EmployeeNumber FirstName LastName Department Email Phone
1 Jerry Johnson Finance JJohnson#yahoo.com 555-5555
1 Jerry Johnson Accounting JJohnson#yahoo.com 555-4444
2 Mary Liz Accounting MLiz#gmail.com 555-4444
is the above table a relational table or nonrelational table?
One of the characteristics of a relation is NO TWO ROWS MAY HOLD IDENTICAL SETS OF DATA VALUES
Jerry Johnson, employee number 100, works both in the accounting and finance department. So he has the same email but different phone number because he works in different departments. I hope that makes sense. Is this a relation? Every value is identical except the Department and Phone number. Even if his phone number was the same (let's say it's his cell phone number and he only has one cell phone,555-5555), his department would still be different. So his EmployeeNumber, FirstName, LastName, Email, and Phone is the same. But department is different because he works in 2 different departments.
So the rule says NO TWO ROWS MAY HOLD IDENTICAL SETS OF DATA VALUES, well... in this case, it's almost identical except for the Department and (phone number too). Or would this be considered repeating because the EmployeeNumber is the same.
I hope I'm making sense. Please help me understand.
also, is this in first normal form?
Related
I have a data set which has 4420 attendances to a medical department from 1120 people. Each person has a unique ID number and other columns are demographics and primary care provider. I want to filter the data so I can work out how many times each person attends the department and then analyse the data by demographics eg primary care provider or age. It shows whether each attendance is primary or duplicate but I can't figure out how to work out attendances per person.
If what you want to do is to count the number of times each person has visited (assuming each one is represented by a single row in the data), use the AGGREGATE command breaking on the ID variable to add the number of instances to the file as a new variable. In the menus, Data>Aggregate, move the ID variable into the box for Break Variable(s), check the box for Number of cases under Aggregated Variables, change the default N_BREAK to another name if you want, and click OK. That will add a new variable to the data with the number of instances for each unique ID.
Im new to Rails and I'm in the middle of sketching up an ERD for my new app. A Yelp-sort of app, where a Client is sorted by price.
So I want one Client to have many priceranges - One Client can both have pricerange $ and Pricerange $$$$ for example. The priceranges are:
$ - $$ - $$$ - $$$$ - $$$$$
How would this look in a table? Would I create a table called PriceRange with Range1, Range2, Range3, Range4, Range5 to be booleans?
Doesn't the PriceRange-table need any foreign/primary keys?
PriceRange
Range1 (Boolean)
Range2 (Boolean)
Range3 (Boolean)
Range4 (Boolean)
Range5 (Boolean)
Look, I'm Brazilian and I'm not very knowledgeable about yelp applications. I do not quite know what it is, but from what I saw, they are systems to assess/measure/evaluate (perhaps the translation is wrong here for you) things, in this case, companies, right?
Following this logic, let's think...
By the description of your problem (context), you have clients (companies), and they can have price ranges, correct? If:
A price interval is represented by textual names, such as "$", "$$",
and so on,
and the same price range may have (numeric) values for different companies,
And the same price range (type) can be (or not) assigned to different
companies,
Then here is what we have:
By decomposing this conceptual model, you would end up with three tables:
Companies
Price Ranges
Price Ranges from Companies
The primary keys of Company and Price Ranges will be passed to Price Ranges from Companies as foreign keys. You can use them as a composite primary key, or use a surrogate key. If using a surrogate key, you will permit/allow a company to have the same kind of price range more than once, which I believe is not the case.
Let's look at another situation, if things are simpler as:
If there is no need to store prices,
and an company may have or not one or more price ranges represented by "$", "$$", and so on,
Then here is what we have:
Similarly, we'll have the same 3 tables. Likewise, you still must pass the primary keys of Companies and Price Ranges to Price Ranges from Companies as foreign keys.
So I want one Client to have many priceranges - One Client can both
have pricerange $ and Pricerange $$$$ for example
Notice how N-N relationships allow us to create optional relationships between entities. This will allow a company to have zero, one, two, (etc.) or all price ranges defined. Again, so that is not allowed a company to have a price range more than once, set the foreign keys as composite primary key in Price Ranges from Companies.
If you have any questions or anything I explained has nothing to do with your context, please do not hesitate to comment.
EDIT
Is the Price ranges from companies what is called a Joint table?
Yes. There are also other terms used, some in different areas of computer science, such as Link Table, or Intermediate Table.
Actually we do not have a table here in the diagram, but an entity. In the Conceptual Model there are no tables, but entities and relationships. Be careful with this terminology when developing the Conceptual Model, or else you may get confused (I say this from experience).
However, yes, once decomposed, we will have a table from this relationship. When decomposed, N-N relationships will always become tables, no exception. Differently, 1-1 and 1-N (or N-1) relationships do not become tables. These tables with these special names (Join/Link/Intermediate Tables) serves to associate records from different tables, hence the name.
And is it necessary to have a column called Price Range Id? I mean
what is it there for?
At where? If you say at the Price Ranges entity, it is rather necessary. Must We not identify records in a table in some way? Here I set what is called a Surrogate Key. If on the other hand, you have a column with unique values for each record in the table, you can also use this column. I highly recommend that you consider the use of surrogate keys. Read the link I gave you.
In the Conceptual Model, we have to define the properties and also the primary keys. During the phase of the conceptual model, natural attributes of entities can become primary keys if you so desire. In this case, we have what is called a Natural Key.
If on the other hand you refer to Price Ranges from Companies entity, so the question is another ("And is it necessary to have a column called Price Range Id?"). Here we have a table with two columns, as I told you. The two are foreign keys. You need it so you can relate rows from the two tables... I think you were not referring to that, is not it? If so, no problem, you can comment and ask more questions. I do not care to answer. To be honest, I did not quite understand your question.
EDIT 2
So that Company 28 can be identified in the Price Ranges (for instance
ID 40) Which would make it easier to call out the price ranges it has?
Maybe my English is not very good, but it seems to me that you have a beginner's doubt/question in relation to the concept of tables and relationships between them. If not that, I apologize because maybe I did not understand. But let's see...
The tables in a database have rows / records. Each line has its own data. Even with this, each line / record needs to be differentiated and identified somehow. That is why we attach to each line an identifier, known as the primary key (this, and this). In summary, the primary key is how we identify, differentiate, separate and organize different records.
Even if all records have different values, you must select a field (column) that represents the primary key of the table. By obligation, every record MUST have a primary key. Although you can choose which field is a primary key, you are allowed to choose one or more fields to serve as the primary key. When this happens, that is, when more than one field participates/serves as the primary key, we have a table with something called Composite Primary Key. Similarly, it has the ability to identify records. Note that, because of that, primary key values must be unique, otherwise you may have 2 identical records.
This is the basic concept so that we can relate tables to each other, in case, records/rows of tables together. If we have a Company identified by the ID 28 (a line/record), and we want to relate it to a Price Range identified by the ID 40, then we need to store somewhere that relationship (28 <--> 40). This is where the role of intermediate/link/join tables comes in (but only to relationships N-N! For 1-N or N-1 relationships it works similarly, but not identical).
My original question was whether it was necessary, and why a company
ID had to link up with a price range ID at all.
With this table storing records which relates to other records (for their primary keys), we can perform a SQL join operation (If you have questions about this, see this image). Depending on how you perform this operation, you'll get:
All companies that have Price Ranges.
All companies that do not have Price Ranges.
All the Price Ranges of a given company.
All companies that have or not a X Price Range.
All price ranges that are given or not to companies.
...
Anyway, you get all this because of the established relationship.
If it could just be taken out and then the table of price ranges would
only involve Pricerange1-5.
This sentence I did not understand. What should be taken out? Could you please explain this sentence better?
I hesitate whether I should add a Country_Dimension or not since I already have a Customer_Dimension which contain some redundant fields such as:
continent_name
country_name
postcode_#
Those could be two completely different things. A country name present in your Customer_Dimension will probably represent the country of an address your customer is using. Most likely the country he's living (or has lived) in. It can change over time since customers can switch addresses.
A dimension representing Countries will do exactly that, it represents countries. I think you first have to decide what the use of your dimension should be.
I'm new to datawarehousing and I have a star schema with a contract fact table. It holds basic contract information like Start date, end date, amount ...etc.
I have to link theses facts to a customer dimension. there's a maximum of 4 customers per contract. So I think that I have two options either I flatten the 4 customers into one row for ex:
DimCutomers
name1, lastName1, birthDate1, ... , name4, lastName4, birthDate4
the other option from what I've heard is to create a bridge table between the facts and the customer dimension. Thus complexifying the model.
What do you think I should do ? What are the advantages / drawbacks of each solution and is there a better solution ?
I would start by creating a customer dimension with all customers in it, and with only one customer per row. A customer dimension can be a useful tool by itself for CRM and other purposes and it means you'll have a single, reliable list of customers, which makes whatever design you then implement much easier.
After that it depends on the relationship between the customer(s) and the contract. The main scenarios I can think of are that a) one contract has 4 customer 'roles', b) one contract has 1-4 customers, all with the same role, and c) one contract has 1-n customers, all with the same role.
Scenario A would be that each contract has 4 customer roles, e.g. one customer who requested the contract, a second who signs it, a third who witnesses it and a fourth who pays for it. In that case your fact table will have one row per contract and 4 customer ID columns, each of which references the customer dimension:
...
RequesterCustomerID int,
SignatoryCustomerID int,
WitnessCustomerID int,
BillableCustomerID int,
...
Of course, if one customer is both a requester and a witness then you'll have the same ID in both RequesterCustomerID and WitnessCustomerID because you only have one row for him in your customer dimension. This is completely normal.
Scenario B is that all customers have the same role, e.g. each contract has 1-4 signatories. If the number of signatories can never be more than 4, and if you're very confident that this will 'always' be true, then the simple solution is also to have one row per contract in the fact table with 4 columns that reference the customer dimension:
...
SignatoryCustomer1 int,
SignatoryCustomer2 int,
SignatoryCustomer3 int,
SignatoryCustomer4 int,
...
Even if most contracts only have 1 or 2 signatories, it's not doing much harm to have 2 less frequently used columns in the table.
Scenario C is where one contract has 1-n customers, where n is a number that varies widely and can even be very large (class action lawsuit?). If you have 50 customers on one contract, then adding 50 columns to the fact table becomes difficult to manage. In this case I would add a bridge table called ContractCustomers or whatever that links the fact table with the customer dimension. This isn't as 'neat' as the other solutions, but a pure star schema isn't very good at handling n:m relationships like this anyway.
There may also be more complex cases, where you mix scenarios A and C: a contract has 3 requesters, 5 signatories, 2 witnesses and the bill is split 3 ways between the requesters. In this case you will have no choice but to create some kind of bridge table that contains the specific customer mix for each contract, because it simply can't be represented cleanly with just one fact and one dimension table.
Either way can work but each solution has different implications. Certainly you need customer and contract tables. A key question is: is it always a maximum of four or may it eventually increase beyond that? If it will stay at 4, then you can have a repeating group of 4 customer IDs in the contract. The disadvantage of this is that it is fixed. If a contract does not have four, there are some empty spaces. If, however, there might be more than 4, then the only viable solution is to use a bridge table because in a bridge table you add more customers by inserting new rows (with no change to the table structure). In the fixed solution, in this case you add more than 4 customers by altering the table. A bridge table is an example of what, for many decades now, ER modeling has called an associative entity. It is the more flexible of the two solutions. However, I worked on a margin system once wherein large margin amounts needed five levels of manager approval. It has been five and will always be five, they told me. Each approving manager represented a different organizational level. In this case, we used a repeating group of five manager IDs, one for each level, and included them in the trade. So it is important to understand the current business rules and the future outlook.
So I am little confused on how to store a list of phone numbers in one column. Here are the requirements:
Users can have more than one phone number.
Phone numbers have to be unique, so if user A added a phone number that is used by user B then a validation error should be displayed.
A default phone number should be selected if the user have more than one phone number.
Solution have to be compatible with Postgresql.
I thought up of four possible solutions:
HStore: making a phone_number field that stores a hash of all the phone numbers. e.g. {1=>"+1-800-123-1234", 2=>"9237492734", "default"=>1}. In this case I need to make lot of queries to make sure that a new phone number is unique, for example I need to query User.where("phone_number #> ('1' => '+1-800-123-1234')") then check in 2 User.where("phone_number #> ('2' => '+1-800-123-1234')") ... etc.
Array of phone numbers in one field: phone_number will store comma separated phone numbers like "+1-800-123-1234,9237492734". Checking a existing phone number would be easy User.where("phone_number LIKE '%+1-800-123-1234%'") but will take a lot of time for the database to pick it up. default_phone will be added to the table as well or making the first phone number as the default one by convention.
Limiting phone numbers to 3 (which is enough) and creating phone_number_1, phone_number_2 and phone_number_3 fields. Checking the uniqueness of a phone number will consume 3 queries. Also will require adding default_phone.
Adding a new table phone_numbers (id:integer, user_id:integer, phone_number:string, default:boolean) and setting has_many relationship with User model. Which is not really seducing ... create a whole table for 1 field. But it have fast lookups and will have limitless phone numbers for each user.
Any ideas, hints and suggestions are greatly appreciated.
Two tables is the solution to go after. You can potentially have multiple users who can be reached at the same phone number, such as a work number or home number that are landlines.
CREATE TABLE users (
id SERIAL PRIMARY KEY,
name TEXT NOT NULL,
-- other bits of information
);
CREATE TABLE phone_numbers (
user_id INTEGER REFERENCES users (id),
phone_number TEXT NOT NULL,
location TEXT NOT NULL, -- Mobile, home, work
PRIMARY KEY (user_id, phone_number),
INDEX (phone_number)
);
If you really want to enforce the 'each person has a unique phone number and that phone number cannot be used to contact anyone else', just add a UNIQUE constraint to the phone_number column.