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Is a Primary Key an index?


Is a Primary Key an index?

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Phil Parkin
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Regarding this

... default candidate key to be used to select particular rows of the table


Can anyone expand on what is meant by 'default' here?


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Phil Parkin (7/16/2014)
Regarding this

... default candidate key to be used to select particular rows of the table


Can anyone expand on what is meant by 'default' here?


When there are multiple candidate keys, the primary key is the one you turn to first. Hence the name primary key.


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Irritating because over-pedantic. Good question nevertheless.

Thanks. The reason I felt motivated to enter it as a QOTD was that I became intrigued by the way that constraints were recorded in the metadata. Unless you appreciate the difference between a key and an index, it can be rather baffling.

I think I ought to follow up with 'Is a Foreign Key an index? If no, is it enforced by an index?'


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Phil Parkin
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Koen Verbeeck (7/16/2014)
Phil Parkin (7/16/2014)
Regarding this

... default candidate key to be used to select particular rows of the table


Can anyone expand on what is meant by 'default' here?


When there are multiple candidate keys, the primary key is the one you turn to first. Hence the name primary key.


Hmm - "the one you turn to first" - what do you mean, practically?


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Regarding this

... default candidate key to be used to select particular rows of the table


Can anyone expand on what is meant by 'default' here?

In addition to what's been said: In SQL, you can define a foreign key by specifying only the table it relates to, leaving out the key. If you do that, the primary key is chosen from the possible candidate keys. There is some debate among relational theorists as to whether a properly normalised table will have more than one candidate key but, heck, we've all seen them, hence the usefulness of having a primary key.


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Phil Parkin
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Phil Factor (7/16/2014)

Regarding this

... default candidate key to be used to select particular rows of the table


Can anyone expand on what is meant by 'default' here?

In addition to what's been said: In SQL, you can define a foreign key by specifying only the table it relates to, leaving out the key. If you do that, the primary key is chosen from the possible candidate keys. There is some debate among relational theorists as to whether a properly normalised table will have more than one candidate key but, heck, we've all seen them, hence the usefulness of having a primary key.


I didn't know this, thank you - 'default' makes sense based on this.


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Ed Wagner
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Knowing that the constraint is indeed different than the index, I thought it was a simple enough question, but I admit to stopping and thinking "hmmm...I wonder how to interpret this" because the clustered index is created when you create the primary key constraint. An interesting question.


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Koen Verbeeck
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Ed Wagner (7/16/2014)
"hmmm...I wonder how to interpret this"


Well, a primary key (constraint) can be considered an object at the logical level, while an index exists purely at the physical level.
So it is 100% possible to decouple the primary key from the index, as the latter one is just a matter of implementation.

Ed Wagner (7/16/2014)
... because the clustered index is created when you create the primary key constraint..


Unless a clustered index was already defined. :-)


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Raghavendra Mudugal
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Koen Verbeeck (7/16/2014)
[quote]
Unless a clustered index was already defined. :-)


how you mean? "already"

this simple statement creates a clustered index
 
CREATE TABLE TT1
(
ID INT PRIMARY KEY,
NAME VARCHAR(10)
);



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Raghavendra Mudugal (7/16/2014)
Koen Verbeeck (7/16/2014)
[quote]
Unless a clustered index was already defined. :-)


how you mean? "already"

this simple statement creates a clustered index
 
CREATE TABLE TT1
(
ID INT PRIMARY KEY,
NAME VARCHAR(10)
);



You can create a table as a heap, create a clustered non-unique index on some columns and then define a primary key.


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