There could be multiple candidate keys for a table and one of the candidate key can be used as Primary Key. If the primary key is composite key, means having multiple columns, what is the technical term for its component columns?
In the relational model, the individual attributes of a compound candidate key (any or every candidate key) are just attributes. But in the context of normalization, attributes are either prime or nonprime. Prime attributes are a part of one or more candidate keys. Nonprime attributes are not part of any candidate key.
Apart from normalization, we usually don't talk about prime and nonprime attributes.
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I couldn't find an clear answer on this during my research. Any help will be appreciated.
A candidate key in relational theory is a column (or columns) that contains unique values, and may be used to uniquely identify a row; a primary key is a candidate key that has been identified as the method of identifying the rows for purposes of building relationships.
Depending on the design, a table may have multiple candidate keys, but only one of them is identified as the primary key. For example, you may have a table of parts for resale; it is possible that the vendor identification for the parts may be unique and different from your own unique resale ids. this table would have two candidate keys, either of which could be used as a primary key (or a third surrogate instead).
Is composite key a subset of a pool of candidate keys? If it is a subset then why there is a new terminology for composite key, if candidate key can also formed with multiple columns.
Update
Simple Key - A simple key is one that has only one attribute.
Candidate Key - a candidate key of a relation is a minimal super key for that relation; that is, a set of attributes such that:
a. The relation does not have two distinct tuples (i.e. rows or records in common database language) with the same values for these attributes (which means that the set of attributes is a super key)
b. There is no proper subset of these attributes for which (1) holds (which means that the set is minimal).
I would like to insist the point that candidate keys are minimal super key.
Compound key - a compound key is a key that consists of two or more attributes that uniquely identify an entity occurrence.
A composite key contains at least one compound key and one more attribute. Composite keys may also include simple keys and non-key attributes.
Now composite key does not satisfy the condition of candidate key of minimal super key.
I may be wrong but please help me in understanding this concept.
I must normalise table data to 3NF.
I have a composite key from 1NF, but all of the non-key attributes appear to be reliant on both of the primary key attributes. I'm trying to take it from 2NF into 3NF. Can I still have a composite key?
You can have a compound key in every normal form.
In fact, when you write functional dependencies in the form A->B, both A and B refer to sets of attributes. That's why they're in uppercase; uppercase letters represent sets in set theory.
...all of the non-key attributes appear to be reliant on Both of the primary keys...
There's only one primary key. In your case, that one primary key has more than one attribute.
There might be more than one candidate key, though. In normalization, every candidate key is equally important. For example, if you're trying to identify transitive dependencies, you need to look for transitivity with respect to every candidate key, not just the primary key.
Defination of Superkey and Primary key in wikipedia
A superkey is a set of attributes within a table whose values can be used to uniquely identify a tuple.
and
The primary key has to consist of characteristics that cannot be duplicated by any other row. The primary key may consist of a single attribute or a multiple attributes in combination.
I've gone through many books and surfed on internet but what i found in them is what is primarykey and what is superkey.
But what i want to know is why superkey is required when we can identify a tuple uniquely through primarykey ?
Superkeys are defined for conceptual completeness. You never need a superkey for reference purposes. A reference to a primary key will do just fine.
The concept of superkeys can be useful when you are analyzing a body of data in order to discover all the functional dependencies in it.
Once you have discovered a key, the next question is whether or not it is a superkey. If is is, you turn your attention to the candidate key contained in the superkey.
Let's define what these terms mean in the first place:
A "superkey" is any set of attributes that, when taken together, uniquely identify rows in the table.
A minimal1 superkey is called "candidate key", or just "key".
All keys in the same table are logically equivalent, but for historical and practical reasons we choose one of them and call it "primary", while the remaining are "alternate" keys.
So, every primary key is key, but not every key is primary. Every key is superkey, but not every superkey is key.
Constraints that physically enforce keys in the database are: PRIMARY KEY constraint (for primary key) and UNIQUE constraint (for alternate key). These constraints should not be created on all superkeys, only on keys.
It is not unusual to have multiple keys in the same table, depending on the nature of your data. For example, a USER table might have unique USER_ID and unique USER_NAME. Since both of them need to be unique on their own, you must create2 both keys, even though only one of them is strictly needed for identification.
1 That is, a superkey that would stop being unique (and therefore, being a superkey) if any of the attributes were removed from it.
2 I.e. create PRIMARY KEY or UNIQUE constraint.
A word key is usually a short for a candidate key.
Superkey means a super-set of a key (key attributes and some more).
Irreducible superkey is called a candidate key. (Irreducible means that if you remove one attribute, it is not a key any more); in general, there is more than one candidate key for a given relation (actually a relational variable).
One candidate key that designer choses to prefer (for some reason) is called the primary key.
This was on a logical level, keys are defined for relational variables, so called relvars.
In physical implementation:
Relvar maps to a table.
Primary key to the primary key of the table.
Other candidate keys (except PK) map to alternate keys (unique not null).
A primary key is a superkey. Having only one such key constraint and only one way to identify tuples isn't necessarily sufficient.
Firstly, the relational model's versatility derives very much from the fact that it does not predetermine how data can or should be accessed in a table. A user or application is free to query a table based on whatever set of attributes may be necessary or convenient at the time. There is no obligation to use a "primary" key, which may or may not be relevant for some queries.
Secondly, uniqueness constraints (usually on candidate keys) are a data integrity feature. They guarantee data isn't duplicated in the key attributes. That kind of constraint is often useful on more than one set of attributes where business rules dictate that things should be unique. Uniqueness of one thing alone obviously doesn't guarantee uniqueness of another thing.
Thirdly, the query optimiser can take advantage of any and all keys as a way of optimising data access through query rewrites. From the optimiser's point of view the more keys it has to work with in a table the better.
I think superkey is just part of the relational algebra abstraction - your primary key is (likely) to be the minimal superkey but you might have other superkeys whereas you only have one primary key.
Is the primary key also a super key and a candidate key? Their definitions are lengthy but I wonder if this is true?
Please note that I'm not asking if they are the same term. I'm just asking in one direction, not the other way round.
Super Key - is a set of one or more columns that can be used to identify a record uniquely in a table
Candidate Key – can be any column or a combination of columns that can qualify as a unique key in database. There can be multiple Candidate Keys in one table. Each Candidate Key can qualify as a Primary Key. You can think of this as the "shortest" super key or minimal super key
Primary Key – is a column or a combination of columns that uniquely identify a record. Only one Candidate Key can be Primary Key.
For a Candidate Key to qualify as a Primary Key, it should be unique and non-null.
So, basically a primary key is just one of the candidate keys, which is a just a minimal super key.
According to dry definitions:
Your primary key is a super key by definition - you can not have two rows with the same primary key.
However, the primary key is not a natural constraint of your business, but an artificial constraint in your data store: for example, you could set a person's birthday as the primary key in your table, and never have two people who were born on the same day. That would be silly, but possible. In that case, the primary key of the table is not a super key of the domain.
However, your primary key is not necessarily a candidate key - you can add redundant columns to your primary key.
Different set of attributes which are able to identify any row in the database is known as super key. And minimal super key is termed as candidate key i.e. among set of super keys one with minimum number of attributes. Primary key could be any key which is able to identify a specific row in database in a unique manner. from this thread
And typing all three keys in google gives about 2,480,000 results
It depends.
The Primary key is the main key the table uses to identify between different elements. It is chosen from the candidate keys.
The candidate keys are all the keys that COULD be the primary key. All the keys that are unique and can be differentiated upon in the table.
The super key is a primary key with additional attributes, this extra information is used to uniquely identify an instance of the entity set.
A candidate key is the most minimal subset of fields that uniquely identifies a tuple. For example if you have a candidate key on the column "user_id" and "pet_id" you'll never have more than 1 tuple with the same user_id and pet_id and neither user_id nor pet_id individually will work as a unique identifier for the tuple.
A super key is a set of fields that contains a key. Using the above example where the combination of "user_id" and "pet_id" uniquely identifies a tuple if we added "pet_name" (which is not key because we can have multiple pets named "fluffy") it would be a super key. Basically it's like a candidate key without the "minimal subset of fields" constraint.
A primary key is a candidate key that you tell the DB to optimize on. There might be multiple ways of referring to a unique tuple (ie. multiple candidate keys) but you can specify one when you're creating the table that you will use the most frequently.
Yes we can simply say that a candiate key is primary key but it must be unique.