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What is a good design for a relational database (RDB)?
You might have seen an excel data sheet filled with lots of columns. I can imagine that kind of excel sheet is very messy. A well designed database should design and create a model, and then implement the model into database.
For RDB, we use ER model. However, we might still design a model with deflects. For example, a table combines student information and department information.
In this case,
DNUM is followed by
D_HEAD are followed by
DNUM. That's dangerous, because it might cause anomalies while inserting, deleting, or modifying a tuple of data.
This is an example of bad designs. So, I am going to introduce to 5 concept of normal forms.
When we create a RDB, we should not only base on ER model, but also need to consider that if the database are normal forms. Then we will have a good design database.
Concept of normal forms were first proposed by Codd in 1972.
The value of an attribute should be an atomic value. It could not be multi-value, array, composite values, or any other relation. (ER model has already defined)
Primary key could be a combination of keys. If a key of the primary key, could determine an attributes in the table, it is a partial relationship between the primary key and non-prime attributes. Then, it violates the second normal form.
In this case, the primary key is
PID, and any attributes should be determined by the two. But,
SNAME is only determined by
SID, so it violates the rule. Therefore, this table should divide in three new tables to follow the concept.
If there are attributes that follow a non-primary key, these attributes should be an another table. Just as the example:
A table is in 4NF if and only if, for every one of its non-trivial multi-valued dependencies X ↠ Y, X is a superkey. That is, X is either a candidate key or a superset thereof.
Removing “bad” multi-valued dependencies help us get into 4th normal.
form, and into better design.
Considering the following example:
Pizza Variety and
Delivery Area are independent to
Restaurant, it violates 4NF. Because, if restaurant
Pizza A1 add a new kind pizza
Cheese Pizza, it need to add three rows for the there locations that in the table.
In other words, adding a new kind of pizza, the table need to insert rows, which is the new pizza with each area, and it is easy to make errors when updating. To eliminate the possibility of these anomalies, 4NF suggests that the table should be separated.
Assume the table meets 1NF to 4NF.
Considering a table that has
Traveling Salesman(primary key),
Imagining a extreme case:
Traveling Salesman has certain
Brands and certain
Product Types in their repertoire. If
Brand B1 and
Brand B2 are in their repertoire, and
Product Type P is in their repertoire, then (assuming
Brand B1 and
Brand B2 both make Product Type P,) the
Traveling Salesman must offer products of Product Type P those made by Brand B1 and those made by Brand B2.
That is to say, the
Product are combined (4NF), the
Salesman is not able to only sale a certain
Brand but exclude one of products of that
Then to solve this, splitting the able to three would make sense.
ER model is good for RDB, but the database might easily makes error if we design not well.
Following these concept of normal forms would reduce the possibility of anomalies, which makes the database more clear and reliable.