on what is the natural way (if any) to approach this problem?
Thanks

Answer 1

You would need a table category with an id and a description

An object can have many categories and a category can have many objects. So we are facing a many to many relationship.

You need a third table, that we call a junction table. This table would have the id of the category and the id of the object.

The design would look like this

Category
Id | Description

Object
Id | Description

Person
id | Type | ...

Categories_Objects
Id | CategoryId | ObjectId

Then if you want to select the objects for a mechanic which would be the category 1 (that’s an example, you need to find the id first)

You would do a select like this

SELECT Object.Description 
FROM Object 
     INNER JOIN Categories_Objects ON Object.id = Categories_Objects.ObjectID 
WHERE CategoryID = 1

OR

SELECT Object.Description 
FROM Object 
     INNER JOIN Categories_Objects ON Object.id = Categories_Objects.ObjectID
     INNER JOIN Category ON Categories_Objects.CategoryId = Category.id
WHERE category.description = "Mechanic"

Note: If a person can use many categories of tools, you will need another junction table like this

Persons_Categories
ID | PersonID | CategoryID