What is Normalisation (or Normalization)? [closed]

Answer 1

Most importantly it serves to remove duplication from the database records. For example if you have more than one place (tables) where the name of a person could come up you move the name to a separate table and reference it everywhere else. This way if you need to change the person name later you only have to change it in one place.

It is crucial for proper database design and in theory you should use it as much as possible to keep your data integrity. However when retrieving information from many tables you're losing some performance and that's why sometimes you could see denormalised database tables (also called flattened) used in performance critical applications.

My advise is to start with good degree of normalisation and only do de-normalisation when really needed

P.S. also check this article: http://en.wikipedia.org/wiki/Database_normalization to read more on the subject and about so-called normal forms

Answer 2

Quoting CJ Date: Theory IS practical.

Departures from normalization will result in certain anomalies in your database.

Departures from First Normal Form will cause access anomalies, meaning that you have to decompose and scan individual values in order to find what you are looking for. For example, if one of the values is the string "Ford, Cadillac" as given by an earlier response, and you are looking for all the ocurrences of "Ford", you are going to have to break open the string and look at the substrings. This, to some extent, defeats the purpose of storing the data in a relational database.

The definition of First Normal Form has changed since 1970, but those differences need not concern you for now. If you design your SQL tables using the relational data model, your tables will automatically be in 1NF.

Departures from Second Normal Form and beyond will cause update anomalies, because the same fact is stored in more than one place. These problems make it impossible to store some facts without storing other facts that may not exist, and therefore have to be invented. Or when the facts change, you may have to locate all the plces where a fact is stored and update all those places, lest you end up with a database that contradicts itself. And, when you go to delete a row from the database, you may find that if you do, you are deleting the only place where a fact that is still needed is stored.

These are logical problems, not performance problems or space problems. Sometimes you can get around these update anomalies by careful programming. Sometimes (often) it's better to prevent the problems in the first place by adhering to normal forms.

Notwithstanding the value in what's already been said, it should be mentioned that normalization is a bottom up approach, not a top down approach. If you follow certain methodologies in your analysis of the data, and in your intial design, you can be guaranteed that the design will conform to 3NF at the very least. In many cases, the design will be fully normalized.

Where you may really want to apply the concepts taught under normalization is when you are given legacy data, out of a legacy database or out of files made up of records, and the data was designed in complete ignorance of normal forms and the consequences of departing from them. In these cases you may need to discover the departures from normalization, and correct the design.

Warning: normalization is often taught with religious overtones, as if every departure from full normalization is a sin, an offense against Codd. (little pun there). Don't buy that. When you really, really learn database design, you'll not only know how to follow the rules, but also know when it's safe to break them.

Answer 3

Normalization a procedure used to eliminate redundancy and functional dependencies between columns in a table.

There exist several normal forms, generally indicated by a number. A higher number means fewer redundancies and dependencies. Any SQL table is in 1NF (first normal form, pretty much by definition) Normalizing means changing the schema (often partitioning the tables) in a reversible way, giving a model which is functionally identical, except with less redundancy and dependencies.

Redundancy and dependency of data is undesirable because it can lead to inconsisencies when modifying the data.

Answer 4

As Martin Kleppman says in his book Designing Data Intensive Applications:

Literature on the relational model distinguishes several different normal forms, but the distinctions are of little practical interest. As a rule of thumb, if you’re duplicating values that could be stored in just one place, the schema is not normalized.

Answer 5

Normalization is one of the basic concepts. It means that two things do not influence on each other.

In databases specifically means that two (or more) tables do not contain the same data, i.e. do not have any redundancy.

On the first sight that is really good because your chances to make some synchronization problems are close to zero, you always knows where your data is, etc. But, probably, your number of tables will grow and you will have problems to cross the data and to get some summary results.

So, at the end you will finish with database design that is not pure normalized, with some redundancy (it will be in some of the possible levels of normalization).

Answer 6

What is Normalization?

Normalization is the process of dividing the large database tables into smaller tables to reduce data redundancy.

What is data redundancy?

Data redundancy is the unnecessary duplication of the data in our database that not only increases the size of our database but also creates the data anomalies.

What are data anomalies?

Anomalies are the problems that occur when there is too much data redundancy which indicates the database is poorly planned and unnormalized. There are three types of data anomalies:

1. Update Anomaly: It happens when we try to update the record in our table. In the employee table, we have a project manager for each project. Now, most probably each project has more than one employee, which means more than one record in our table. For each record, we are repeating the project code, project name, and project manager. Now If we have to change the project manager, let’s say for the GLO project from Qadir Shaikh to Hamza Imran. We have to change it for every record where the project is GLO. There is a chance we might miss one or two entries leaving the database in an inconsistent state. Where in some records project manager is Qadir Shaikh and for others, it is Hamza Imran.
2. Insertion Anomaly: Let’s say we want to add a new employee who has been hired recently and currently is in training so hasn’t been assigned any project yet. We can’t add employees without the project details or we have to fill the project fields with null values. OR let’s say we want to add a new project in our database that we have received from the client but we haven’t started working on it yet, so currently, there is no employee assigned to this project. But we can’t store the project details without any employee since employee_id (primary key) is linked with the employee.
3. Deletion Anomaly: Let’s say our project NJAS is completed and we want to remove it from our table so we will remove all the entries of the table where the project is NJAS. But If we do so we will also remove the record of all the employees (Umer shaikh all others) who were working on this project We can remove these anomalies by normalizing our Employees table. There are five rules to normalize the table. Each rule is called the normal form. There is also another normal form called Boyce Codd normal form which is also called 3.5 normal form. Let’s look at this one by one.

Type of Normal Form:

• 1st Normal Form says each should have more than one value
• 2nd Normal Form removes Partial Dependency
• 3rd Normal Form removes Transitive Dependency
• Boyce Codd Normal Form (the strict version of 3NF)
• 4th Normal Form removes Multi-valued Dependency
• 5th Normal Form removes Join Dependency

For more details, you can read this article to learn about all the normal forms and how to apply them

What is Normalization? | How to apply Normalization up to the 5th Normal Form?