Differnce Between Primary Key & Unique Key

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In relational databases and flat file databases, a table is a set of data elements (values) that is organized using a model of vertical columns (which are identified by their name) and horizontal rows. A table has a specified number of columns, but can have any number of rows[citation needed]. Each row is identified by the values appearing in a particular column subset which has been identified as a candidate key. Table is another term for relations; although there is the difference in that a table is usually a multi-set (bag) of rows whereas a relation is a set and does not allow duplicates. Besides the actual data rows, tables generally have associated with them some meta-information, such as constraints on the table or on the values within particular columns.
The data in a table does not have to be physically stored in the database. Views are also relational tables, but their data are calculated at query time. Another example are nicknames, which represent a pointer to a table in another database.

Primary Key..

In relational database design, a unique key can uniquely identify each row in a table, and is closely related to the Superkey concept. A unique key comprises a single column or a set of columns. No two distinct rows in a table can have the same value (or combination of values) in those columns if NULL values are not used. Depending on its design, a table may have arbitrarily many unique keys but at most one primary key.

  • 1.It will not accept null values.

  • 2.There will be only one primary key in a table.

  • 3.Clustered index is created in Primary key.

  • 4.Primary key allows each row in a table to be uniquely identified and ensures that no duplicate rows exist.

Unique Key..

Definition: The primary key of a relational table uniquely identifies each record in the table. It can either be a normal attribute that is guaranteed to be unique (such as Social Security Number in a table with no more than one record per person) or it can be generated by the DBMS (such as a globally unique identifier, or GUID, in Microsoft SQL Server). Primary keys may consist of a single attribute or multiple attributes in combination.

  • 1.Null values are accepted.

  • 2.More than one unique key will be there in a table.

  • 3.Non-Clustered index is created in unique key.

  • 4.Unique key constraint is used to prevent the duplication of key values within the rows of a table and allow null values.


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