I'm taking a DBMS course and I need to design and build my own DB. I have a database for a hospital where doctors,nurses,support staff etc are in a ISA relationship to an Employee entity with the rest of the data like the name, address , salary and the rest of the employee data.
Designing a form, I want to be able to add an employee with all of their data in one form.
Is there a way to do a "conditional table" of sorts where if i select "doctor" from a drop-down i get to add to the doctor table too, and same for the rest of the entities under the ISA relationship?
Thx!
As a general rule, when dealing with data, you do NOT flip or switch tables for a given form or relatonal database design.
So, for example. If I have a table of customers. Well, now if I want to mark some of the customers as plumbers, and others as doctors? I don't create two tables.
All I would do is add ONE column to that customers table and it would simply allow me to set the type of customer. The reason for this design is "many" but some significant reasons are:
For each new type of customer, you would not create a new table. Worse, all of the forms, the reports, the SQL, the code you write? Well, all of that code would have to be modified EACH time you create a new table. So, you SIMPLY cannot adopt a design in which the concept of changing a table is part of that process.
Forms are bound to ONE table. For related data, you in most cases will use a sub form.
So, think of even a accounting system. They can have huge numbers of customers, and as a result, you can "query" that table to give you all customers. Or you might ask how many accounting firms are in the customer list. Or make a report that summeries by customer type a "count" of each type of customer.
So, buidling forms, or reports? They cannot on the fly "change" the tables they are using.
So, in place of a tables called:
SalesJan
SalesFeb
SalesMar
etc.
Well, now you can't query sales from Jan to mar, because the data is in different tables.
So, what you do is have ONE table called "sales", and you add ONE column of the date. Now, at the start of each new month, you don't have to create a new table.
Now, of course in some cases it makes sense to create a separate table. For example, a table of customers, and a table of employees in a database is just fine. It makes sense in this case to use two tables, since the information about a customer and what they can do and the kind of information is VERY different then how you would deal with employees.
So, with above? Well, if I need to print mailing labels for all customers and all employees? That would require two different reports. And very likely the table structure for the two tables is different.
Bottom line:
If you working on design or form or report? And you needing to try and change the table that the form/report/code etc is going to operate on? This is a sign that your design approach has gone complete off the rails and is the wrong design.
So, in the case of doctors, nurses etc.? Well, they are all hospital staff, and MOST of the basic information about such employees will be common, much the same, and thus a SINGLE table of "employees" makes the most sense. You would only need a nice "employee type" combo box on that one form, and thus you can add/enter/edit/search any employee in that one table.
The fact that you "want to search" for a employee show that all these people "are" employees and thus belong in one table. And the basic information about all employees is going to be the same anyway. If you find you are attempting to create a new table but with near identical structures over and over, then just like a new table for each month sales, or a new table for each new kind of employee? Simply add the "one" column that allows you to make that distinguish, and not a whole new table.
Now one COULD even attempt to put patients in the same table, but then again, dealing with patents as opposed employees is a considerable different kind of "thing".
So employees are employees - even different kinds. (manager, cleaning staff etc.).
And patients are patients - even different kinds (long term care, emergency etc.).
Related
I am currently going to be designing an app in vb.net to work with an access back-end database. I have been trying to think of ways to reduce down data redundancy
and I have an example scenario below:
Lets imagine, for an example purpose, I have a customers table and need to highlight all customers in WI and send them a letter. The customers table would
contain all the customers and properties associated with customers (Name, Address, Etc) so we would query for where the state is "WI" in the table. Then we would
take the results of that data, and append it into a table with a "completion" indicator (So from 'CUSTOMERS' to say 'WI_LETTERS' table).
Lets assume some processing needs to be done so when its completed, mark a field in that table as 'complete', then allow the letters to be printed with
a mail merge. (SELECT FROM 'WI_LETTERS' WHERE INDICATOR = COMPLETE).
That item is now completed and done. But lets say, that every odd year (2013) we also send a notice to everyone in the table with a state of "WI". We now query the
customers table when the year is odd and the customer's state is "WI". Then append that data into a table called 'notices' with a completion indicator
and it is marked complete.
This seems to keep the data "task-based" as the data is based solely around the task at hand. However, isn't this considered redundant data? This setup means there
can be one transaction type to many accounts (even multiple times to the same account year after year), but shouldn't it be one account to many transactions?
How is the design of this made better?
You certainly don't want to start creating new tables for each individual task you perform. You may want to create several different tables for different types of tasks if the information you need to track (and hence the columns in those tables) will be quite different between the different types of tasks, but those tables should be used for all tasks of that particular type. You can maintain a field in those tables to identify the individual task to which each record applies (e.g., [campaign_id] for Marketing campaign mailouts, or [mail_batch_id], or similar).
You definitely don't want to start creating new tables like [WI_letters] that are segregated by State (or any client attribute). You already have the customers' State in the [Customers] table so the only customer-related attribute you need in your [Letters] table is the [CustomerID]. If you frequently want to see a list of Letters for Customers in Wisconsin then you can always create a saved Query (often called a View in other database systems) named [WI_Letters] that looks like
SELECT * FROM Letters INNER JOIN Customers ON Customers.CustomerID=Letters.CustomerID
WHERE Customers.State="WI"
I'm looking at this company's database design, and would like to know the purpose of their design, ie store relationship in one table and the data in another, why do this?
They have this,
EMPLOYEE
Id (PK)
DepartmentId
EMPLOYEE_DATA
EmployeeId (PK)
First Name
Last Name
Position
etc...
Rather than this...
EMPLOYEE
Id (PK)
DepartmentId
First Name
Last Name
Position
etc...
...OR this...(employee can belong to many departments)
EMPLOYEE
Id (PK)
First Name
Last Name
etc...
EMPLOYEE_DEPARTMENT
Id
EmployeeId
DepartmentId
Position
That's a link table, or join table, or cross table.. lots of different names.
How would you assign an employee to two different departments with your design? You can't. You can only assign them to one.
With their design, they can assign the same ID to multiple departments by creating multiple records with the employee ID and different department ID's.
EDIT:
You need to be more specific about what you're asking. Your first question seemed to be asking what the purpose of mapping table was. Then you changed it, then you changed it again.. none of which makes much sense.
It seems now that you are asking what the better design is, which is a totally different question than what you originally asked. Please state specifically what question you want answered so we don't have to guess.
EDIT2:
Upon re-reading, if this is the actual design, then no.. It does not support multiple department id's. Such a design makes little sense, except for one situation. If the original design did not include a department, this would allow them to add a department ID without modifying the original EMPLOYEE_DATA table.
They may not have wanted to update legacy code to support the Employee id, so they added it this way.
Purpose of design is determined by business rules.
Business rules dictate entity (logical model perspective) / table (physical model perspective) design. No design is "bad" if it is built according to the requirements that were determined based on business rules. Those rules can however change over time -- foreseeing such changes and building to accommodate/future-proof the data model can really save time, effort and ultimately money.
The first and third example are the same -- the third example has an extraneous column (EMPLOYEE_DEPARTMENT.id). ORMs don't like composite keys, so a single column is used in place of.
The second example is fine if:
employees will never work for more than one department
there's no need for historical department tracking
Conclusion:
The first/third example is more realistic for the majority of real-world situations, and can be easily customized to provide more value without major impact (re-writing entire table structure). It uses what is most commonly referred to as a many-to-many relationship to allow many employees to relate to many departments.
If an employee can be in more than one department, then you would need a mapping table but I'd do it like the following:
EMPLOYEE
Id (PK)
First Name
Last Name
DEPARTMENT
Id (PK)
Name
EMPLOYEE_DEPARTMENT
EmployeeId_fk (PK)
DepartmentId_fk (PK)
Position
This would allow for multiple positions in multiple departments.
You would do this if an employee can be a member of multiple departments. With the latter table, each employee can only belong to one department.
The only remotely good reason for doing this is to implement an extension model where the master table identifying unique customers does not include all the data for customers that is not always necessary. Instead, you create one core table with the core employee data and and extension table with all the supplementary fields. I've seen people take this approach to avoid creating large tables with many columns that are rarely needed. However, in my experience it's typically premature optimization, and I wouldn't recommend it.
In contrast to many responses, the model included does not support multiple departments per employee - it is not a many to many mapping approach.
I am working on a VB.net (VS-2010, Win XP Pro 2 SP3), Employee Management Project. I need to keep track of Employee Leave Attendance and also each Equipment assigned to an Employee. How can I achieve this using SQLlite.
It will be very useful if you could provide me with examples as I am completely new to the field of SQL and VB.net
I think this can be done with two tables where one has the primary key while the other has a foreign key, but I am not sure. Also how many tables will I need for storing data in Leave and Equipment Form.
I went through other questions but I was unable to figure out a solution for my problem.
(Sorry, I cannot provide with images as this site prevents me from posting images without 10 reps)
Most problems are only as complex, and as simple as you make them. Out of habbit, nearly all tables end up with a unique ID field. There are exceptions, which I will call "link" tables, eg, ones that provide connection details between two data tables.
Now, in your senario
You would need a "holiday" table, where each row will contain the employee unique ID and either a start/finish date, eg, if they take half a day, it needs to be visible, or, just a year and value, eg in 2011, I booked, 2 lots of 35 hours, and 1 lot of 4 hours eg, Ive taken 2 weeks and half a day.
For the equipment, you would need a data table, since an item can only got to 1 employee, it depends if you're going to use this for booking or not, but if its just like a library, eg I currently have a loaner laptop, then you can just have an employee field in the equipment table. If you need a booking system, then you would require link tables and more complex.
Best way to work out your tables is to try and group your data, and then write the items on peices of paper and see how you as a human do it. After a while you end up able to do so in your head.
I am working on a project where there are several types of users (students and teachers). Currently to store the user's information, two tables are used. The users table stores the information that all users have in common. The teachers table stores information that only teachers have with a foreign key relating it to the users table.
users table
id
name
email
34 other fields
teachers table
id
user_id
subject
17 other fields
In the rest of the database, there are no references to teachers.id. All other tables who need to relate to a user use users.id. Since a user will only have one corresponding entry in the teachers table, should I just move the fields from the teachers table into the users table and leave them blank for users who aren't teachers?
e.g.
users
id
name
email
subject
51 other fields
Is this too many fields for one table? Will this impede performance?
I think this design is fine, assuming that most of the time you only need the user data, and that you know when you need to show the teacher-specific fields.
In addition, you get only teachers just by doing a JOIN, which might come in handy.
Tomorrow you might have another kind of user who is not a teacher, and you'll be glad of the separation.
Edited to add: yes, this is an inheritance pattern, but since he didn't say what language he was using I didn't want to muddy the waters...
In the rest of the database, there are no references to teachers.id. All other tables who need to relate to a user
use users.id.
I would expect relating to the teacher_id for classes/sections...
Since a user will only have one corresponding entry in the teachers table, should I just move the fields from the teachers table into the users table and leave them blank for users who aren't teachers?
Are you modelling a system for a high school, or post-secondary? Reason I ask is because in post-secondary, a user can be both a teacher and a student... in numerous subjects.
I would think it fine provided neither you or anyone else succumbs to the temptation to reuse 'empty' columns for other purposes.
By this I mean, there will in your new table be columns that are only populated for teachers. Someone may decide that there is another value they need to store for non-teachers, and use one of the teacher's columns to hold it, because after all it'll never be needed for this non-teacher, and that way we don't need to change the table, and pretty soon your code fills up with things testing row types to find what each column holds.
I've seen this done on several systems (for instance, when loaning a library book, if the loan is a long loan the due date holds the date the book is expected back. but if it's a short loan the due date holds the time it's expected back, and woe betide anyone who doesn't somehow know that).
It's not too many fields for one table (although without any details it does seem kind of suspicious). And worrying about performance at this stage is premature.
You're probably dealing with very few rows and a very small amount of data. You concerns should be 1) getting the job done 2) designing it correctly 3) performance, in that order.
It's really not that big of a deal (at this stage/scale).
I would not stuff all fields in one table. Student to teacher ratio is high, so for 100 teachers there may be 10000 students with NULLs in those 17 fields.
Usually, a model would look close to this:
I your case, there are no specific fields for students, so you can omit the Student table, so the model would look like this
Note that for inheritance modeling, the Teacher table has UserID, same as the User table; contrast that to your example which has an Id for the Teacher table and then a separate user_id.
it won't really hurt the performance, but the other programmers might hurt you if you won't redisign it :) (55 fielded tables ??)
Consider a database with tables Products and Employees. There is a new requirement to model current product managers, being the sole employee responsible for a product, noting that some products are simple or mature enough to require no product manager. That is, each product can have zero or one product manager.
Approach 1: alter table Product to add a new NULLable column product_manager_employee_ID so that a product with no product manager is modelled by the NULL value.
Approach 2: create a new table ProductManagers with non-NULLable columns product_ID and employee_ID, with a unique constraint on product_ID, so that a product with no product manager is modelled by the absence of a row in this table.
There are other approaches but these are the two I seem to encounter most often.
Assuming these are both legitimate design choices (as I'm inclined to believe) and merely represent differing styles, do they have names? I prefer approach 2 and find it hard to convey the difference in style to someone who prefers approach 1 without employing an actual example (as I have done here!) I'd would be nice if I could say, "I'm prefer the inclination-towards-6NF (or whatever) style myself."
Assuming one of these approaches is in fact an anti-pattern (as I merely suspect may be the case for approach 1 by modelling a relationship between two entities as an attribute of one of those entities) does this anti-pattern have a name?
Well the first is nothing more than a one-to-many relationship (one employee to many products). This is sometimes referred to as a O:M relationship (zero to many) because it's optional (not every product has a product manager). Also not every employee is a product manager so its optional on the other side too.
The second is a join table, usually used for a many-to-many relationship. But since one side is only one-to-one (each product is only in the table once) it's really just a convoluted one-to-many relationship.
Personally I prefer the first one but neither is wrong (or bad).
The second would be used for two reasons that come to mind.
You envision the possibility that a product will have more than one manager; or
You want to track the history of who the product manager is for a product. You do this with, say a current_flag column set to 'Y' (or similar) where only one at a time can be current. This is actually a pretty common pattern in database-centric applications.
It looks to me like the two model different behaviour. In the first example, you can have one product manager per product and one employee can be product manager for more than one product (one to many). The second appears to allow for more than one product manager per product (many to many). This would suggest the two solutions are equally valid in different situations and which one you use would depend on the business rule.
There is a flaw in the first approach. Imagine for a second, that the business requirements have changed and now you need to be able to set 2 Product Manager to a product. What will you do? Add another column to the table Product? Yuck. This obviously violates 1NF then.
Another option the second approach gives is an ability to store some attributes for a certain Product Manager <-> Product relation. Like, if you have two Product Manager for a product, then you can set one of them as a primary...
Or, for example, an employee can have a phone number, but as a product manager he/she can have another phone number... This also goes to the special table then.
Approach 1)
Slows down the use of the Product table with the additional Product Manager field (maybe not for all databases but for some).
Linking from the Product table to the Employee table is simple.
Approach 2)
Existing queries using the Product table are not affected.
Increases the size of your database. You've now duplicated the Product ID column to another table as well as added unique constraints and indexes to that table.
Linking from the Product table to the Employee table is more cumbersome and costly as you have to ink to the intermediate table first.
How often must you link between the two tables?
How many other queries use the Product table?
How many records in the Product table?
in the particular case you give, i think the main motivation for two tables is avoiding nulls for missing data and that's how i would characterise the two approaches.
there's a discussion of the pros and cons on wikipedia.
i am pretty sure that, given c date's dislike of this, he defines relational theory so that only the multiple table solution is "valid". for example, you could call the single table approach "poorly typed" (since the type of null is unclear - see quote on p4).