Columns represent Entity Attributes

• Column Domain: A domain specifies the permissible set of values that a column can hold in a relational database. It acts as a constraint on the type of data that can be entered into a column.
• Type: Each column in a table is associated with a data type, which is part of its domain definition. Examples include INTEGER, VARCHAR, DATE, BOOLEAN, etc.

• Data Integrity: Domains help maintain data integrity by ensuring that only valid data types and values are entered into a column. This prevents data anomalies and inconsistencies.
• Consistency: By defining domains, you ensure that the data across different rows and tables remains consistent and reliable.
• Validation: Domains provide a way to validate data before it is stored in the database, reducing errors and improving data quality.

• Data Type: Specifies the kind of data (e.g., integer, string, date) that can be stored in the column.
• Constraints: Additional rules that further restrict the values. These can include:
  • NOT NULL: Ensures that the column cannot have NULL values.
  • CHECK: Defines a condition that each row must satisfy.
  • UNIQUE: Ensures that all values in the column are unique across the table.
  • DEFAULT: Provides a default value for the column if none is specified.
  
  
• Range of Values: Sometimes, a domain can also specify a range or a specific set of allowable values. For example, a column for days of the week might restrict values to "Monday", "Tuesday", etc.

• Creating a Table with Domains: When creating a table, you define the domain for each column using data types and constraints.
  

  CREATE TABLE Employees (
      EmployeeID INT NOT NULL,
      FirstName VARCHAR(50) NOT NULL,
      LastName VARCHAR(50) NOT NULL,
      BirthDate DATE,
      Salary DECIMAL(10, 2) CHECK (Salary > 0),
      Email VARCHAR(100) UNIQUE
  );
  

  CREATE TABLE Products (
      ProductID INT PRIMARY KEY,
      ProductName VARCHAR(100) NOT NULL,
      Price DECIMAL(10, 2) CHECK (Price >= 0),
      Stock INT CHECK (Stock >= 0)
  );
  

• Enhanced Data Quality: By restricting the values that can be entered, domains help ensure high-quality data.
• Simplified Data Management: Domains make it easier to manage data by providing clear rules for what constitutes valid data.
• Improved Query Accuracy: Queries yield more accurate results when data integrity is maintained through strict domain enforcement.
• Better Documentation: Domains serve as documentation for the expected data format and constraints, making the database schema easier to understand and maintain.