sql - Subset of a candidate key -

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As we know that a candidate key is a combination of a column or column that identifies rows in a relation .

Suppose I have a relation in which the candidate is a combination of the key column. I want to ask,

Is it possible that a subset of the candidate key is also specially Recognize?

or

A candidate key is a key for which there is no subset that uniquely identifies the row?

A candidate key is a combination of a column or column or more correctly set of one or more columns . Or more a set of columns correctly.

A candidate is a set of key columns that uniquely identifies the rows and does not contain any small ("proper") subgroups that uniquely identifies the rows.

P> is a set of superkey columns that uniquely identifies the rows, so a candidate key is a superkey, in which there is no small suprise.

In SQL, you can not declare an empty candidate key. In addition, there is no unique and primary key (which are not only unique in the context of constraints), but not the keys, if a small column set in the column set of such an announcement is not declared as superkey, So the superketer announcing is a candidate key.

Your question basically said that there is a candidate key regarding you "combination of keys" that you mean something, some small subset of a superkey (Superman) is a superkey or candidate key is.

No, a candidate key can not be a small candidate key, on the other hand, a unique / PK declaration column can be a unique / PK declaration on the small subset of the column. But then there will be no candidate key later.


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