SQL Alternate Key

Keys are basically the backbone of maintaining data integrity and uniqueness in a relational database world. Many of us would be familiar with Primary Keys and Foreign Keys, but the A

lternate Key is yet another fundamental type that people less often discuss. Knowing about alternate keys would make this reader better design robust database schemas also ensure all potential unique identifiers are put in proper use. This article will introduce the concept of Alternate Keys in SQL. As mentioned above, it would be different from other types of keys such as primary keys and other keys. The syntax for creating and administering alternate keys in SQL will also be presented here with examples.

What is an Alternate Key in SQL?

Alternate Key is a column or a group of columns within a database table that can uniquely identify a record but not chosen to be a primary key. For instance, if there are plenty of unique identifiers other than the primary key, for a table, then they are known as alternate keys.

Though a primary key is selected to uniquely identify each row, any other unique columns (or a group of columns) that can be used to uniquely identify rows are referred to as alternate keys.

Definition of Alternate Key

• Alternate Key: A unique identifier that is not selected as the primary key.
• An alternate key enforces the uniqueness constraint on a column or a group of columns, ensuring that duplicate values are not allowed.
• Alternate keys act as backup keys if the primary key is not available.

Creating an Alternate Key in SQL

Alternate Key Constraint in SQL

The alternate key constraint in SQL is an essential feature that allows for the unique identification of records in a database table, serving as a secondary means of enforcing uniqueness alongside the primary key. While the primary key is designated to uniquely identify each record, the alternate key constraint in SQL ensures that other columns can also maintain unique values without being selected as the primary key. For instance, in a table containing customer information, both an email address and a phone number could serve as alternate keys, allowing for flexibility in data retrieval while enforcing data integrity. By defining an alternate key constraint in SQL using the UNIQUE keyword, you can ensure that these fields remain distinct across all records. This capability is particularly useful in scenarios where multiple identifiers are necessary for different operations, enhancing the overall robustness of the database design. Understanding how to implement and utilize the alternate key constraint in SQL is crucial for creating well-structured relational databases that uphold data integrity and facilitate efficient querying.

An alternate key definition is done using the UNIQUE constraint. UNIQUE constraint ensures that all of the values within the column are unique for the entire table. While a primary key uniquely identifies each row, alternate keys offer other choices on uniqueness.

Syntax for Creating an Alternate Key

The UNIQUE constraint is used to declare an alternate key. Here’s the basic syntax:

CREATE TABLE table_name (
    column1 datatype,
    column2 datatype,
    ...,
    CONSTRAINT constraint_name UNIQUE (column_name)
);

Example of Creating an Alternate Key

Let’s consider an example that has a table Customers. Each customer has a unique customer_id which is the primary key, but both the columns of email and phone_number should be unique in order to prevent duplication. In this case, email and phone_number can be defined as alternate keys.

CREATE TABLE Customers (
    customer_id INT PRIMARY KEY,
    email VARCHAR(100),
    phone_number VARCHAR(15),
    CONSTRAINT email_unique UNIQUE (email),
    CONSTRAINT phone_unique UNIQUE (phone_number)
);

In this example:

• customer_id is the primary key, ensuring that each customer has a unique identifier.
• email and phone_number are alternate keys, ensuring that no two customers share the same email or phone number.

Difference Between Alternate Key and Primary Key

While both alternate keys and primary keys enforce uniqueness, they serve different purposes in a database schema.

Key Differences

Feature	Primary Key	Alternate Key
Purpose	Uniquely identifies each record in a table	Acts as a backup unique identifier
Number of Keys Allowed	Only one primary key per table	A table can have multiple alternate keys
NULL Values	Cannot contain NULL values	Can contain NULL values (unless specified)
Constraint	Defined using the PRIMARY KEY keyword	Defined using the UNIQUE constraint
Usage	Chosen as the main key for table identification	Not selected as the primary key but ensures uniqueness

Example of Alternate Key vs Primary Key

In a Users table, the user_id can be used as the primary key. However, username and email can both be alternate keys since they are also unique fields.

CREATE TABLE Users (
    user_id INT PRIMARY KEY,
    username VARCHAR(50),
    email VARCHAR(100),
    password VARCHAR(50),
    CONSTRAINT username_unique UNIQUE (username),
    CONSTRAINT email_unique UNIQUE (email)
);

In this example:

• user_id is the primary key, ensuring uniqueness for each user.
• username and email are alternate keys that also enforce uniqueness but are not chosen as the primary key.

Examples of Alternate Keys in SQL

Let’s look at more practical Examples of Alternate Keys in SQL across different scenarios.

Example 1: Employee Table

In the Employees table, employee_id is the primary key, although since it is unique for each employee, social_security_number (SSN) could also be considered an alternate key.

CREATE TABLE Employees (
    employee_id INT PRIMARY KEY,
    first_name VARCHAR(50),
    last_name VARCHAR(50),
    social_security_number VARCHAR(11),
    CONSTRAINT ssn_unique UNIQUE (social_security_number)
);

Here:

• employee_id is the primary key.
• social_security_number is an alternate key ensuring that no two employees share the same SSN.

Example 2: Product Table

The product_id is your primary key. If the SKU (Stock Keeping Unit) and barcode also need to be unique, then in a Products table you might have:.

CREATE TABLE Products (
    product_id INT PRIMARY KEY,
    product_name VARCHAR(100),
    SKU VARCHAR(50),
    barcode VARCHAR(13),
    CONSTRAINT sku_unique UNIQUE (SKU),
    CONSTRAINT barcode_unique UNIQUE (barcode)
);

In this case:

• product_id is the primary key.
• SKU and barcode serve as alternate keys ensuring that no two products have the same SKU or barcode.

Managing Alternate Keys in SQL

How to Add an Alternate Key in SQL

You can add an alternate key to an existing table by using the ALTER TABLE command.

Syntax for Adding an Alternate Key

ALTER TABLE table_name
ADD CONSTRAINT constraint_name UNIQUE (column_name);

Example of Adding an Alternate Key

Let’s add an alternate key to the Customers table for the email column:

ALTER TABLE Customers
ADD CONSTRAINT email_unique UNIQUE (email);

This command adds a unique constraint to the email column, making it an alternate key.

Dropping an Alternate Key in SQL

If you need to remove an alternate key, you can drop the unique constraint using the ALTER TABLE command.

Syntax for Dropping an Alternate Key

ALTER TABLE table_name
DROP CONSTRAINT constraint_name;

Example of Dropping an Alternate Key

Let’s drop the alternate key on the email column in the Customers table:

ALTER TABLE Customers
DROP CONSTRAINT email_unique;

This command removes the unique constraint, allowing duplicate values in the email column.

When to Use Alternate Keys in SQL

Alternate keys are useful if you have a table that has more than one unique field and you want to ensure the fields are unique without declaring the fields as a primary key.

Example of When to Use Alternate Keys

Let’s take an example Student Enrollment System that has as a primary key student_id for each student but should also look after the uniqueness of email and student_number.

CREATE TABLE Students (
    student_id INT PRIMARY KEY,
    student_number VARCHAR(10),
    email VARCHAR(100),
    first_name VARCHAR(50),
    last_name VARCHAR(50),
    CONSTRAINT student_number_unique UNIQUE (student_number),
    CONSTRAINT email_unique UNIQUE (email)
);

In this example:

• student_id is the primary key.
• Both student_number and email are alternate keys, ensuring that no two students share the same number or email address.

Advantages of SQL Alternate Key

In SQL, an alternate key is any candidate key of the table that is not designated as the primary but still allows one to uniquely identify records. Though a primary key is primarily a unique identifier for a table, alternate keys also play an extremely important role in maintaining integrity and aiding the optimization of queries. The important advantages of using alternate keys in SQL are listed below.

1. Ensures Data Uniqueness

Alternate keys ensure uniqueness in a table. Even though one column is guaranteed to be unique by making it the primary key, alternate keys can ensure other columns or any combination of columns remain unique as well. It prevents duplicating some fields, which helps keep information in a database more accurate and reliable.

2. Supports More Flexible Querying

Alternate Keys. Alternate keys provide more flexible ways of querying. Using different unique identifiers, you can query records using a column other than the primary key. This might be necessary because some columns are not ideal for search in the record lookup. When alternate keys exist, you can speed up your queries of large records when you have them indexed.

3. Complex Relationships

In relational database systems, alternate keys can be defined to specify relationships between different tables, exactly like the primary keys. It helps the database to maintain complex relationships and allows the creation of unique constraints of various types. For instance, a table can contain an email address and a username; while the email may become the primary key, the username may be treated as an alternate key in order to uniquely identify users.

4. Alternates in Selecting the Primary Key

When a table contains more than one candidate key, then alternate keys help in selection of a suitable key for the primary key of the table. The extra candidate keys are used as alternates keys, and you can select any of the candidate keys as the primary key for your table considering query performance and readability and storage.

5. Data Integrity Improved

Similar to a primary key, an alternate key forces uniqueness constraints on column(s) it is applied to. That’s important for integrity reasons since important fields of data (like email, phone number, or social security number) are unique across rows and prevent errors and duplicates from getting into the system.

6. Performance Improves with Indexing

Indexing an alternate key will greatly improve query performance. An alternate key index allows for faster searches, lookups, and joins, especially when the alternate key is often used in query conditions. This can optimize database operations when dealing with tables having multiple columns or queries that span several columns.

7. Backup for Primary Key

An alternate key can serve as a backup in situations wherein the primary key might be vulnerable to intrusion or is just not the best fit to be used for some operations directly anyway. For example, if the primary is an auto-incremented integer, then the alternate key would be something like an email or a product code that would be more meaningful as an identifier in other contexts.

8. Suitable when composite keys

In tables where composite keys are used (where multiple columns in combination form a unique key), alternate keys can also be defined. This provides even more flexibility in maintaining uniqueness on different column combinations. It is particularly helpful where an application requires that the combination of multiple fields be unique-for example, an application requiring that a user_id, when combined with a role, forms a unique combination in a permissions table.

9. Implements Recovery and Analysis

Alternate keys are helpful in a recovery or migration case in an alternate route to uniquely identify records. This might be especially important if the primary key is not sufficient or if the database administrator will require more identifiers to match records between systems. Alternate keys can be also of help in data analysis and reporting by possible precise queries dependent upon unique values.

10. Improves Application Logic

Alternate keys help enforce business logic rules at the database level from the viewpoint of the application design. Such examples are ensuring that usernames, email addresses, or order numbers are unique in the system for proper application functioning. Alternate keys thus ensure intrinsic means for enforcing these rules rather than custom validations in the application level.

Disadvantages of SQL Alternate Key

Alternate keys in SQL are helpful to enforce uniqueness and promote data integrity, yet they also have some disadvantages. Knowing these disadvantages will help in arriving at more informed choices about when and how to make use of alternate keys in the design of databases. Below are the major disadvantages of using SQL alternate keys:

1. Increased Complexity in Database Design

Introducing multiple alternate keys in the database increases the complexity of its schema. Management and maintenance of multiple keys, especially in extremely large systems, make the design very hard to understand or update. Confusion among database administrators or developers working on the system later increases because of this.

2. Overhead in Performance

Although alternate keys are useful to enforce uniqueness and support on queries, they introduce significant overhead concerning data insertion, updates, and deletions. If alternate keys are indexed, every change to the data of a table will need to modify the indexes of that table for those keys. This can slow down operations over writes, especially in large tables with much-changing data.

3. Higher Storage Usage

Alternate keys need indexes, and these indices take additional space in the database. For every alternate key, storage space is wasted by the index of the alternate key. This means an increased size for the database. In cases where multiple alternate keys are defined together, this creates huge overhead in storage, especially for large-sized databases or in systems that lack sufficient storage.

4. Increases the Complexity in Optimizing Queries

Sometimes the use of many alternative keys can make queries difficult to optimize. While using alternative keys can improve certain types of queries, it can introduce some performance trade-offs as the database optimizer needs to choose between multiple keys for table-joining operations or filtering data. Using incorrect indexes may introduce inefficient plans for your queries, thereby slower performance in some scenarios.

5. Ineffective Data Maintenance

The frequent use of different alternate keys will make it troublesome in data maintenance like migration, backup, or recovery. Managers need to maintain assurance that alternate keys are all consistent and unique for the application on multiple environments. This makes errors and sometimes fails during restoration or synchronization procedures.

6. Functionality Redundancy

Poor planning of alternate keys will introduce redundancy through duplicate functionality with primary keys or other candidate keys. For instance, two very highly correlated alternate keys might perform the same functionality in different contexts. This creates unnecessary complexity and confusion, especially when alternate keys overlap functionalities with foreign keys and unique constraints.

7. More Work to Validate Data Entry

While alternate keys ensure data uniqueness, they impose rigid validation rules on data entry. This can require even more work when entering data because, in order for the values for the columns of an alternate key to be entered, they have to be unique. If the columns of an alternate key are anchored on some business logic-names and addresses- or usernames or product codes-say-it could require more frequent conflict resolutions on the part of the users or the system.

8. Bulk Operations Are Slow

Maintenance activities for alternate keys on high volume operations, such as mass inserts, updates, or deletes will be slower because the alternate key indexes must be kept up to date. Also, any import of large volumes of data will be slower because the system checks all the constraints of alternate keys on new records for no duplicate record.

9. Complexity in Maintaining Dynamic Data Models

Alternate keys become cumbersome to maintain as the schema or business requirement changes. If the alternate key were to be modified, dropped, or replaced, this could likely result in complex migration and batch-level re-indexing processes on large datasets and possibly result in downtimes or failures in data migration processes.

10. Unnecessary Constraints

Defining an alternate key may sometimes impose a constraint on the data that is not truly needed and which would be limiting in the future. For example, in a system, where some fields might have to allow for duplication in the near future, such as email addresses for multiple accounts, the imposition of an alternate key very early may become too limiting and may force the redesign of the schema or the removal of the key down the road.