Constraints and Relationships in DB2

Introduction

In DB2, constraints and relationships are essential elements of database design that ensure data integrity and enforce business rules. Constraints define rules or conditions that must be met by the data, while relationships establish connections between tables. In this tutorial, we will explore the various types of constraints and relationships supported by DB2 and learn how to use them effectively in database design.

1. Constraints

Constraints help maintain the integrity and accuracy of data in DB2. Here are a few commonly used constraints:

• Primary Key (PK): Ensures the uniqueness and non-nullability of a column or a combination of columns.
• Foreign Key (FK): Establishes a relationship between two tables by enforcing referential integrity.
• Unique Constraint: Ensures that the values in a column or a combination of columns are unique.
• Check Constraint: Specifies a condition that must be satisfied by the data in a column.
• Not Null Constraint: Ensures that a column does not contain null values.

These constraints play a vital role in maintaining data integrity and enforcing business rules within the database.

2. Relationships

Relationships define the associations between tables in DB2. The most common type of relationship is the parent-child relationship established using foreign keys. Here's an example:

CREATE TABLE Orders (


OrderID INT PRIMARY KEY,
CustomerID INT,
OrderDate DATE,
-- Other columns
);

CREATE TABLE Customers (
CustomerID INT PRIMARY KEY,
FirstName VARCHAR(50),
LastName VARCHAR(50),
-- Other columns
);

ALTER TABLE Orders
ADD CONSTRAINT FK_Orders_Customers
FOREIGN KEY (CustomerID)
REFERENCES Customers(CustomerID);

In the above example, the Orders table has a foreign key relationship with the Customers table, linking the CustomerID column in the Orders table to the CustomerID column in the Customers table. This relationship ensures referential integrity, preventing the creation of orphan records.

Common Mistakes to Avoid

• Not defining primary keys for tables.
• Forgetting to create foreign key constraints to enforce referential integrity.
• Using inappropriate check constraints that do not accurately represent the business rules.
• Overusing or underusing constraints, leading to unnecessary restrictions or data integrity issues.
• Not considering the performance impact of constraints, especially when dealing with large datasets.

Frequently Asked Questions (FAQs)

1. Q: Can a table have multiple foreign keys?

   A: Yes, a table can have multiple foreign keys that establish relationships with different tables.

2. Q: Can constraints be modified or dropped after they are created?

   A: Yes, constraints can be modified or dropped using the ALTER TABLE statement. However, this may require careful consideration and planning to ensure data integrity.

3. Q: What is the purpose of the unique constraint?

   A: The unique constraint ensures that the values in a column or a combination of columns are unique, preventing duplicate entries.

4. Q: Can constraints be disabled temporarily?

   A: Yes, constraints can be temporarily disabled using the ALTER TABLE statement. However, this should be done with caution, as it may compromise data integrity during the disabled period.

5. Q: Can constraints be defined at the column level and table level?

   A: Yes, constraints can be defined at both the column level and table level. Column-level constraints apply to specific columns, while table-level constraints apply to the entire table.

Summary

In this tutorial, we explored the concept of constraints and relationships in DB2. Constraints help maintain data integrity and enforce business rules, while relationships establish connections between tables. We discussed different types of constraints and demonstrated the creation of a foreign key relationship. We also highlighted common mistakes to avoid and provided answers to frequently asked questions related to constraints and relationships. By properly implementing constraints and relationships, you can ensure the accuracy, consistency, and reliability of your DB2 databases.