Prevent denormalization

Question

I was wondering how to prevent denormalization in this case:

Table Categories:

- id
- name

Table Category_types:

- id
- name

Association table: (category and category types are in a many-to-many relationship)

- id
- id_cat
- id_cat_type

I have a table Request where I have to store info about the Category of this Request and the CategoryType. At the moment the schema is:

Request:

 - id
 - title
 - desc
 - id_cat
 - id_cat_type

By the way I think this is a bad idea, because there is a functional dependency (I guess?) between id_cat and id_cat_type. I think I can solve this problem storing the id of the association table

Request:

 - id
 - title
 - desc
 - id_association_table

I know that splitting tables maybe would be the best option here, but I was wondering if there is some other approach to solve this kind of problems.

Thanks

Answer 1

-- Category CAT exists.
--
category {CAT}
      PK {CAT}

-- Category type TYP exists.
--
ctype {TYP}
   PK {TYP}

For each category, that category may be of more than one category type. For each category type, more than one category may be of that category type.

-- Category CAT is of type TYP.
--
category_ctype {CAT, TYP}
            PK {CAT, TYP}

FK1 {CAT} REFERENCES category {CAT}
FK2 {TYP} REFERENCES ctype    {TYP}

-- Request REQ is of category CAT, category type TYP.
--
request {REQ, CAT, TYP}
     PK {REQ}

FK {CAT, TYP} REFERENCES category_ctype {CAT, TYP}

Note:

All attributes (columns) NOT NULL

PK = Primary Key
FK = Foreign Key