What is the difference between "Primary Key" and "Unique Key" in SQL?

I've had a chance to with a Postgres database and came across a table with two primary keys. This intrigues me to look up a definition of primary and unique key. I always thought there would only be one primary key in a table, and the values have to be unique. But, what I saw in the PostgreSQL v8.1.23 was something otherwise. It allowed duplicate entries, and also allowed multiple primary keys in a table. Is this possible? Yes, it's called composite Primary Keys. Composite primary keys (multiple primary keys) makeup uniqueness in a table row -- which means composite primary keys work together to provide uniqueness.

Here is the definition of primary key and unique key constraints in a SQL table.

Primary Key

Primary Key is an attribute or a set of attributes in a table that uniquely identifies a record (row) in a table and no two records (rows) in the table can have the same values for all the columns comprising the primary key.

A primary key can consist of one or more columns of a table. When two or more columns are used as a primary key, they are called a composite key. Each single column's data can be duplicated but the combination values of these columns cannot be duplicated.

• A primary key is a column or a set of columns in a table that uniquely identifies each record.
• It must have a unique value for each row, and it cannot contain NULL values.
• There can be only one primary key in a table.
• It is used to establish relationships between tables, often as a foreign key in other tables.
• Primary keys are automatically indexed for efficient retrieval.

Unique Key

A unique key is a key that stores unique values (no duplicates) for that particular column, and it accepts a null value and multiple null values do not violate the unique constraints.

• A unique key is a constraint that ensures the values in a column or a set of columns are unique across the entire table.
• Unlike a primary key, a unique key can allow NULL values, but if a column is defined as NOT NULL, the unique constraint enforces uniqueness with respect to non-NULL values.
• There can be multiple unique keys in a table.
• Unique keys are used to enforce the integrity of the data by preventing duplicate values in specific columns.

Differences between Primary Key and Unique Key

• A Unique key can store a null value and a primary key cannot store any null value.
• A primary key can be referenced to another table as a Foreign Key.
• A table can have only one primary key, but have multiple unique keys.
• Primary is not always a single-column key, and may consist of multiple columns to make a composite key.