I understand the difference between aggregation and composition but I am struggling a bit with association. My current understanding is that an association exists between classes when ‘they use each other’, for example, one object is passed to the other during a method call. See also:
http://www.codeproject.com/Articles/330447/Understanding-Association-Aggregation-and-Composit
Both objects exist independently and, in contrast to aggregation, no object is a container class of the other. Does this mean that both objects MUST have a copy of the other(s) (e.g. 1:m relationship) or how else is the association ‘stored’. Any feedback would be very much appreciated.
From the UML Superstructure 2.4.1:
An association declares that there can be links between instances of the associated types. A link is a tuple with one value for each end of the association, where each value is an instance of the type of the end. (UML Superstructure, Page 37)
Nothing more, nothing less. and very vague. Because of this, it is also very hard to understand. What I defined (In a course I teach) is a hierarchy of links from dependency to composition where:
Dependency from A to B means that A uses B but indirectly (say by receiving instances of it and forwarding them to other objects).
Association from A to B means that A uses B directly, (for example by calling methods)
Aggregation from A to B means that B is part of A (semantically) but B can be shared and if A is deleted, B is not deleted. Note that this says nothing about how the "is part" is implemented.
Composition from A to B is like Aggregation, where B cannot be shared and if A is deleted, all of its aggregates (Bs) are deleted also.
Aggregation is an Association relationship where the Association can be considered the containing class 'Owning' the contained class, and the lifetime of that relationship is not defined.
Association is an 'Has-A' relationship.
Example:-
public class Person
{
private final Name name;
private Address currentAddress;
//...
}
In this case, the Person Has-A name and Has-A Address, so there is an Association between Person and Name, and Person and Address.
An association describes a relationship between instances of one or more classes. In the words of the UML Reference Manual, "Associations are the glue that holds together a system."
Aggregation is a form of association in which there is a "whole-part" relationship. You may say that if a class Airplane has a class Engine then this forms a "whole-part" relationship.
Aggregation
Let's set the terms. The Aggregation is a metaterm in the UML standard, and means BOTH composition and shared aggregation, simply named shared. Too often it is named incorrectly "aggregation". It is BAD, for composition is an aggregation, too. As I understand, you meant you understand "shared aggregation and composition".
From UML standard:
Precise semantics of shared aggregation varies by application area and
modeler.
I haven't found a word about that aggregation supposed multiplicity, for example.
Association.
A definition from UML 3.4.1 standard:
An association describes a set of tuples whose values refer to typed
instances. An instance of an association is called a link. A link is a
tuple with one value for each end of the association, where each value
is an instance of the type of the end.
Aggregated relationship is a subclass of Association.
Association is based on relationship. IT is the glue for models.
But your feelings didn't lie - as the shared aggregation is not strictly defined, there is also NO any strictly defined boundary between Association and Aggregated association. Authors of tools and modellers have to set it themselves.
Association
It represents a relationship between two or more objects where all objects have their own lifecycle and there is no owner. The name of an association specifies the nature of relationship between objects. This is represented by a solid line.
Let’s take an example of relationship between Teacher and Student. Multiple students can associate with a single teacher and a single student can associate with multiple teachers. But there is no ownership between the objects and both have their own lifecycle. Both can be created and deleted independently.
Aggregation
It is a specialized form of Association where all object have their own lifecycle but there is ownership. This represents “whole-part or a-part-of” relationship. This is represented by a hollow diamond followed by a line.
Let’s take an example of relationship between Department and Teacher. A Teacher may belongs to multiple departments. Hence Teacher is a part of multiple departments. But if we delete a Department, Teacher Object will not destroy.
It depends on the context.
Association: A man drives a car, focus on the caller and callee relationship.
Aggregation: A man has a car, focus on the owner and member relationship.
Composition: A man has a mouth, focus on the owner & member but the owner consists of members, it means that they shared the same life cycle.
Feels like I'm speaking Chinglish.
Association
Association is a relationship where all objects have their own life-cycle and there is no owner. Let’s take the example of Teacher and Student. Multiple students can associate with a single teacher and a single student can associate with multiple teachers but there is no ownership between the objects and both have their own life-cycle. Both can create and delete independently.
Aggregation
the objects in Aggregation have their own life-cycle but there is ownership. Child object can not belong to another parent object. Let’s take an example of Department and teacher. A single teacher can not belongs to multiple departments, but if we delete the department teacher object will not destroy. We can think about the “has-a” relationship.
Composition
It is a strong type of Aggregation. Child object does not have their life-cycle and if parent object deletes all child object will also be deleted. Let’s take again an example of the relationship between House and rooms. House can contain multiple rooms there is no independent life of room and any room can not belongs to two different houses if we delete the house room will automatically delete.
An association between object types classifies relationships between objects of those types. For instance, the association Committee-has-ClubMember-as-chair, which is visualized as a connection line in the class diagram shown below, may classify the relationships FinanceCommittee-has-PeterMiller-as-chair, RecruitmentCommittee-has-SusanSmith-as-chair and AdvisoryCommittee-has-SarahAnderson-as-chair, where the objects PeterMiller, SusanSmith and SarahAnderson are of type ClubMember, and the objects FinanceCommittee, RecruitmentCommittee and AdvisoryCommittee are of type Committee.
See also my alternative CodeProject article.
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Academia has it that table names should be the singular of the entity that they store attributes of.
I dislike any T-SQL that requires square brackets around names, but I have renamed a Users table to the singular, forever sentencing those using the table to sometimes have to use brackets.
My gut feel is that it is more correct to stay with the singular, but my gut feel is also that brackets indicate undesirables like column names with spaces in them etc.
Should I stay, or should I go?
I had same question, and after reading all answers here I definitely stay with SINGULAR, reasons:
Reason 1 (Concept). You can think of bag containing apples like "AppleBag", it doesn't matter if contains 0, 1 or a million apples, it is always the same bag. Tables are just that, containers, the table name must describe what it contains, not how much data it contains. Additionally, the plural concept is more about a spoken language one (actually to determine whether there is one or more).
Reason 2. (Convenience). it is easier come out with singular names, than with plural ones. Objects can have irregular plurals or not plural at all, but will always have a singular one (with few exceptions like News).
Customer
Order
User
Status
News
Reason 3. (Aesthetic and Order). Specially in master-detail scenarios, this reads better, aligns better by name, and have more logical order (Master first, Detail second):
1.Order
2.OrderDetail
Compared to:
1.OrderDetails
2.Orders
Reason 4 (Simplicity). Put all together, Table Names, Primary Keys, Relationships, Entity Classes... is better to be aware of only one name (singular) instead of two (singular class, plural table, singular field, singular-plural master-detail...)
Customer
Customer.CustomerID
CustomerAddress
public Class Customer {...}
SELECT FROM Customer WHERE CustomerID = 100
Once you know you are dealing with "Customer", you can be sure you will use the same word for all of your database interaction needs.
Reason 5. (Globalization). The world is getting smaller, you may have a team of different nationalities, not everybody has English as a native language. It would be easier for a non-native English language programmer to think of "Repository" than of "Repositories", or "Status" instead of "Statuses". Having singular names can lead to fewer errors caused by typos, save time by not having to think "is it Child or Children?", hence improving productivity.
Reason 6. (Why not?). It can even save you writing time, save you disk space, and even make your computer keyboard last longer!
SELECT Customer.CustomerName FROM Customer WHERE Customer.CustomerID = 100
SELECT Customers.CustomerName FROM Customers WHERE Customers.CustomerID = 103
You have saved 3 letters, 3 bytes, 3 extra keyboard hits :)
And finally, you can name those ones messing up with reserved names like:
User > LoginUser, AppUser, SystemUser, CMSUser,...
Or use the infamous square brackets [User]
I prefer to use the uninflected noun, which in English happens to be singular.
Inflecting the number of the table name causes orthographic problems (as many of the other answers show), but choosing to do so because tables usually contain multiple rows is also semantically full of holes. This is more obvious if we consider a language that inflects nouns based on case (as most do):
Since we're usually doing something with the rows, why not put the name in the accusative case? If we have a table that we write to more than we read, why not put the name in dative? It's a table of something, why not use the genitive? We wouldn't do this, because the table is defined as an abstract container that exists regardless of its state or usage. Inflecting the noun without a precise and absolute semantic reason is babbling.
Using the uninflected noun is simple, logical, regular and language-independent.
If you use Object Relational Mapping tools or will in the future I suggest Singular.
Some tools like LLBLGen can automatically correct plural names like Users to User without changing the table name itself. Why does this matter? Because when it's mapped you want it to look like User.Name instead of Users.Name or worse from some of my old databases tables naming tblUsers.strName which is just confusing in code.
My new rule of thumb is to judge how it will look once it's been converted into an object.
one table I've found that does not fit the new naming I use is UsersInRoles. But there will always be those few exceptions and even in this case it looks fine as UsersInRoles.Username.
Others have given pretty good answers as far as "standards" go, but I just wanted to add this... Is it possible that "User" (or "Users") is not actually a full description of the data held in the table? Not that you should get too crazy with table names and specificity, but perhaps something like "Widget_Users" (where "Widget" is the name of your application or website) would be more appropriate.
What convention requires that tables have singular names? I always thought it was plural names.
A user is added to the Users table.
This site agrees:
http://vyaskn.tripod.com/object_naming.htm#Tables
This site disagrees (but I disagree with it):
http://justinsomnia.org/writings/naming_conventions.html
As others have mentioned: these are just guidelines. Pick a convention that works for you and your company/project and stick with it. Switching between singular and plural or sometimes abbreviating words and sometimes not is much more aggravating.
How about this as a simple example:
SELECT Customer.Name, Customer.Address FROM Customer WHERE Customer.Name > "def"
vs.
SELECT Customers.Name, Customers.Address FROM Customers WHERE Customers.Name > "def"
The SQL in the latter is stranger sounding than the former.
I vote for singular.
I am of the firm belief that in an Entity Relation Diagram, the entity should be reflected with a singular name, similar to a class name being singular. Once instantiated, the name reflects its instance. So with databases, the entity when made into a table (a collection of entities or records) is plural. Entity, User is made into table Users. I would agree with others who suggested maybe the name User could be improved to Employee or something more applicable to your scenario.
This then makes more sense in a SQL statement because you are selecting from a group of records and if the table name is singular, it doesn't read well.
I stick with singular for table names and any programming entity.
The reason? The fact that there are irregular plurals in English like mouse ⇒ mice and sheep ⇒ sheep. Then, if I need a collection, i just use mouses or sheeps, and move on.
It really helps the plurality stand out, and I can easily and programatically determine what the collection of things would look like.
So, my rule is: everything is singular, every collection of things is singular with an s appended. Helps with ORMs too.
IMHO, table names should be plural like Customers.
Class names should be singular like Customer if it maps to a row in the Customers table.
Singular. I don't buy any argument involving which is most logical - every person thinks his own preference is most logical. No matter what you do it is a mess, just pick a convention and stick to it. We are trying to map a language with highly irregular grammar and semantics (normal spoken and written language) to a highly regular (SQL) grammar with very specific semantics.
My main argument is that I don't think of the tables as a set but as relations.
So, the AppUser relation tells which entities are AppUsers.
The AppUserGroup relation tells me which entities are AppUserGroups
The AppUser_AppUserGroup relation tells me how the AppUsers and AppUserGroups are related.
The AppUserGroup_AppUserGroup relation tells me how AppUserGroups and AppUserGroups are related (i.e. groups member of groups).
In other words, when I think about entities and how they are related I think of relations in singular, but of course, when I think of the entities in collections or sets, the collections or sets are plural.
In my code, then, and in the database schema, I use singular. In textual descriptions, I end up using plural for increased readability - then use fonts etc. to distinguish the table/relation name from the plural s.
I like to think of it as messy, but systematic - and this way there is always a systematically generated name for the relation I wish to express, which to me is very important.
I also would go with plurals, and with the aforementioned Users dilemma, we do take the square bracketing approach.
We do this to provide uniformity between both database architecture and application architecture, with the underlying understanding that the Users table is a collection of User values as much as a Users collection in a code artifact is a collection of User objects.
Having our data team and our developers speaking the same conceptual language (although not always the same object names) makes it easier to convey ideas between them.
I personaly prefer to use plural names to represent a set, it just "sounds" better to my relational mind.
At this exact moment i am using singular names to define a data model for my company, because most of the people at work feel more confortable with it.
Sometimes you just have to make life easier to everyone instead of imposing your personal preferences.
(that's how i ended up in this thread, to get a confirmation on what should be the "best practice" for naming tables)
After reading all the arguing in this thread, i reached one conclusion:
I like my pancakes with honey, no matter what everybody's favorite flavour is. But if i am cooking for other people, i will try to serve them something they like.
Singular. I'd call an array containing a bunch of user row representation objects 'users', but the table is 'the user table'. Thinking of the table as being nothing but the set of the rows it contains is wrong, IMO; the table is the metadata, and the set of rows is hierarchically attached to the table, it is not the table itself.
I use ORMs all the time, of course, and it helps that ORM code written with plural table names looks stupid.
I've actually always thought it was popular convention to use plural table names. Up until this point I've always used plural.
I can understand the argument for singular table names, but to me plural makes more sense. A table name usually describes what the table contains. In a normalized database, each table contains specific sets of data. Each row is an entity and the table contains many entities. Thus the plural form for the table name.
A table of cars would have the name cars and each row is a car. I'll admit that specifying the table along with the field in a table.field manner is the best practice and that having singular table names is more readable. However in the following two examples, the former makes more sense:
SELECT * FROM cars WHERE color='blue'
SELECT * FROM car WHERE color='blue'
Honestly, I will be rethinking my position on the matter, and I would rely on the actual conventions used by the organization I'm developing for. However, I think for my personal conventions, I'll stick with plural table names. To me it makes more sense.
I don't like plural table names because some nouns in English are not countable (water, soup, cash) or the meaning changes when you make it countable (chicken vs a chicken; meat vs bird).
I also dislike using abbreviations for table name or column name because doing so adds extra slope to the already steep learning curve.
Ironically, I might make User an exception and call it Users because of USER (Transac-SQL), because I too don't like using brackets around tables if I don't have to.
I also like to name all the ID columns as Id, not ChickenId or ChickensId (what do plural guys do about this?).
All this is because I don't have proper respect for the database systems, I am just reapplying one-trick-pony knowledge from OO naming conventions like Java's out of habit and laziness. I wish there were better IDE support for complicated SQL.
We run similar standards, when scripting we demand [ ] around names, and where appropriate schema qualifiers - primarily it hedges your bets against future name grabs by the SQL syntax.
SELECT [Name] FROM [dbo].[Customer] WHERE [Location] = 'WA'
This has saved our souls in the past - some of our database systems have run 10+ years from SQL 6.0 through SQL 2005 - way past their intended lifespans.
If we look at MS SQL Server's system tables, their names as assigned by Microsoft are in plural.
The Oracle's system tables are named in singular. Although a few of them are plural.
Oracle recommends plural for user-defined table names.
That doesn't make much sense that they recommend one thing and follow another.
That the architects at these two software giants have named their tables using different conventions, doesn't make much sense either... After all, what are these guys ... PhD's?
I do remember in academia, the recommendation was singular.
For example, when we say:
select OrderHeader.ID FROM OrderHeader WHERE OrderHeader.Reference = 'ABC123'
maybe b/c each ID is selected from a particular single row ...?
The system tables/views of the server itself (SYSCAT.TABLES, dbo.sysindexes, ALL_TABLES, information_schema.columns, etc.) are almost always plural. I guess for the sake of consistency I'd follow their lead.
I am a fan of singular table names as they make my ER diagrams using CASE syntax easier to read, but by reading these responses I'm getting the feeling it never caught on very well? I personally love it. There is a good overview with examples of how readable your models can be when you use singular table names, add action verbs to your relationships and form good sentences for every relationships. It's all a bit of overkill for a 20 table database but if you have a DB with hundreds of tables and a complex design how will your developers ever understand it without a good readable diagram?
http://www.aisintl.com/case/method.html
As for prefixing tables and views I absolutely hate that practice. Give a person no information at all before giving them possibly bad information. Anyone browsing a db for objects can quite easily tell a table from a view, but if I have a table named tblUsers that for some reason I decide to restructure in the future into two tables, with a view unifying them to keep from breaking old code I now have a view named tblUsers. At this point I am left with two unappealing options, leave a view named with a tbl prefix which may confuse some developers, or force another layer, either middle tier or application to be rewritten to reference my new structure or name viewUsers. That negates a large part of the value of views IMHO.
Tables: plural
Multiple users are listed in the users table.
Models: singular
A singular user can be selected from the users table.
Controllers: plural
http://myapp.com/users would list multiple users.
That's my take on it anyway.
I once used "Dude" for the User table - same short number of characters, no conflict with keywords, still a reference to a generic human. If I weren't concerned about the stuffy heads that might see the code, I would have kept it that way.
I've always used singular simply because that's what I was taught. However, while creating a new schema recently, for the first time in a long time, I actively decided to maintain this convention simply because... it's shorter. Adding an 's' to the end of every table name seems as useless to me as adding 'tbl_' in front of every one.
This may be a bit redundant, but I would suggest being cautious. Not necessarily that it's a bad thing to rename tables, but standardization is just that; a standard -- this database may already be "standardized", however badly :) -- I would suggest consistency to be a better goal given that this database already exists and presumably it consists of more than just 2 tables.
Unless you can standardize the entire database, or at least are planning to work towards that end, I suspect that table names are just the tip of the iceberg and concentrating on the task at hand, enduring the pain of poorly named objects, may be in your best interest --
Practical consistency sometimes is the best standard... :)
my2cents ---
As others have mentioned here, conventions should be a tool for adding to the ease of use and readability. Not as a shackle or a club to torture developers.
That said, my personal preference is to use singular names for both tables and columns. This probably comes from my programming background. Class names are generally singular unless they are some sort of collection. In my mind I am storing or reading individual records in the table in question, so singular makes sense to me.
This practice also allows me to reserve plural table names for those that store many-to-many relationships between my objects.
I try to avoid reserved words in my table and column names, as well. In the case in question here it makes more sense to go counter to the singular convention for Users to avoid the need to encapsulate a table that uses the reserved word of User.
I like using prefixes in a limited manner (tbl for table names, sp_ for proc names, etc), though many believe this adds clutter. I also prefer CamelBack names to underscores because I always end up hitting the + instead of _ when typing the name. Many others disagree.
Here is another good link for naming convention guidelines: http://www.xaprb.com/blog/2008/10/26/the-power-of-a-good-sql-naming-convention/
Remember that the most important factor in your convention is that it make sense to the people interacting with the database in question. There is no "One Ring to Rule Them All" when it comes to naming conventions.
Possible alternatives:
Rename the table SystemUser
Use brackets
Keep the plural table names.
IMO using brackets is technically the safest approach, though it is a bit cumbersome. IMO it's 6 of one, half-a-dozen of the other, and your solution really just boils down to personal/team preference.
My take is in semantics depending on how you define your container. For example, A "bag of apples" or simply "apples" or an "apple bag" or "apple".
Example:
a "college" table can contain 0 or more colleges
a table of "colleges" can contain 0 or more collegues
a "student" table can contain 0 or more students
a table of "students" can contain 0 or more students.
My conclusion is that either is fine but you have to define how you (or people interacting with it) are going to approach when referring to the tables; "a x table" or a "table of xs"
I think using the singular is what we were taught in university. But at the same time you could argue that unlike in object oriented programming, a table is not an instance of its records.
I think I'm tipping in favour of the singular at the moment because of plural irregularities in English. In German it's even worse due to no consistent plural forms - sometimes you cannot tell if a word is plural or not without the specifying article in front of it (der/die/das). And in Chinese languages there are no plural forms anyway.
I only use nouns for my table names that are spelled the same, whether singular or plural:
moose
fish
deer
aircraft
you
pants
shorts
eyeglasses
scissors
species
offspring
I did not see this clearly articulated in any of the previous answers. Many programmers have no formal definition in mind when working with tables. We often communicate intuitively in terms of of "records" or "rows". However, with some exceptions for denormalized relations, tables are usually designed so that the relation between the non-key attributes and the key constitutes a set theoretic function.
A function can be defined as a subset of a cross-product between two sets, in which each element of the set of keys occurs at most once in the mapping. Hence the terminology arising from from that perspective tends to be singular. One sees the same singular (or at least, non-plural) convention across other mathematical and computational theories involving functions (algebra and lambda calculus for instance).
I always thought that was a dumb convention. I use plural table names.
(I believe the rational behind that policy is that it make it easier for ORM code generators to produce object & collection classes, since it is easier to produce a plural name from a singular name than vice-versa)