What is the difference between UNIQUE and PRIMARY KEY in Oracle? [duplicate]

PRIMARY KEY defines what the primary key of the table is. That is, what is the set of columns that should identify a tuple in a unique and unambiguous way. In addition, the table is usually organized internally according to how the primary key is defined. In addition, what is exported to other tables as a foreign key is the primary key.

However, there may be more than one way to uniquely identify a tuple, and that is where UNIQUE enters.

For example, let's assume that a usuario table has the following fields: name, id, CPF, RG, email, and date of birth.

Let's see, id would be the primary key. However, we can not have two users with the same email, so the email is UNIQUE . Also we can not have two users with the same RG or the same CPF, which are UNIQUE as well.

In addition, from time to time a person can change RG. If the RG was the primary key, you would have to come out cascading the changes on all the foreign keys, a problem that does not exist in columns UNIQUE that are not PRIMARY KEY .

Another difference is that UNIQUE columns can be NULL , whereas PRIMARY KEY columns can not. For example, in that case I gave up, this would be useful if you are to register a foreigner who does not have a CPF and RG. If one of these was a primary key, you would get screwed.

As the SOEN

The primary key:

• There can be only one per table
• Oracle can not be NULL
• Is unique and is the primary means of relating the tables .

The unique key:

• There may be more than one in a table

• You can get NULL values

In addition, the use of primary and foreign guarantors ensure compliance and integrity of the data and the structure of tables EX: You can not delete a table or a data that is dependent on another, which prevents any nonconformity.

Primary Key :

• There can only be one in a table
• Primary Key is a unique identifier of a record in a table

Unique Key :

• Are unique by table record
• There may be more than Unike Key in a table
• Allow null values
• You are a candidate to become a Primary Key
• A Unique Key can be null and, in the case of null, not unique to the registry

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