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I am reading about candidate keys and composite keys. I came to know that
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 ?
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Primary Key is a unique and non-null key which identify a record uniquely in table. A table can have only one primary key. Candidate key is also a unique key to identify a record uniquely in a table but a table can have multiple candidate keys. Primary key column value can not be null.
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.
While a primary key and a composite key might do the same things, the primary key will consist of one column, where the composite key will consist of two or more columns.
Secondary Key is the key that has not been selected to be the primary key. However, it is considered a candidate key for the primary key. Therefore, a candidate key not selected as a primary key is called secondary key. Candidate key is an attribute or set of attributes that you can consider as a Primary key.
The primary Key is one or more columns in a table that uniquely identifies each row in the table. If there is more than 1 column that makes the row unique, it's called Composite Primary Key. A candidate key is one that can identify each row of a table uniquely.
Primary Key is a arbitrary selected Candidate key. There must be only and only One primary key. We can choose any candidate key as a Primary key. Other candidate keys which are not chosen as Primary are called Alternate Keys. If Primary Key have more then one column (or attributes) ,it is called Composite Key.
Can a candidate key be composite, I am quite confused as some sources say that it should be a single column and some say that it can be composite? Keys can be composite. Nothing wrong with them.
if EmpID and project id are put in projectionHours table then combination of empid and project ID called as composite key because combination of these two act as primary key in projectionHours table. All keys in a table that become unique called as candidate key.
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.
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