Database Design

Effective database design is crucial for the performance, integrity, and maintainability of any SQL Server application. This section covers the fundamental principles and best practices for creating robust and efficient database schemas.

Fundamentals of Database Design

A well-designed database minimizes data redundancy, ensures data consistency, and facilitates efficient data retrieval. Key considerations include understanding the data requirements, identifying entities and their attributes, and defining the relationships between them.

Normalization

Normalization is a process used to organize data in a database. It involves structuring tables in a way that reduces data redundancy and improves data integrity. The primary goals of normalization are to eliminate:

• Repeating groups of data: Information that appears multiple times for the same record.
• Redundant data: The same piece of information stored in multiple places.
• Data dependencies: Ensuring that attributes not directly related to the primary key are not stored in the same table.

Common normal forms include First Normal Form (1NF), Second Normal Form (2NF), and Third Normal Form (3NF).

Tables and Columns

A relational database is composed of tables, which are collections of related data organized in rows and columns. Each column represents an attribute of the entity being stored, and each row represents an instance of that entity.

Best practices for table and column naming:

• Use clear, descriptive names.
• Avoid spaces and special characters; use underscores (e.g., OrderDate or order_date).
• Be consistent with casing (e.g., PascalCase or snake_case).
• Use singular nouns for table names and descriptive names for columns.

Keys and Relationships

Keys are fundamental to defining relationships between tables and ensuring data integrity. They are columns or sets of columns that uniquely identify rows or link related data.

Key types include:

• Primary Key: Uniquely identifies each record in a table. It cannot contain NULL values and should ideally be immutable.
• Foreign Key: A column or set of columns in one table that refers to the primary key in another table, establishing a link between them.
• Unique Key: Ensures that all values in a column or set of columns are unique, but it can contain NULL values.

Relationships are typically defined as:

• One-to-One: Each record in table A corresponds to at most one record in table B, and vice versa.
• One-to-Many: Each record in table A can correspond to multiple records in table B, but each record in table B corresponds to only one record in table A.
• Many-to-Many: Each record in table A can correspond to multiple records in table B, and vice versa. This is typically implemented using an intermediary junction table.

Example: One-to-Many Relationship

Consider two tables: Customers and Orders. A customer can place many orders, but an order belongs to only one customer.

-- Customers Table
CREATE TABLE Customers (
    CustomerID INT PRIMARY KEY,
    FirstName VARCHAR(50),
    LastName VARCHAR(50),
    Email VARCHAR(100) UNIQUE
);

-- Orders Table
CREATE TABLE Orders (
    OrderID INT PRIMARY KEY,
    OrderDate DATETIME,
    CustomerID INT,
    TotalAmount DECIMAL(10, 2),
    FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID)
);

Indexes

Indexes are special lookup tables that the database search engine can use to speed up data retrieval operations. By creating an index on one or more columns, you can significantly improve the performance of queries that filter or sort data based on those columns.

Types of indexes:

• Clustered Index: Determines the physical order of data in the table. A table can have only one clustered index.
• Non-Clustered Index: Does not affect the physical order of data but creates a separate structure with pointers to the actual data rows. A table can have multiple non-clustered indexes.
• Unique Indexes
• Filtered Indexes

Considerations for indexing:

• Index columns frequently used in WHERE, JOIN, and ORDER BY clauses.
• Avoid over-indexing, as it can impact write performance (INSERT, UPDATE, DELETE).
• Regularly review and maintain indexes.

Constraints

Constraints are rules enforced on data columns to ensure the accuracy and reliability of the data in the database. Common constraints include:

• PRIMARY KEY
• FOREIGN KEY
• UNIQUE
• CHECK: Enforces a condition on the values in a column (e.g., CHECK (Price > 0)).
• DEFAULT: Assigns a default value to a column when no value is specified during an insert operation.
• NOT NULL: Ensures that a column cannot have a NULL value.

Data Types

Choosing appropriate data types for columns is essential for data integrity, storage efficiency, and query performance. SQL Server provides a rich set of data types.

Choosing the right data type:

• Use the smallest data type that can accurately store the data.
• Use exact numeric types (DECIMAL) for financial data.
• Use NVARCHAR for text that may contain international characters.
• Consider date and time precision needs when choosing between DATETIME and DATETIME2.