Third Normal Form (3NF)
Jul 31, · Third Normal Form (3NF) is considered adequate for normal relational database design because most of the 3NF tables are free of insertion, update, and deletion anomalies. Moreover, 3NF always ensures functional dependency preserving and facetimepc.coted Reading Time: 2 mins. A table is in a third normal form when the following conditions are met ?. It is in second normal form. All nonprimary fields are dependent on the primary key. The dependency of these non-primary fields is between the data. For example, in the following table – the street name, city and the state are unbreakably bound to their zip code.
The third post focused on the second normal formits definition, and examples to hammer it home. Once a table is in second normal form, we are guaranteed that every column is dependent on the primary key, or as I like to say, the table serves a single purpose. But what about relationships among the columns? Could there be dependencies between columns that could cause an inconsistency? How are these addressed? By the third normal form. What does non-transitively dependent mean? When something is transitivethen a meaning or relationship is the same in the middle as it is across the whole.
Since ten is greater than five, and five is greater than three, you can infer that ten is greater than three. In this case, the greater than comparison is transitive. An object has a dependence on another object when it relies upon it. In the case of databases, when we say that a column has a dependence on another column, we mean that the value can be derived from the other.
For example, my age is dependent on my birthday. Dependence also plays an important role in the definition of the second normal form. Consider three columns: AuthorNationality, Author, and Book.
Column values for AuthorNationality and Author rely on the Book; once the book is known, you can find out the Author or AuthorNationality. But also notice that the AuthorNationality relies upon Author. That is, once you know the Author, you can determine their nationality.
In this sense then, the AuthorNationality relies upon Book, via Author. This is a transitive dependence. This can be generalized as being three columns: A, B and PK. That is A is transitively dependent on PK. To be non-transitively dependent, then, means that all the columns are dependent on the primary key a criteria for 2 nd normal form and no other columns in the table.
Generally speaking a postal how to fix light scratches on car applies to one city. Although all the columns are dependent on the primary key, CustomerID, there is an opportunity for an update anomaly as you could update the CustomerPostalCode without making a corresponding update to the CustomerCity. In order for our model to be in third normal form, we how to paint neon lights to remove the transitive dependencies.
As we stated our dependency what is third normal form. The CustomerPostalCode remains in the customer table. The CustomerPostalCode can then be designated a foreign key. In this way, through the relation, how to play poker online with friends city and postal code is still known for each customer. Now each column in the customer table is dependent on the primary key. Their only dependency is on the primary key.
At this point our data model fulfills the requirements for the third normal form. For most practical purposes this is usually sufficient; however, there are cases where even further data model refinements can take place.
Can database normalization be taken too far? You bet! I think you should normalize if you feel that introducing update or insert anomalies can severely impact the accuracy or performance of your database application. If not, then determine whether you can rely on the user to recognize and update the fields together. If you need to present summarized or complied data to a user, and that data is very time consuming or resource intensive to create, it may make sense to maintain this data separately.
It was a database wide what is third normal form list. The task list was rebuilt on-the-fly in real-time using views. Performance was fine for a couple of years, but as the user base grew, more and more DB resources were being spent to rebuild the list each time the user visited the home page. I finally had to redesign the DB. I replaced what kind of mineral oil to use on cutting board view with a separate table that was initially populated with the view data and then maintained with code to avoid anomalies.
We needed to create complicated application code to ensure it was always up-to-date. For the user experience it was worth it. We traded off complexity in dealing with update anomalies for improved user experience. This post concludes our series on normalization. If you want to start from the beginning, click here. More tutorials are to follow!
I want to remind you all that if you have other questions you want answered, then post a comment or tweet me. What other topics would you like to know more about?
Kris Wenzel has been working with databases over the past 28 years as a developer, analyst, and DBA. Kris has written hundreds of blog articles and many online courses. He loves helping others learn SQL. Just some info in case someone is thinking about taking your advice… zip codes can cross city lines, county lines, and even state lines.
That example still holds. Since postal code is the primary key it is OK for a city to be assigned more than one postal code. This is the extreme example of zip codes spanning multiple states — it is much more common for them to span multiple counties and cities. Hi Tony, thanks for brining up this example. It points out some interesting things with the way people can use and intrepret a data model!
Bottom line, when working with data it is really important to understand its meaning and intended use before your model is complete.
Yeah I agree, not a good example of 3NF since zip code is not directly related to city. In fact, in many rural unincorporated areas there may be no city at all. Thanks a lot Kris Wenzel. I think you are the best one on internet to teach Normalization this simple. The site also structured in good manner. Hello sir, As we did in the customer table to break the table into customer and postal code tables. Hi — It all depends on whether office number is transitively dependent on office.
In my opinion it think it makes sense to keep them the same. I would break office number into a separate table if there how to promote pay per call offers more than one office number per office location.
IF there are any other hard or difficult to explain topics you want me to tackle, just let me know. Please let your lecturer know about the site and article. They are more than welcome to direct other students to the site if it would help them out.
If there are other topics you would like me to cover, please let me know. Thanks for such simple and clear explanations. Would like to get some problem solving excersise and solution for practice purposes. Thanksfor the suggestion. HI I have an issue with your Postalcode table in that your primary key, postalcode, breaks the rules for primary keys — it must be unique for every record in that table. Your first two records have the same postal code.
This is the issue with your plan to separate them out as more than one address has the same postal code. Probably similar to the UK. Here, in a street, one post code will cover about 15 houses. Hi the postal codes are unique. Also, it is OK for the non primary keys to be not be unique. So I can have different zip codes for the same city. This happens all the time for large cities such as New What size bottle nipple for 4 month old and Chicago.
Great Work Kris. Very helpful info here. As I was reading through your explanation of 3NF, I had the same thought as Tony — a single zip code can, indeed, span several municipalities!
Case in point, I live in a township which is immediately adjacent to a major city. How to break your ribs share the same zip code.
My mail is addressed to the major city not the township. Therefore, your example holds to 3NF. Thanks for this refresher on data normalization! Nicely explained: It was easy to follow the plain-english DB normalization process. Great examples and simplified to the essence! However, in the Customer to PostalCode tables, you store the postal code in the Customer table.
Nothing is worse what is third normal form, being excited to learn a new tool but not knowing where to start, wasting time learning the wrong features, and being overwhelmed.
Techopedia Explains Third Normal Form (3NF)
Third Normal Form (3NF) Third Normal Form is an upgrade to Second Normal Form. When a table is in the Second Normal Form and has no transitive dependency, then it is in the Third Normal Form. Before moving forward with Third Normal Form, check these topics out to understand the concept better: First Normal Form (1NF). Create A Website SQL Data Warehousing CSS PHP HTML Database Normalization 3rd Normal Form Definition A database is in third normal form if it satisfies the following conditions: It . Third Normal Form (3NF) For a relation to be in third normal form: it should meet all the requirements of both 1NF and 2NF If there is any columns which are not related to primary key, then remove them and put it in a separate table, relate both the table by means of foreign key i.e.; there should not be any transitive facetimepc.coted Reading Time: 1 min.
Before moving forward with Third Normal Form, check these topics out to understand the concept better :. In our last tutorial, we learned about the second normal form and even normalized our Score table into the 2nd Normal Form.
So let's use the same example, where we have 3 tables, Student , Subject and Score. In the Score table, we need to store some more information, which is the exam name and total marks, so let's add 2 more columns to the Score table.
For example, a mechanical engineering student will have Workshop exam but a computer science student won't. And for some subjects you have Prctical exams and for some you don't. Does it depend on our Score table's primary key? For example, practicals are of less marks while theory exams are of more marks. This is Transitive Dependency. When a non-prime attribute depends on other non-prime attributes rather than depending upon the prime attributes or primary key.
Again the solution is very simple. Studytonight available on:. Third Normal Form 3NF. Studytonight Forum NEW. Ask Question.
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