What is a Composite Primary Key?

Technology - What is a Composite Primary Key

In a database, a composite primary key is a primary key that consists of two or more columns. A primary key is a unique identifier for a record in a table, and it is used to ensure that each record in the table can be uniquely identified.

A composite primary key is used when a single column cannot uniquely identify a record in the table, but a combination of columns can. For example, let’s say we have a table of student course registrations where each student can register for multiple courses. A single column, such as student ID, may not be enough to uniquely identify each record in the table, as a student can register for the same course multiple times. In this case, a composite primary key consisting of both student ID and course ID can be used to ensure that each registration record is uniquely identified.

The syntax for creating a composite primary key in SQL is as follows:

CREATE TABLE table_name (
column1 datatype,
column2 datatype,

CONSTRAINT pk_constraint_name PRIMARY KEY (column1, column2, …);

In the above example, pk_constraint_name is the name of the primary key constraint, and (column1, column2, …) specifies the columns that make up the composite primary key.

Like a Primary Key, what a composite Primary Key is depends on the database.  Essentially a Composite Primary Key:

  • Is a combination of Fields (columns) that uniquely identifies every row.
  • Is an index in database systems that use indexes for optimization
  • Is a type of table constraint
  • It is applied with a data definition language (DDL) alter command
  • And may define parent-child relationship between tables
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