Do Composite Primary Keys Exist in Table Without Many to Many Relationships?

Yes, a composite primary key can exist in a table that has no many to many relationships. The PRIMARY KEY constraint provides a unique identifier to a table, ensuring that there are no duplicate entries. This constraint can be defined on one or more columns, which is known as a composite primary key. In this article, we will explore the concept of composite primary keys, why they are important, and provide examples to illustrate their usage.

Understanding the Role of Composite Primary Keys

A primary key is a column or a set of columns in a table that uniquely identifies each row. While many-to-many relationships often require a junction table with a composite primary key, a composite primary key can exist independently without any relationship constraints. This key ensures data integrity by preventing duplicates and facilitating efficient data retrieval.

A composite primary key is a combination of two or more columns that together form a unique identifier for each row in the table. Unlike a simple primary key, which is a single column, a composite primary key can be made up of multiple columns. The combination of these columns guarantees the uniqueness of each record in the table.

Benefits of Using Composite Primary Keys

Composite primary keys offer several advantages:

Data Integrity: They ensure that no two rows in the table have the same combination of values, thereby maintaining data integrity. Efficient Indexing: The uniqueness enforced by a composite primary key often leads to the creation of a unique index, which can significantly speed up query performance. Reference Integrity: They can be used in a one-to-many or many-to-many relationship, ensuring that related data is consistently and correctly linked.

Examples of Using Composite Primary Keys

Example 1: Company Departments

In a company, departments must be uniquely identified. A composite primary key can be defined using a sequence for the department number, which ensures that each department is uniquely referenced.

CREATE TABLE DEPARTMENT (
  DEPT_NO INTEGER PRIMARY KEY,
  DEPT_NAME VARCHAR200 NOT NULL,
  SUMMARY VARCHAR2000 NOT NULL
)
CREATE SEQUENCE DEPT_NO_SEQ
  INCREMENT BY 100
  START WITH 100

Example 2: Family Members

Family members must have different names if they are born on the same date. A composite primary key can be used to ensure that no two family members share the same name on the same date, preventing confusion at family reunions.

CREATE TABLE FAMILY (
  FIRST VARCHAR30 NOT NULL,
  LAST VARCHAR30 NOT NULL,
  BIRTHDAY DATE NOT NULL,
  PRIMARY KEY (FIRST, BIRTHDAY)
)

Note that when using a composite primary key, both columns must be specified in the WHERE clause for the query to utilize the index properly.

SELECT * FROM FAMILY
WHERE FIRST  'JANE' AND BIRTHDAY BETWEEN '2010-01-01' AND '2015-12-31'

Composite Primary Keys in One-to-Many Relationships

Composite primary keys are not limited to tables without relationships. They can also be used in the 'many' side of a one-to-many relationship. For instance, in a financial market database, daily stock data for equities can be related to the equity table via a composite primary key.

Consider a database that tracks daily open/close/high/low/volume data for equities. The equity table has a primary key on ticker_symbol, while the daily table has a composite primary key on ticker_symbol and latest_timestamp.

CREATE TABLE EQUITY (
  Ticker_symbol VARCHAR20 PRIMARY KEY,
  Company_Name VARCHAR100 NOT NULL,
  Industry VARCHAR50 NOT NULL
)
CREATE TABLE DAILY (
  Ticker_symbol VARCHAR20,
  Latest_Timestamp TIMESTAMP,
  Open decimal(9,2),
  Close decimal(9,2),
  High decimal(9,2),
  Low decimal(9,2),
  Volume BIGINT,
  PRIMARY KEY (Ticker_symbol, Latest_Timestamp)
)

The composite primary key in the daily table ensures that each daily entry is uniquely linked to the correct equity, maintaining the integrity of the data relationship.

Conclusion

A composite primary key is a powerful tool for ensuring data integrity and improving query performance. While often associated with many-to-many relationships, these keys can be effectively used in tables that have no such relationships. By leveraging composite primary keys, database administrators can maintain the integrity of their data, enhance query efficiency, and ensure a robust data management system.