Can a table contain only a primary key and 1 or more foreign keys? Or it will violate the normalization design?
For example:
PK: SKILL_NAME
FK: SKILL_ID
FK: EMPL_ID
Yes. This would be the typical structure, for instance, for an association/junction table that implements a n-m relationship between two entities.
That said, almost all tables I create also have:
createdAt -- insertion time for the record
createdBy -- who inserted the record
And sometimes:
createdOn -- the system/database where the record was created
Your example may not be a good one because i don't understand the scenario that skill_name is the PK, instead of skill_id.
What you mentioned is actually similar to EAV data model, i see some open source produce (magento) using it. But it's too normalized and need denormalized caching table for the performance.
I'm developing a system for a retailer and I've hit a bit of a conundrum when it comes to deciding how to represent the orders in the database. The schema for my Order table so far is as follows:
Id - PK
AccountId - FK (Nullable)
ShippingAddressId - FK (Nullable)
BillingAddressId - FK (Nullable)
ShippingMethod - (Nullable)
Type - (Nullable)
Status
Date
SubTotal
Tax
Total
My problem is I'm not sure whether I should represent online purchases and in-store purcahses in separate tables or not. If I were to store them in the same table, all non-nullable fields would be the only ones applicable for in-store purchases.
Another design pattern that crossed my mind is something like this:
Online order table:
PurchaseId - PK, FK
AccountId - FK
ShippingAddressId - FK
BillingAddressId - FK
ShippingMethod
Type
Purchase table:
Id - PK
Status
Date
SubTotal
Tax
Total
And for in-store purchases, there would simply be no reference from the online orders table.
Thoughts?
I would make a second table for location, with a primary key and location information. That could be online as well. Then use a foriegn key in your main table. You would then just fill the fields require for the application you are doing(in store, or online). This would also allow For the business to grow to more locations just by simply adding it into the location table.
I'm going with the original design. Likely more maintainable and efficient as well.
Your second design is very close to an Entity Sub-typing pattern. If the primary key of your online order table was the foreign key to your purchase table then you would have entity sub-typing.
Your original design is a practical design for the physical implementation of your database because it is simple to use. Entity sub-typing would be the preferred design at the logical level because it clearly represents your rules about which predicates (columns) belong to which logical tables.
Some people would also use the entity sub-typing pattern for their physical model too because they have an aversion to nulls.
I'm designing a database which will store information about some artists. These artists can belongs to one or more organizations. From these organizations i just want to store their names and i'm thinking in create a table with these organizations which just have the names as primary key and nothing else. Is the fact of having a table with just the fieds of the primary key a conceptual error? In this case, i will appreciate some suggestions to solve that.
Is the fact of having a table with just the fieds of the primary key a conceptual error?
Not by itself. There are perfectly legitimate situations where all fields comprise a PK.
In this particular case, the organization name is a key, but that doesn't necessarily mean it should be primary key - you could "invent" another key that is smaller (typically integer) and easier to maintain and make it primary, like this:
The organizarion_id is called a "surrogate key", and some pros of doing that include:
Child FKs will be slimmer (since only the integer is migrated to the child, not the whole string).
You can update the organization_name without updating the organization_id, and consequently without cascading this update to children.
A small integer surrogate may be friendlier to ORMs than a more complex natural key.
Cons:
May require more JOINing.
Requires one more index, and each additional index brings overhead (even in heap-based tables, but especially in clustered tables).
As you can see, it's a matter of balance and you are the only one who has enough domain knowledge to make the right decision.
NOTE: Order of fields in organization_artist matters. Use the order shown above if you need to efficiently query for artists of a given organization and reverse it if you need organizations of a given artist. If you need both directions, you'll need another composite index on these two fields (beside the index underneath PK), but in opposite order. If you can live with only one index, consider clustering this table (if your DBMS supports it).
You want an OrganizationId, to handle the situations where the Organization name changes.
You might also have situations where different organizations seem to have the same name. How many "Museum of Modern Art"s are there? (Well, to a New Yorker, only one ;-)
Your organization table might expand over time, with columns such as shortname, address, contact person, prefered language, and so on. So, the table should look like:
create table Organizations (
OrganizationId int not null identity(1,1),
Name varchar(255),
CreatedBy varchar(255) default system_user,
CreatedAt datetime default getdate()
)
In a mature database, you would even recognize that organizations change names, merge, and sometimes split. You can handle this by adding effective dates and end dates to the records.
The standard practice for something like this would be to have 1 table for the artists, 1 table for the organizations, and 1 association table to associate the artist with 1 or more organization.
ARTIST (id, firstName, lastName)
ORGANIZATION (id, name)
ARTIST_ORGANIZATION(artist_id, org_id)
Even though the organization name may/should be unique, it's good to have a numeric id as the primary key so you can do associations. And querying the association with id is better performance than searching for a string.
I have two tables: Companies and Employees.
I also have a relation table Employs which contains the foreign keys company_id, employee_id as a composite primary key. Note: employees can work at multiple companies.
I would like to have another table EmployeeSchedules which simply contains schedules (company_id:integer,employee_id:integer,start_time:datetime, end_time:datetime) for employees working at a company. This will end up being displayed in a calendar widget of some sort.
However, with this design I would have to verify at the application level that the employee actually works at the company before adding a schedule.
I was wondering if there would be a better way to represent this at the database level or just stick with verifying at the application level? For example, if there was a way to link the EmployeeSchedules pkey (company_id,employee_id) with the Employs pkey (company_id, employee_id). Any other design recommendations are welcome!
I would re-define the schema, and add another table:
Person(id, name)
Company(id);
Employee(id, companyId, personId);
Schedules(id, employeeId, startTime, endTime);
That means a an employee record can only be bound to one company. A person can have multiple employee records however. All the "id" columns are unique, and are the primary key of the table. "companyId" refers to the primary key of the company table and so on.
I have three basic types of entities: People, Businesses, and Assets. Each Asset can be owned by one and only one Person or Business. Each Person and Business can own from 0 to many Assets. What would be the best practice for storing this type of conditional relationship in Microsoft SQL Server?
My initial plan is to have two nullable foreign keys in the Assets table, one for People and one for Businesses. One of these values will be null, while the other will point to the owner. The problem I see with this setup is that it requires application logic in order to be interpreted and enforced. Is this really the best possible solution or are there other options?
Introducing SuperTypes and SubTypes
I suggest that you use supertypes and subtypes. First, create PartyType and Party tables:
CREATE TABLE dbo.PartyType (
PartyTypeID int NOT NULL identity(1,1) CONSTRAINT PK_PartyType PRIMARY KEY CLUSTERED
Name varchar(32) CONSTRAINT UQ_PartyType_Name UNIQUE
);
INSERT dbo.PartyType VALUES ('Person'), ('Business');
SuperType
CREATE TABLE dbo.Party (
PartyID int identity(1,1) NOT NULL CONSTRAINT PK_Party PRIMARY KEY CLUSTERED,
FullName varchar(64) NOT NULL,
BeginDate smalldatetime, -- DOB for people or creation date for business
PartyTypeID int NOT NULL
CONSTRAINT FK_Party_PartyTypeID FOREIGN KEY REFERENCES dbo.PartyType (PartyTypeID)
);
SubTypes
Then, if there are columns that are unique to a Person, create a Person table with just those:
CREATE TABLE dbo.Person (
PersonPartyID int NOT NULL
CONSTRAINT PK_Person PRIMARY KEY CLUSTERED
CONSTRAINT FK_Person_PersonPartyID FOREIGN KEY REFERENCES dbo.Party (PartyID)
ON DELETE CASCADE,
-- add columns unique to people
);
And if there are columns that are unique to Businesses, create a Business table with just those:
CREATE TABLE dbo.Business (
BusinessPartyID int NOT NULL
CONSTRAINT PK_Business PRIMARY KEY CLUSTERED
CONSTRAINT FK_Business_BusinessPartyID FOREIGN KEY REFERENCES dbo.Party (PartyID)
ON DELETE CASCADE,
-- add columns unique to businesses
);
Usage and Notes
Finally, your Asset table will look something like this:
CREATE TABLE dbo.Asset (
AssetID int NOT NULL identity(1,1) CONSTRAINT PK_Asset PRIMARY KEY CLUSTERED,
PartyID int NOT NULL
CONSTRAINT FK_Asset_PartyID FOREIGN KEY REFERENCES dbo.Party (PartyID),
AssetTag varchar(64) CONSTRAINT UQ_Asset_AssetTag UNIQUE
);
The relationship the supertype Party table shares with the subtype tables Business and Person is "one to zero-or-one". Now, while the subtypes generally have no corresponding row in the other table, there is the possibility in this design of having a Party that ends up in both tables. However, you may actually like this: sometimes a person and a business are nearly interchangeable. If not useful, while a trigger to enforce this will be fairly easily done, the best solution is probably to add the PartyTypeID column to the subtype tables, making it part of the PK & FK, and put a CHECK constraint on the PartyTypeID.
The beauty of this model is that when you want to create a column that has a constraint to a business or a person, then you make the constraint to the appropriate table instead of the party table.
Also, if desired, turning on cascade delete on the constraints can be useful, as well as an INSTEAD OF DELETE trigger on the subtype tables that instead delete the corresponding IDs from the supertype table (this guarantees no supertype rows that have no subtype rows present). These queries are very simple and work at the entire-row-exists-or-doesn't-exist level, which in my opinion is a gigantic improvement over any design that requires checking column value consistency.
Also, please notice that in many cases columns that you would think should go in one of the subtype tables really can be combined in the supertype table, such as social security number. Call it TIN (taxpayer identification number) and it works for both businesses and people.
ID Column Naming
The question of whether or not to call the column in the Person table PartyID, PersonID, or PersonPartyID is your own preference, but I think it's best to call these PersonPartyID or BusinessPartyID—tolerating the cost of the longer name, this avoids two types of confusion. E.g., someone unfamiliar with the database sees BusinessID and doesn't know this is a PartyID, or sees PartyID and doesn't know it is restricted by foreign key to just those in the Business table.
If you want to create views for the Party and Business tables, they can even be materialized views since it's a simple inner join, and there you could rename the PersonPartyID column to PersonID if you were truly so inclined (though I wouldn't). If it's of great value to you, you can even make INSTEAD OF INSERT and INSTEAD OF UPDATE triggers on these views to handle the inserts to the two tables for you, making the views appear completely like their own tables to many application programs.
Making Your Proposed Design Work As-Is
Also, I hate to mention it, but if you want to have a constraint in your proposed design that enforces only one column being filled in, here is code for that:
ALTER TABLE dbo.Assets
ADD CONSTRAINT CK_Asset_PersonOrBusiness CHECK (
CASE WHEN PersonID IS NULL THEN 0 ELSE 1 END
+ CASE WHEN BusinessID IS NULL THEN 0 ELSE 1 END = 1
);
However, I don't recommend this solution.
Final Thoughts
A natural third subtype to add is organization, in the sense of something that people and businesses can have membership in. Supertype and subtype also elegantly solve customer/employee, customer/vendor, and other problems similar to the one you presented.
Be careful not to confuse "Is-A" with "Acts-As-A". You can tell a party is a customer by looking in your order table or viewing the order count, and may not need a Customer table at all. Also don't confuse identity with life cycle: a rental car may eventually be sold, but this is a progression in life cycle and should be handled with column data, not table presence--the car doesn't start out as a RentalCar and get turned into a ForSaleCar later, it's a Car the whole time. Or perhaps a RentalItem, maybe the business will rent other things too. You get the idea.
It may not even be necessary to have a PartyType table. The party type can be determined by the presence of a row in the corresponding subtype table. This would also avoid the potential problem of the PartyTypeID not matching the subtype table presence. One possible implementation is to keep the PartyType table, but remove PartyTypeID from the Party table, then in a view on the Party table return the correct PartyTypeID based on which subtype table has the corresponding row. This won't work if you choose to allow parties to be both subtypes. Then you would just stick with the subtype views and know that the same value of BusinessID and PersonID refer to the same party.
Further Reading On This Pattern
Please see A Universal Person and Organization Data Model for a more complete and theoretical treatment.
I recently found the following articles to be useful for describing some alternate approaches for modeling inheritance in a database. Though specific to Microsoft's Entity Framework ORM tool, there's no reason you couldn't implement these yourself in any DB development:
Table Per Hierarchy
Table Per Type (this is what I advocate above as the only fully normalized method of implementing inheritance in a database)
Table Per Concrete Class
Or a more brief overview of these three ways: How to choose an inheritance strategy
P.S. I have switched, more than once, my opinion on column naming of IDs in subtype tables, due to having more experience under my belt.
You don't need application logic to enforce this. The easiest way is with a check constraint:
(PeopleID is null and BusinessID is not null) or (PeopleID is not null and BusinessID is null)
You can have another entity from which Person and Business "extend". We call this entity Party in our current project. Both Person and Business have a FK to Party (is-a relationship). And Asset may have also a FK to Party (belongs to relationship).
With that said, if in the future an Asset can be shared by multiple instances, is better to create m:n relationships, it gives flexibility but complicates the application logic and the queries a bit more.
ErikE's answer gives a good explanation on how to go about the supertype / subtype relationship in tables and is likely what I'd go for in your situation, however, it doesn't really address the question(s) you've posed which are also interesting, namely:
What would be the best practice for storing this type of conditional relationship in Microsoft SQL Server?
...are there other options?
For those I recommend this blog entry on TechTarget which has an excerpt from excerpt from "A Developer's Guide to Data Modeling for SQL Server, Covering SQL Server 2005 and 2008" by Eric Johnson and Joshua Jones which addresses 3 possible options.
In summary they are:
Supertype Table - Almost matches what you've proposed, have a table with some fields that will always be null when others are filled. Good when only a couple of fields aren't shared. So depending on how different Business and People are you could possibly combine them into one table, Owners perhaps, and then just have OwnerID in your Asset table.
Subtype Tables - Basically the opposite of what Supertype tables are and is what you have just now. Here we have lots of unique fields and maybe one or two the same so we just have the repeated fields appear in each table. As you are finding this isn't really suitable for your situation.
Supertype and Subtype Tables - A combination of both of the above where the matching fields are placed in a single table and the unique ones in separate tables and matching IDs are used to join the record from one table to the other. This matches ErikE's proposed solution and, as mentioned, is the one I would favour as well.
Sadly it doesn't go on to explain which, if any, are best practice but it is certainly a good read to get an idea of the options that are out there.
YOu can enforce the logic with a trigger instead. Then no matter how the record is changed, only one of the fileds will be filled in.
You could also have a PeopleAsset table and a BusinessAsset table, but stillwould have the problem of enforcing that only one of them has a record.
An asset would have a foreign key to the owning person, and you should setup an association table to link assets and businesses. As said in other comments, you can use triggers and/or constraints to ensure that the data stays in a consistent state. ie. when you delete a business, delete the lines in your association table.
Table People, Businesses both can use UUID as primary key, and union both to a view for sql join purpose.
so you can simply use one foreign key column in Assets relation to both People and Businesses, because UUID is nearly unique. And you can simply query like：
select * from Assets
join view_People_Businesses as v on v.id = Assets.fk