What is the difference between foreign key and candidate key

So you cannot have the same column with primary key and foreign key. If true, it can be the primary key for the table. Foreign keys reference typically the primary key on another table, although in theory any necessarily unique combination of columns i.

One thing that is to be noted here is that the foreign key of one table may or may not be the primary key. But it should be the primary key of another table. There an be more than one candidate key in a relation. These Candidate keys are the candidates that can qualify to become a primary key. Though each candidate key qualifies to become a primary key, only one can be chosen as a primary key.

One of the Candidate Keys gets qualified to become a primary key. The rules that a candidate key must qualify to become primary are that the key value should never be NULL and it must be unique for all tuples. The combination of columns guarantees uniqueness, though individually uniqueness is not guaranteed.

Hence, they are combined to uniquely identify records in a table. The difference between compound and the composite key is that any part of the compound key can be a foreign key, but the composite key may or maybe not a part of the foreign key. They do not lend any meaning to the data in the table. Surrogate key in DBMS is usually an integer. A surrogate key is a value generated right before the record is inserted into a table.

Above, given example, shown shift timings of the different employee. In this example, a surrogate key is needed to uniquely identify each employee.

Skip to content. Primary Key — is a column or group of columns in a table that uniquely identify every row in that table. Candidate Key — is a set of attributes that uniquely identify tuples in a table. Alternate Key — is a column or group of columns in a table that uniquely identify every row in that table.

Foreign Key — is a column that creates a relationship between two tables. Compound Key — has two or more attributes that allow you to uniquely recognize a specific record. Composite Key — is a combination of two or more columns that uniquely identify rows in a table. The combination of columns guarantees uniqueness, though individual uniqueness is not guaranteed. Surrogate Key — An artificial key which aims to uniquely identify each record is called a surrogate key.

What is a Primary Key? What is the Alternate key? What is a Candidate Key? What is the Foreign key? What is the Compound key? What is the Composite key?

In this article, we have made a comparison between primary and candidate key. Here we conclude that it is optional to specify a primary key for a relation. But if we are declaring a relationship, candidate keys must be present in that relation. JavaTpoint offers too many high quality services. Mail us on [email protected] , to get more information about given services.

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