In SQL, a user-defined data type is a custom data type that you can create to meet specific requirements. It allows you to define your own data types based on existing data types provided by the database management system (DBMS).
Creating User-Defined Data Types
To create a user-defined data type in SQL, you can use the CREATE TYPE statement. This statement allows you to define the structure and behavior of your custom type.
First, let’s consider an example of creating a user-defined data type called EmployeeType. This data type will represent information about an employee, including their name, age, and department.
CREATE TYPE EmployeeType AS ( name VARCHAR(50), age INT, department VARCHAR(50) );
In the above example, we defined a new user-defined data type called EmployeeType. It consists of three attributes: name, age, and department, each with its own data type.
Using User-Defined Data Types
Once you have created a user-defined data type, you can use it in various SQL statements such as CREATE TABLE, INSERT INTO, and SELECT. Let’s see how we can utilize our EmployeeType in some examples:
Create Table with User-Defined Data Type:
CREATE TABLE Employees ( id INT, employee_info EmployeeType );
In the above example, we created a table called Employees. It has two columns: id, which is of type INT, and employee_info, which is of our user-defined data type EmployeeType.
Insert into Table using User-Defined Data Type:
INSERT INTO Employees (id, employee_info) VALUES (1, ('John Doe', 30, 'Marketing'));
In the above example, we inserted a record into the Employees table. We provided the values for both id and employee_info. Notice how we enclosed the values for the user-defined data type within parentheses.
Select from Table using User-Defined Data Type:
SELECT id, employee_info.name FROM Employees;
In this example, we selected the id and name attributes from the Employees table. Note that we accessed the attributes of the user-defined data type using dot notation (e.g.,
employee_info.name) to retrieve specific information.
Dropping User-Defined Data Types
If you no longer need a user-defined data type, you can drop it using the DROP TYPE statement.
DROP TYPE EmployeeType;
In this example, we dropped the user-defined data type called
User-defined data types in SQL allow you to create custom data structures tailored to your specific needs. By defining your own types, you can enhance code readability and simplify database management. Remember to use proper naming conventions and consider how these types can improve your SQL queries and data modeling.