What are Database Keys and their Types?

Have you ever wondered how a massive database keeps track of millions of individual records? The secret lies in database keys, which act like unique identification tags for each piece of data. Just like a key unlocks a specific door, database keys allow for efficient retrieval and organization of information. This article delves into the world of database keys, exploring the different types and their crucial roles in maintaining data integrity and enabling smooth information flow.

What are Database Keys?

Keys are unique identifiers used to establish relationships between tables and ensure data integrity. Keys are essential for identifying and retrieving specific records within a table. The most common types of keys include primary keys and foreign keys.

Database keys offer several advantages:

• Unique Identification: The primary function of keys is to uniquely identify each row (record) in a table. This is similar to a person's social security number, ensuring no two entries are identical.‍
• Data Integrity: Keys help maintain data accuracy and consistency. Enforcing uniqueness prevents duplicate entries and ensures each record represents a distinct piece of information.‍
• Efficient Data Retrieval: Keys act as an indexing system for tables. When you query the database for specific data, you can use the key to quickly locate the relevant rows, significantly improving search speed and efficiency.

Types of Keys in Database

A database management system has several types of keys, each serving a specific purpose. 

Primary Key

The primary key is the chosen candidate key that uniquely identifies each record within a table. 

It cannot contain duplicate values or NULL values. The primary key is typically indexed, allowing for faster search and retrieval operations. In relational database systems, the primary key plays a pivotal role in establishing relationships between tables through foreign keys.

For example, the below Students table has StudentID as the primary key since it uniquely identifies each student record.

In this example, "StudentID" is the primary key. It has these characteristics:

1. Unique: Each StudentID is different for every student.

2. Not null: Every student must have a StudentID.

3. Unchanging: The StudentID doesn't change over time.

CREATE TABLE Students (
  StudentID INT PRIMARY KEY,
  FirstName VARCHAR(50) NOT NULL,
  LastName VARCHAR(50) NOT NULL,
  ... (other student attributes)
);

Candidate Key

A candidate key represents a unique combination of one or more columns within a table that can uniquely identify each row and can potentially become the primary key.  Each candidate key ensures that no two rows in the table can have the same combination of values for the candidate key columns. 

Designing candidate keys is a fundamental step in database schema design, as they lay the groundwork for establishing the primary means of identifying records within a table.

For instance, in a Students table, possible candidate keys could be StudentID, SocialSecurityNumber, and a combination of FirstName and LastName. 

In this table:

• "SocialSecurityNumber" is a candidate key as it uniquely identifies each student.
• The combination of "FirstName" and "LastName" can also be a candidate key if it ensures uniqueness throughout the information.

CREATE TABLE Students (
  StudentID INT PRIMARY KEY, -- Unique identifier (chosen as primary key)
  SocialSecurityNumber VARCHAR(11) UNIQUE, -- Unique SSN (privacy concerns)
  FirstName VARCHAR(50) NOT NULL,
  LastName VARCHAR(50) NOT NULL,
  UNIQUE (FirstName, LastName) -- Combination of names (alternative candidate key)
);

Unique Key 

A unique key constraint ensures that all values in a column or a combination of columns are unique within a table. Similar to a primary key, a unique key guarantees uniqueness but may allow NULL values. Unique keys provide additional data integrity by preventing duplicate values in specified columns. 

While distinct from primary keys, unique keys are critical in database design for enforcing uniqueness constraints and supporting various data modeling requirements. They offer flexibility in ensuring data consistency while accommodating different business rules and constraints.

For example, in the Users table given below, the Email column is assigned as a unique key constraint to ensure each user's email address is unique. However, some users might not provide an email address (NULL value).

In this example, the "Email" column has a unique constraint, ensuring every email copes with is precise, although it allows NULL values.

CREATE TABLE Users (
  UserID INT PRIMARY KEY,
  Email VARCHAR(50) NOT NULL UNIQUE,  -- Email is unique and NOT NULL
  FirstName VARCHAR(50) NOT NULL,
  LastName VARCHAR(50) NOT NULL,
  ... (other user attributes)
);

Foreign Key

A foreign key is a column or set of columns within a table that establishes a relationship with the primary key or a unique key in another table. It serves to maintain referential integrity by enforcing a link between related tables. 

The values in the foreign key columns must match the values in the referenced primary key or unique key column(s). This relationship allows for the creation of relational databases, where data across multiple tables can be interconnected and queried in meaningful ways. Foreign keys are instrumental in maintaining consistency and coherence in database systems.

For example, in an Orders table, CustomerID is a foreign key referencing the CustomerID primary key column in a Customers table, establishing a relationship between orders and customers.

Customers Table:

Orders Table:

In this example, "CustomerID" inside the Orders table is a foreign key that references "CustomerID" inside the Customers table.

CREATE TABLE Customers (
  CustomerID INT PRIMARY KEY,
  FirstName VARCHAR(50) NOT NULL,
  LastName VARCHAR(50) NOT NULL,
  ... (other customer attributes)
);

CREATE TABLE Orders (
  OrderID INT PRIMARY KEY,
  CustomerID INT NOT NULL,
  OrderDate DATE,
  ... (other order attributes),
  FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID)
    ON DELETE RESTRICT
    ON UPDATE CASCADE
);

Super Key

The super key is a set of one or more columns whose combined values can uniquely identify each record within a table. It represents a broader concept than a primary key and can include more columns than necessary for unique identification. 

Every super key guarantees uniqueness, but not all super keys are selected as primary keys. While primary keys are specifically chosen as the main means of identifying records, super keys provide a foundation for understanding the various potential identifiers within a table.

For example, in a Students table, a super key could be a combination of StudentID, FirstName, and LastName.

In this example, a super key may be a combination of "StudentID," "FirstName," and "LastName."

CREATE TABLE Students (
  StudentID INT PRIMARY KEY,
  FirstName VARCHAR(50) NOT NULL,
  LastName VARCHAR(50) NOT NULL,
  ... (other student attributes)
);

Alternate Key 

An alternate key is a candidate key within a table that is not chosen as the primary key. It represents an alternative means of uniquely identifying records but is not designated as the primary means of identification. An alternate key is useful when you need to enforce unique constraints on multiple tables or when you need to reference a table from multiple other tables using different columns.

In database design, considering alternate keys alongside primary keys ensures flexibility and accommodates different querying and indexing requirements. 

For instance, if StudentID is chosen as the primary key in a Students table, then SocialSecurityNumber could be an alternate key. Because it meets the criteria of uniqueness, identification, and potential significance within the context of the Students table.

In this example, "SocialSecurityNumber" is an alternate key because it uniquely identifies every student however is not the primary key.

CREATE TABLE Students (
  StudentID INT PRIMARY KEY,
  SocialSecurityNumber VARCHAR(11) UNIQUE, -- Enforce uniqueness for SSN (privacy concerns)
  FirstName VARCHAR(50) NOT NULL,
  LastName VARCHAR(50) NOT NULL,
  ... (other student attributes)
);

Composite Key

The composite key is a combination of two or more columns within a table that uniquely identifies each record. While individual columns may not be unique on their own, their combination ensures uniqueness. 

For instance, in an Orders table, a composite key might consist of OrderID and ProductID to identify each order item uniquely.

In this example, the combination of "OrderID" and "ProductID" serves as a composite key to uniquely pick out every order item.

CREATE TABLE Orders (
  OrderID INT NOT NULL,
  ProductID INT NOT NULL,
  Quantity INT,
  ... (other order item attributes),
  PRIMARY KEY (OrderID, ProductID) -- Composite primary key
);

Airbyte: Your Data Symphony - All The Right Keys In Perfect Harmony

By now, you might already have understood database keys and their uniqueness. It's now time to apply them to your databases to optimize storage and make retrieval easier. But first, you must consolidate your data in one single location. However, this question might come to your mind—what platform will allow you to do this? The answer is Airbyte.

Airbyte is a data integration platform that helps you consolidate your data scattered in different places into a unified destination. You can specify how Airbyte should handle any schema changes in the source for each connection. This process helps ensure accurate and efficient data syncs, minimizing errors and saving you time and effort in managing your data pipelines.

Here are some features of Airbyte: 

• Airbyte offers 350+ pre-built connectors for various source and destination systems. This extensive library ensures effortless data transfer from one platform to another.
• Airbyte’s PyAirbyte is a Python library that packs most Airbyte connectors into a single code. This streamlines data integration by allowing you to leverage a wide range of Airbyte-supported connectors with Python programming.
• If the pre-built connector list doesn't cover a specific connector, Airbyte’s Connector Development Kit (CDK) allows you to build custom connectors.
• You can specify how Airbyte must handle the source schema changes for each connection. This process helps ensure accurate and efficient data syncs, minimize errors, and save time and effort when handling your data pipelines.

Conclusion 

Database keys serve as the foundation for data organization and retrieval within relational databases. They offer a multifaceted approach to ensuring data integrity, efficient access, and establishing relationships between tables. Understanding the various key types—primary, candidate, super, foreign, and alternate—empowers you to create well-structured and efficient databases. By leveraging these key concepts, you can build a robust system that guarantees clean, consistent, and readily accessible data.

FAQs

1. How do I choose a database key?

Selecting a database key includes identifying a column or a set of columns that uniquely identifies each record inside the table. It has to be unique, not null, and preferably immutable to preserve data integrity.

2. Why are keys needed in a database?

Keys are critical in a database to ensure that each record can be uniquely identified, preventing duplicate entries and maintaining data integrity. They also assist in establishing relationships among distinct tables, facilitating efficient data retrieval and manipulation.

3. Do all databases need a primary key?

While not all databases require a primary key, it is exceptionally encouraged to have one to ensure every record is unique and to enhance data integrity and query performance. Primary keys also help set up clear relationships among tables in relational databases.

4. What is the difference between a primary key and a unique key?

A primary key uniquely identifies each record in a table and does not contain null values, while a unique key also guarantees uniqueness but can contain null values.

5. Can a foreign key be null?

Yes, a foreign key can be null if the relationship it represents is optional. This means that the referenced record in the parent table may only sometimes be required for some records inside the child table.