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This lecture covers the different phases of the database design process, including discovery, design, development, distribution, and documentation. It also introduces the concept of normalization and how to apply normalization rules for better database design.
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(Spring 2015) Instructor: Craig Duckett Lecture 05: Tuesday, April 12, 2015 Normalization
PHASE 1: DISCOVERY DUE: Thursday, April 21st , uploaded to Team Web Siteand ZIPPED and uploaded to StudentRackerby Phase 1 Project Manager Phase 2: Design due Thursday, May 5th(with TEAM POWERPOINT PRESENTATION)
Five Phase Due Dates • One (1) Team Project for a Client (3-to-4 Members on Team) 1000 points Total • Phase 1: Discovery (200 Points) DUE THURSDAY, APRIL 21st • Phase 2: Design (200 Points) DUE THURSDAY, MAY 5th(with Presentation) • Phase 3: Develop (200 Points) DUE THURSDAY, MAY 19th • Phase 4: Distribute (200 Points) DUE TUESDAY, MAY 31st • Phase 5: Documentation (200 Points) DUE THURSDAY, JUNE 9th(Last Day of Class)
Conceptual Overview The first phasein the database design process is to define a mission statement and mission objective. This establishes the purpose of the database and provides a focus for the developer. The second phaseinvolves analyzing the current database, if one exists. It will typically be a legacy (one that has been in use for several years) or paper-based (forms, index cards, folders, etc.) database. It is very important to conduct interviews with users and management to identify how they interact with the database on a daily basis. With this information, you then compile a list of fields. This list will be refined as the design is developed. The third phaseis creating the data structures: tables, fields, establishing keys and defining field specifications. Tables are the first structures you define in the database. Once subjects are identified, they’re established as tables, then fields are associated with the appropriate tables. Tables should be reviewed to be sure they represent only one subject and that no fields are duplicated. Next, fields are reviewed to be sure there are no multipart or multivalued fields. If so, you modify those fields so each fields stores a single value. Then a Primary Key is established, making sure it uniquely identifies each record within the table. Finally, field specifications are established. Interviews should be conducted with users to help identify any specific field characteristics that may be important to them.
Conceptual Overview In the fourth phase, table relationships are established. Interviews are conducted with users and management to identify relationships, relationship characteristics and establish relationship-level integrity. Once relationships have been identified, it is necessary to establish the logical connection for each relationship. Depending upon the type of relationship, you would use either a Primary Key or a “linking” or “composite” table to make the connection between a pair of tables based upon the type of relationship you want to establish. The fifth phaseof the database design process is to define the business rules. How an organization views and uses its data will determine limitations and requirements that must be built into the database. Again, this information is gained through interviews with users and management. Next, validation tables are defined. For example, if certain fields are found to have a finite range of values due to the way they are used by an organization, validation tables are used to ensure the consistency and validity of the values stored in those fields. The sixth phaseof the database design process is determining and establishing views. Interviewing users and management will help identify the different ways data is viewed. One group may view data from a different perspective than another group. Another group may only need to view one specific field from a certain table. The seventh phaseis reviewing the final database structure for data integrity. First, each table is reviewed to ensure that it meets proper design criteria. Then field specifications are reviewed and checked. Then, you test the validity of each relationship. Finally, business rules are reviewed and confirmed.
Normalization Once we've started to plan out our tables, our columns, and relationships, we do something called Database Normalization. This is a process where you take your database design, and you apply a set of formal criteria of rules called Normal Forms. These were developed about 45 years ago mainly by Edgar Codd, the father of relational databases. And we step through them 1, 2, 3, first normal form, second normal form, and third normal form. There are others but these are the important ones.
Normalization Normalization should be carried out for every database you design. It's really not that hard, even though, yes, when you first start reading about database normalization, you'll run into phrases like: But you don't have to get into all this language unless you are mathematically inclined. The entire point of normalization is to make your database easier and more reliable to work with. You usually will end up creating a few new tables as part of the process. But the end result is your database will contain a minimum of duplicate or redundant data. It will contain data that's easy to get to, easier to edit, and maintain, and you can preform operations, even difficult ones on your database without creating garbage in it, without invalidating the state of it. If you're a working database administrator or database designer, you can do normalization in your sleep. It's a core competency of the job. It's important. And as you'll see, we've already been doing a little of it.
First Normal Form (1NF) Before we apply the first set of criteria, what's called first normal form, often shortened to 1NF, I'm taking as a given that we already have our columns and our primary keys specified. First normal form says that each of your columns and each of your tables should contain one value, just one value, and there should be no repeating groups. Okay, what does this actually mean?
First Normal Form (1NF) Well, let's say I begin developing a database for my company and one of my tables is an Employee table, very simple stuff, EmployeeID, LastName, FirstName, and so on. And we allocate every employee a computer. I want to keep track of that, so we'll add a ComputerSerial column to keep track of who has what. Now, this is actually okay right now. This technically is in first normal form. Here's the problem.
First Normal Form (1NF) Let's say I figured out that some of our employees need a Mac and a PC to do the testing. Others need a desktop and a laptop. So, several people have multiple computers, and I want to keep track of all of them. There is a couple of ways that I could deal with this. I could just start stuffing extra data into that one column. We could start putting commas or vertical bars or any other delimiter and put in multiple values in the one ComputerSerial column. This is just something you just don't do in Relational Database Design. We're violating first normal form. Understand the relational databases will happily deal with hundreds of tables. Each table could have hundreds of columns and millions of rows. But they do not want columns that have a variable amount of values. You would find it hard to search directly for a serial number. You'd find it hard to sort. You'd find it hard to maintain. So, it's not in first normal form if you do this because first normal form demands that every column, every field contains one and only one value.
First Normal Form (1NF) So, what we might do then is go back to the original way, and instead start adding new columns. So, ComputerSerial2, ComputerSerial3, this is what's called a repeating group, and there should be no repeating groups. The classic sign of a repeating group column is a column with the same name, and the number tacked onto the end of it just to make it unique, because usually this is a sign of an inflexible design. Sure, if we could guarantee that there would only ever be two or three, that's fine. But what happens when we want to add the tablet and the smart phone? What happens when one employee manages testing and needs to be associated with six computers? We don't want to require a change to the database schema just because we buy a new computer. So, what do we do here?
First Normal Form (1NF) Well, what we do is the same thing for a lot of these normalization steps. We'll take this data out of the Employee table, and put it in its own table called Computer.
First Normal Form (1NF) This then has relationships. We create a one-to-many relationship between Employee, and this new Computer, or it could be called an asset table or whatever else makes sense. And it has a foreign key back to the Employee table. I can take any EmployeeID like 551, follow it to the Computer table, and find his two computers or 553, find his three computers, there are no repeating values, no repeating groups in either table. And this will get us into first normal form.
First Normal Form (1NF) So, it's very common that the solution to a normalization issue is to create a new table. Sometimes, it's a one-to-many relationship like this, other times it might even require a many-to-many with a linking table.
Second Normal Form (2NF) Before you attempt to go into second normal form or 2NF, well first, you have to be in first normal form. You don't pick and choose between them. You go through this one two three. Now whereas first normal form is about the idea of repeating values in a particular column, second normal form, and third normal form are all about the relationship between your columns that are your keys, and your other columns that aren't your keys. The second normal form has the rather puzzling official description that any non-key field should be dependent on the entire primary key. And that is about as simple as it can get phrased.
Second Normal Form (2NF) Now, when I say the word field, it usually refers to the idea that the actual value in a particular column position for a particular row. But what does this actually mean? Well, for most of what we've done in this course, this actually won't be an issue for us. Second normal form is only ever a problem when we're using a Composite Primary Key. That is a primary key made of two or more columns. So, let me show you a table that currently is in first normal form but not in second normal form. Going back to the idea of a database for a training center, I have an Events table here that has an ID of a Course, a Date, CourseTitle, Room, Capacity, AvailableSeats, and so on. Now, what's actually happening here is this table has been defined to use two columns as the primary key. It's a composite primary key.
Second Normal Form (2NF) Now, the issue with second normal form is that if you use a composite key, you need to look closely at the other columns in this table. So, going along to my non-key columns, I have CourseTitle, SQL Fundamentals, Room 4A, Capacity is 12, there are 4 seats available. A lot of this information would be unique to this one entry, this one course on this particular date. That's fine. But second normal form asks that all of my non-key columns, everything that isn't part of the key, so Course Title, Room, Capacity, Available, they all have to be dependent on the entire primary key. Now, that is the case for Room and Capacity and Available. These are unique values based on the fact that we're running this particular Date, this particular Course, and this particular room with a certain number of seats available. It will always be different. But CourseTitle, well, I could get that just from half of the key. I could get that just from the first part of the key. It has no connection to the Date whatsoever. SQL Fundamentals will always be based on SQL101.
Second Normal Form (2NF) It doesn't matter if it's being run in March or April or May. Now, this might sound a little bit ivory tower. But here would be the impact. What happens if somebody reached into this table, and they changed that Course ID, but they didn't change the title? Now, we've got a conflict. We might have the wrong title for the wrong piece of data. That's because my data now isn't in second normal form, and we're trying to fix that conflict from ever happening. So, how do we fix it?
Second Normal Form (2NF) Well, once again, we're going to rip out the CourseTitle. We're going to create a separate Courses table, where we want to again map the ID of the course into its own row. So, we'll always have one specific title for one specific ID. And then we create a one-to-many relationship between Events and Course. And removing that from the Event table means that everything in that table is now based on the entire key, particular course, at a particular date which may have a different room or different capacity, different number of available seats.
Third Normal Form (3NF) Now, let's take a look at the third normal form. Well, as plainly as this can be described, it's that no non-key field, meaning, a column that is not part of the primary key is dependent on another non-key field. It is in a way similar to second normal form. Second normal form asks can I figure out any of the values in this row from just part of the composite key? While third normal form asks can I figure out any of the values in this row from any of the other values in this row? And I shouldn't be able to do that.
Third Normal Form (3NF) Let's take a look at an example. I've got this updated version of the Events and Courses table from the previous example. So, it's in both first normal form, it doesn't have any repeating values or repeating groups, and it's in second normal form. Meaning, there's no part of this that's dependent on just on a piece of the key. What I need to do for third normal form is look at my non-key fields, Room, Capacity, Availability. If I scan the entire row, let's take the first row, we've got SQL101 course occurring on the 1st of March. There is apparently 4 seats available. It's in Room 4A with a capacity of 12. Now, this is at a first look at it perfectly acceptable, because this course could be being scheduled in a different room every time with a different number of available seats as we start to sell different seats for a particular date. That's all okay.
Third Normal Form (3NF) Here's the problem. It's between Room and Capacity. These are both non-key fields. These columns aren't part of the primary key. But if I look down the column for Room, I see 4A has 12 seats capacity, 4A has 12 seats, 7B has 14 seats. So, if every time we're in Room 4A, we always have 12 seats or every time we're in 7B, we always have 14 seats. I don't need to repeat that information. I could figure out capacity from Room and Room alone. I have one non-key field that is based on another non-key field. So, we don't need these to be stored in the same table. What we need to do is, you guessed it, split some of this information out into its own table.
Third Normal Form (3NF) So, we need to pull out Capacity from the Event table, and just keep Room. And that's as long as Room will always tell us a fixed capacity, we'd create our own table for it, 4A always has 12, 7B always has 14, and so on. Now, we're in third normal form, no non-key field is dependent on another non-key field. Now, as you're seeing, it's all about the redundancy of the information. This is what we're trying to do with normalization.
Third Normal Form (3NF) Now, another example of third normal form would be something like this, which is very common. Let's say we've got an OrderItem table, which is calculating different parts of an invoice. So, it has a ProductID with a Quantity, a UnitPrice, and a Total.
Third Normal Form (3NF) Now, you don't have to worry about how this might relate to different tables. All I'm interested in looking at is this part. We've got Quantity for UnitPrice of $10, Total is $40. Here is the issue. We can see that Total is based purely as on Quantityx UnitPrice. Now, Quantity and UnitPrice are bothnon-key fields. So we're figuring out Total from these other two non-key fields. We don't need to do this. We don't need to store this in the database. We don't want to store information in your table that's easily ascertained by adding other non-key fields together, or in this case multiplying them. One of the main reasons for this is to prevent any conflicts. If in this example I have a row that says we have a Quantity of 4 and the UnitPrice of 10, but the Total says 50, well, where is the problem? There is a problem. How do we do it? How would we fix it?
Third Normal Form (3NF) Is the Total wrong or is the Quantity wrong? Your data doesn't make sense anymore. So, we would remove that Total column form this table. We can figure it out when we need to figure it out. Now, third normal form will help you figure out these potential problems. Now just a quick side bar, in cases like this where you might find a total useful in the table, many database systems offer you the option of defining a computed or calculated column. It's not actually stored in the database, it is a convenient read-only fiction. Its value is automatically calculated based on the other columns in the table, and you may find that useful from time to time.
Database Denormalization So, we should always take our database design through the first, second, and third normal forms. There are more criteria available. There are fourth, fifth, and sixth normal forms. There's something called Boyce-Codd normal form. But taking it to third normal form is the usual expectation in a business environment, and certainly all we need to cover in a course like this one. Now, you will actually find a lot of tables out there intentionally break normalization rules and some others seem like they do but they actually don't.
Denormalization Here's one example. Let's say we've got an Employee table, and I'm storing an Email and a Phone number. Technically, this can be described as breaking first normal form. It's a repeating group. But in practice, you may find it more convenient to just allow an Email and Email2 column or perhaps a HomePhone and MobilePhone column rather than splitting everything out into multiple tables and having to follow relationships every single time you read or write this data. This will be referred to as a de-normalization decision. You're consciously making the choice that something could be normalized out into another table. You could follow the official rules. But for convenience and/or for performance, you're not going to.
Denormalization Normalizing a table like this, thinking that I've immediately spotted a non-key field dependency, that would actually be taking it too far and making things more inconvenient. And the question is you really want to understand your data before you can make all these choices whether to normalize or de-normalize. And you might de-normalize to make things a bit more efficient, but do it knowingly instead of accidentally.
Denormalization These really the three steps that we would go through, first normal form, second normal form, and third normal form. First being about having no repeating values and no repeating groups, second normal form, no values based on just part of say half of a composite key, and third normal form, none of your non-key values should be based on or determined from another non-key value. Taking your database design through these three central criteria will vastly improve the quality of your data.
Denormalization one example that can seem like a normalization and/or de-normalization issue but really isn't any table that's full of address information. This situation can be a little deceptive. If I look at a table like this, and I can see I've got Zip code being stored as the last column here. Theoretically, I could figure out what the City, and the State are just from the Zip Code, if I separated them out into their own table. So technically, I have non-key fields, City and State that are dependent on another non-key field, Zip, that could be figured out from Zip alone. However, this kind of case is not the full story because while it might be true 99% of the time that a Zip code maps to a particular City or Town, there are some cases where multiple towns or cities are allowed in the same zip code, some Zip codes even cross multiple states.
