I am reading about candidate keys and composite keys. I came to know that
a candidate key can qualify as a primary key and it can be a single column or combination of columns
and a composite key is also a combination of columns.
For composite key I have referred this link
how do I make a composite key with SQL Server Management Studio?
Thus when both the candidate key and composite key are a combination of columns, they can qualify as a primary key. Then what is the exact difference ? Can you please explain with examples ?
As I know candidate key is a unique key that can be used as a primary key. but not necessarily used as one.
Composite key is a key of two or more attributes that uniquely identifies the row.
A key is a set of columns that can be used to uniquely identify each row within a table.
Every table has at least one key. Let's say we've identified each possible key for the table. Each of these keys is a candidate key.
As we examine each of these keys, the key may consist of no columns (!), one column, or more than one column, which when considered together uniquely identify each row. The term composite key specifically refers to a key which consists of more than one column.
In SQL, it was decided that one key should be selected and treated "more equal" than the other keys of the table. This key is called the primary key. Other keys can also be declared on the table, these are usually referred to as Unique Contsraints.
(!) In SQL, you aren't allowed to declare a key with no columns - although it would occasionally be useful (think of a table that should only ever have a single row, where each column is representing configuration information)
As an example of a table which has multiple keys, all of which are composite. Imagine an appointment system, where a client and a counsellor meet in a room at a particular time:
CREATE TABLE Appointments (
ClientID int not null,
CounsellorID int not null,
RoomID int not null,
AppointmentTime datetime not null
)
The candidate keys for this table are {ClientID,AppointmentTime}, {CounsellorID,AppointmentTime} and {RoomID,AppointmentTime}. Any of those combinations of columns could be used to uniquely identify a row in the table, and all of them are composite keys.
Which one we choose to declare as primary key will depend (probably) on our own interpretation of the main "focus" of the system. Are we mainly concerned with Room usage, or Clients, or Counsellors? In any case, we'll select one and declare it the primary key. We'll also hopefully declare the other keys as unique constraints.
Or, we could decide to use a surrogate, and declare an AppointmentID column, using whatever auto-numbering facilities are available in the database. That could then be the (non-composite) primary key. But we should still declare the other keys for the table.
Candidate Key: A nominee for primary key field is known as candidate key.
Composite Key: Creating more than one primary key is jointly known as composite key.
Update :
A candidate key is a unique key that can be used as a primary key.
Composite key is a key of two or more attributes that uniquely identifies the row.
A key is a set of columns that can be used to uniquely identify each row within a table.
CANDIDATE KEY :- Candidate key is a unique key and is a "Candidate" for being a primary key.
COMPOSITE KEY :- "Composition" of two or more columns as primary key, is consider as Composite key.
A candidate key is just that: a column or combination of columns that could be used as the primary key, ie a candidate or potential primary key, A composite key is by definition two or more columns that could be used to identify a row. Most commonly when talking about composite keys the question is whether to have it as the primary key to have a surrogate key instead. On occassion you may hear people refer to a composite key that is not a primary key (ie not unique for all rows), in which case it is (hopefully) just a way to refer to a multi-colmn join on non-primary key fields (instead of doing cross join and filtering the rows in the where clause).
CANDIDATE KEY : Candidate key is the single column/multiple column(combined) which helps us to uniquely identify rows in a table.
We can have more than 1 candidate keys
<----- Now among all these candidate keys, one of them is made primary key(which depends upon the developer which one is suited for them) ------>
COMPOSITE KEY : Composite key is set of 2 or more columns (it can't be single column) which helps us to identify rows in a table.
So, yes composite keys are candidate key
In case Composite key is made as primary key, we call it composite primary key(primary key made up of more than 1 column combined).
-Hope this helps
Lets keep it simple.
Primary Key: Uniquely identifies each row using one column
Composite Key: Uniquely identifies each row using more than one columns
Candidate Key: Can be a Primary key or a Composite key, but is not being used as one. As the name suggests it is only a 'candidate'.
Related
If the two columns both have unique values of their own whether it is possible to declare both the columns are primary keys ? PS Not talking about declaring a combination of both the columns as composite key.
A primary key in a table has three properties:
The key is NOT NULL (all components).
The key is unique.
There is only one per table.
This is the definition of a primary key in a relational database. The third condition makes it pretty clear that you cannot have two of them in the same table.
On the other hand, you could have one primary key and another key that is defined as UNIQUE and NOT NULL. You can do this as many times as you like.
I would like to ask in which cases its proper to use UNIQUE keyword in SQL. I know that if I declare a column as a primary key has uniqueness on its own but what happens with other attributes like country? Is it proper to use a unique constraint there?
The unique keyword in sql is used whenever you want each and every row entry of that column to be different from each other. A primary key column is automatically unique but there are some cases in which you may want more columns to be unique.
For example if you have a product_id as primary key it will ensure that no other row will have a product with product_id as that row. And in addition to that, you want that no two rows should have the same product_imei, then you can make the product_imei unique.
You can make a composite primary key like Primary Key(column1,column2) but that will mean that the combination you get from product_id and product_imei will be unique.
For example
(DLK-22,356938035643809) and (DLK-22, 11111111111111) both can exist in a table if (product_id,product_imei) is the primary key.
So you can use a unique constraint on as much columns as you like and its need depends on the scenario of the problem you are facing. You can use the unique constraint with the country if that helps you, there is no problem in doing so
The UNIQUE constraint ensures that all values in a column are different. Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of columns. A PRIMARY KEY constraint automatically has a UNIQUE constraint.
As any other constraint the UNIQUE constraint enforces some level of data quality. If you add this constraint to a column, then all values on that column will be different.
For example, on a table where EMPLOYEE_PK is already unique (because it's the PK) you may want to enforce the CARD_NUMBER column is also unique; that's the number displayed on the employee card. In your model the card number may be different from the PK, and you may also need to make sure it's unique.
Another extra benefit of a UNIQUE constraint is that other tables can link foreign keys to it. Since a UNIQUE column effectively acts as a "table key", any other table can establish a foreign key pointing to it. I've met many people who [wrongly] think that foreign keys can only point to primary keys.
If a table has a composite primary key made of attribute A, B and this composite primary key is referenced in another table, can that other table have attribute A as a primary key and NOT attribute b? And maybe, Attribute A with some other attribute in the same table as another composite primary key?
If the second table has both attributes A and B, then an FK constraint can reference those same attributes in the first table (since they're uniquely constrained as the PK), regardless of whether A and/or B are part of the PK of the second table or not.
Yes. One fairly common example is a general ledger. The Item table has a composite key, consisting of Transaction Number and Item Number (within transaction). The Transaction table has a key of Transaction Number, referenced by one of the key fields in Item.
General ledger systems aren't always designed this way, but they often are.
I need to understand the difference between super key and composite key. The examples I found made more confused. Can you please simply clarify what is the difference? Thanks
The accepted answer is not entirely accurate...
A superkey is any set of columns that, combined together, are unique. There are typically many superkeys per table and same column may be shared by many superkeys. They are not very useful by themselves, but are more of a mental tool for identifying candidate keys (see below).
A candidate key is a minimal superkey - if any column is removed it would no longer be unique. There are typically significantly fewer candidate keys than superkeys.
A key is just a synonym for a candidate key.
A composite1 key is a key that has more than one column. In other words, it's a minimal superkey that has multiple columns.
Few more points:
Every key is unique, so calling it "unique key" is redundant. Just "key" is enough.
At the DBMS level, a key is enforced through a PRIMARY KEY or UNIQUE2 constraint.
An index is usually present underneath the key (PRIMARY KEY or UNIQUE constraint), for performance reasons. But despite often going together, key and index are separate concepts: key is a logical concept (changes the meaning of data) and index is a physical concept (doesn't change the meaning of data, just performance).
1 Aka. compound, complex or concatenated.
2 On NOT NULL columns.
Yes, I agree with #Branko, the accepted answer is not the accurate and not clear.
I'll take example of an Employee table:
CREATE TABLE Employee (
Employee ID,
FullName,
SSN,
DeptID
);
And to know difference b/w Super & Candidate keys, let's check first what are other types of keys?
1. Candidate Key: are individual columns in a table that qualifies for uniqueness of all the rows. Here in Employee table EmployeeID & SSN are Candidate keys.
2. Primary Key: is the columns you choose to maintain uniqueness in a table. Here in Employee table you can choose either EmployeeID or SSN columns, EmployeeID is preferable choice, as SSN is a secure value.
3. Alternate Key: Candidate column other the Primary column, like if EmployeeID is PK then SSN would be the Alternate key.
4. Super Key: If you add any other column/attribute to a Primary Key then it become a super key, like EmployeeID + FullName is a Super Key.
5. Composite Key: If a table don't have any individual columns that qualifies for a Candidate key, then you have to select 2 or more columns to make a row unique. Like if there is no EmployeeID or SSN columns, then you can make FullName + DateOfBirth as Composite primary Key. But still there can be a narrow chance of duplicate row.
Reference
A superkey is a set of one or more attributes that, taken collectively, allow us to identify uniquely an entity in the entity set.
For example, the customer-id attribute of the
entity set customer is sufficient to distinguish one customer entity from another. Thus,customer-id is a superkey.
Composite Key is a combination of more than one fields/columns of a table. It can be a Candidate key, Primary key.
A super key uniquely identifies a row. It could be made up of one column or many. A composite key is a key made of more than one column.
If a Super Key is made of more than one column it is also a composite.
If a composite key uniquely identifies a row it is also a Super key.
I don't see the name 'Super key' used too much: it's usually just called a 'Unique key'.
Is it that a primary key is the selected candidate key chosen for a given table?
Candidate Key – A Candidate Key can be any column or a combination of columns that can qualify as unique key in database. There can be multiple Candidate Keys in one table. Each Candidate Key can qualify as Primary Key.
Primary Key – A Primary Key is a column or a combination of columns that uniquely identify a record. Only one Candidate Key can be Primary Key.
More on this link with example
John Woo's answer is correct, as far as it goes. Here are a few additional points.
A primary key is always one of the candidate keys. Fairly often, it's the only candidate.
A table with no candidate keys does not represent a relation. If you're using the relational model to help you build a good database, then every table you design will have at least one candidate key.
The relational model would be complete without the concept of primary key. It wasn't in the original presentation of the relational model. As a practical matter, the use of foreign key references without a declared primary key leads to a mess. It could be a logically correct mess, but it's a mess nonetheless. Declaring a primary key lets the DBMS help you enforce the data rules. Most of the time, having the DBMS help you enforce the data rules is a good thing, and well worth the cost.
Some database designers and some users have some mental confusion about whether the primary key identifies a row (record) in a table or an instance of an entity in the subject matter that the table represents. In an ideal world, it's supposed to do both, and there should be a one-for-one correspondence between rows in an entity table and instances of the corresponding entity.
In the real world, things get screwed up. Somebody enters the same new employee twice, and the employee ends up with two ids. Somebody gets hired, but the data entry slips through the cracks in some manual process, and the employee doesn't get an id, until the omission is corrected. A database that does not collapse the first time things get screwed up is more robust than one that does.
Primary key -> Any column or set of columns that can uniquely identify a record in the table is a primary key. (There can be only one Primary key in the table)
Candidate key -> Any column or set of columns that are candidate to become primary key are Candidate key. (There can be one or more candidate key(s) in the table, if there is only one candidate key, it can be chosen as Primary key)
A Primary key is a special kind of index in that:
there can be only one;
it cannot be nullable
it must be unique.
Candidate keys are selected from the set of super keys, the only thing we take care while selecting the candidate key is: It should not have any redundant attribute.
Example of an Employee table:
Employee (
Employee ID,
FullName,
SSN,
DeptID
)
Candidate Key: are individual columns in a table that qualifies for the uniqueness of all the rows. Here in Employee table EmployeeID & SSN are Candidate keys.
Primary Key: are the columns you choose to maintain uniqueness in a table. Here in Employee table, you can choose either EmployeeID or SSN columns, EmployeeID is a preferable choice, as SSN is a secure value.
Alternate Key: Candidate column other the Primary column, like if EmployeeID is PK then SSN would be the Alternate key.
Super Key: If you add any other column/attribute to a Primary Key then it becomes a super key, like EmployeeID + FullName, is a Super Key.
Composite Key: If a table does not have a single column that qualifies for a Candidate key, then you have to select 2 or more columns to make a row unique. Like if there is no EmployeeID or SSN columns, then you can make FullName + DateOfBirth as Composite primary Key. But still, there can be a narrow chance of duplicate row.
There is no difference. A primary key is a candidate key. By convention one candidate key in a relation is usually chosen to be the "primary" one but the choice is essentially arbitrary and a matter of convenience for database users/designers/developers. It doesn't make a "primary" key fundamentally any different to any other candidate key.
A table can have so many column which can uniquely identify a row. This columns are referred as candidate keys, but primary key should be one of them because one primary key is enough for a table. So selection of primary key is important among so many candidate key. Thats the main difference.
Think of a table of vehicles with an integer Primary Key.
The registration number would be a candidate key.
In the real world registration numbers are subject change so it depends somewhat on the circumstances what might qualify as a candidate key.
Primary key -> Any column or set of columns that can uniquely identify a record in the table is a primary key. (There can be only one Primary key in the table) and
the candidate key-> the same as Primary key but the Primary Key chosen by DB administrator's prospective for example(the primary key the least candidate key in size)
A primary key is a column (or columns) in a table that uniquely identifies the rows in that table.
CUSTOMERS
CustomerNo FirstName LastName
1 Sally Thompson
2 Sally Henderson
3 Harry Henderson
4 Sandra Wellington
For example, in the table above, CustomerNo is the primary key.
The values placed in primary key columns must be unique for each row: no duplicates can be tolerated. In addition, nulls are not allowed in primary key columns.
So, having told you that it is possible to use one or more columns as a primary key, how do you decide which columns (and how many) to choose?
Well there are times when it is advisable or essential to use multiple columns. However, if you cannot see an immediate reason to use multiple columns, then use one. This isn't an absolute rule, it is simply advice. However, primary keys made up of single columns are generally easier to maintain and faster in operation. This means that if you query the database, you will usually get the answer back faster if the tables have single column primary keys.
Next question — which column should you pick? The easiest way to choose a column as a primary key (and a method that is reasonably commonly employed) is to get the database itself to automatically allocate a unique number to each row.
In a table of employees, clearly any column like FirstName is a poor choice since you cannot control employee's first names. Often there is only one choice for the primary key, as in the case above. However, if there is more than one, these can be described as 'candidate keys' — the name reflects that they are candidates for the responsible job of primary key.
If superkey is a big set than candidate key is some smaller set inside big set and primary key any one element(one at a time or for a table) in candidate key set.
First you have to know what is a determinant?
the determinant is an attribute that used to determine another attribute in the same table.
SO the determinant must be a candidate key. And you can have more than one determinant.
But primary key is used to determine the whole record and you can have only one primary key.
Both primary and candidate key can consist of one or more attributes