Integrity Constraints in SQL

Domain Integrity Constraints in SQL

Domain Integrity Constraints define the permissible values for a given column. By applying these constraints, you can restrict the data entered into a specific column, ensuring consistent data values across your database.

Some commonly used domain integrity constraints include:

• Data type – The column must contain values of a specific data type
• Data format – The format of the values in a column must follow a defined pattern
• Range – The values must fall within a specified range
• Enumeration – The values in the column can only be taken from a predefined set of values

Entity Integrity Constraints in SQL

Entity Integrity Constraints involve uniquely identifying the rows in a database table, such that there are no duplicate or null values in a primary key column. A primary key is a unique column in a table that uniquely identifies every row in the table. This constraint helps maintain the uniqueness and integrity of data by preventing the existence of duplicate rows.

Referential Integrity Constraint in SQL

Referential Integrity Constraint ensures that relationships between tables are maintained consistently. It is enforced by using foreign keys, which are columns in a table that refer to a primary key in another table. The foreign key helps to maintain the referential integrity between two related tables by making sure that changes in one table's primary key are reflected in the corresponding foreign key in another table.

There are two main rules to uphold when it comes to Referential Integrity Constraints:

• If a primary key value is updated or deleted, the corresponding foreign key values in the related table must be updated or deleted as well.
• Any new foreign key value added to the related table must have a corresponding primary key value in the other table.

Implementing Complex Integrity Constraints in SQL

Complex Integrity Constraints in SQL involve implementing custom constraints and using triggers for specific scenarios. These constraints enable the database to handle more complex data requirements than the standard constraints allow. Implementing complex constraints allows you to maintain data integrity, consistency, and accuracy in the database effectively while addressing specific business needs.

Creating Custom Constraints

Creating Custom Constraints involves defining rules based on specific requirements that cannot be addressed using the standard predefined constraints. These constraints help to maintain the data validity and consistency according to the specific business logic. Custom constraints can be implemented using SQL CHECK constraints.

Here are a few essential aspects to consider when creating custom constraints:

• Custom constraints must be based on columns within the same table.
• The constraint must consist of a logical expression, not a SQL query.
• The constraint must be verified and maintained during all INSERT, UPDATE and DELETE operations on the table.

To create a custom constraint in SQL, you can use the CHECK keyword followed by the desired constraint condition.

CREATE TABLE Employees (
   employee_id INT PRIMARY KEY,
   salary NUMERIC(10,2),
   join_date DATE,
   CHECK (join_date > '2000-01-01' AND salary >= 20000)
);

By creating custom constraints, you can maintain data integrity and enforce specific business rules that are not achievable using standard SQL constraints.

Using Triggers for Complex Constraints

Triggers are a powerful tool in SQL that facilitate the implementation of complex integrity constraints. SQL triggers are attached to specific events such as INSERT, UPDATE, DELETE or even multiple events on specific tables or views. Triggers are automatically invoked when the defined events occur, making it possible to verify constraints and apply alterations automatically.

When you need to apply complex constraints that involve multiple tables or complex calculations, triggers become a suitable method to maintain data integrity.

Triggers can be classified into two types:

• Before Triggers – These triggers are executed before the specified event. They are useful for maintaining complex constraints by validating and manipulating data before it is permanently stored in the database.
• After Triggers – These triggers are executed after the specified event. They can be used for maintaining data integrity by performing additional actions like updating other tables or generating log entries for maintaining an audit trail.

CREATE TRIGGER check_project_budget
AFTER INSERT ON tasks
FOR EACH ROW
BEGIN
  DECLARE project_total_cost NUMERIC(10,2);
  DECLARE project_budget NUMERIC(10,2);
  
  SELECT SUM(estimated_cost)
  INTO project_total_cost
  FROM tasks
  WHERE project_id = NEW.project_id;

  SELECT budget
  INTO project_budget
  FROM projects
  WHERE project_id = NEW.project_id;

  IF project_total_cost > project_budget THEN
    SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = 'Project budget exceeded!';
  END IF;
END;

In summary, using triggers allows you to maintain complex constraints involving multiple tables and calculations, ensuring data integrity, consistency, and adherence to business rules. By understanding when and how to use custom constraints and triggers, you can build a robust and optimized database for your specific needs.

Ensuring Data Consistency with Integrity Constraints in SQL

Integrity constraints play a vital role in ensuring data consistency within SQL databases by enforcing rules to maintain data integrity, uniqueness, and referential relationships across tables. This section will delve deep into the aspects of altering and dropping constraints and maintaining referential integrity during updates and deletes.

Altering and Dropping Constraints

When dealing with an evolving database, you might need to modify or remove constraints to accommodate changes in the data structure or business requirements. In SQL, you can easily do this by using the ALTER TABLE statement to alter or drop constraints.

However, when altering or dropping constraints, it's essential to follow certain methods and best practices to avoid compromising your data's integrity and consistency. Let us explore how to perform these actions safely and correctly with the following steps:

1. Identify the constraint to be altered or dropped - You can usually find this information in the table schema or by querying the INFORMATION_SCHEMA.
2. Ensure data integrity - Before altering or removing a constraint, make sure the existing data adheres to the new constraint requirements, or address any potential conflicts if the constraint is dropped.
3. Perform the alteration or dropping using the ALTER TABLE statement - Issue the appropriate SQL commands to apply the changes to the constraints as needed.
4. Verify the changes - Confirm that the constraints have been altered or dropped successfully by examining the updated table schema or running test queries.

1. Identify the constraint:

   SELECT * FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS
        WHERE TABLE_NAME = 'Employees';
       

2. Ensure that all existing salary values are equal to or greater than 18,000.
3. Perform the alteration using the ALTER TABLE statement:

   ALTER TABLE Employees
        DROP CONSTRAINT salary_constraint;
       ALTER TABLE Employees
        ADD CONSTRAINT salary_constraint CHECK (salary >= 18000);
       

4. Verify the changes by checking the table schema and running test queries.

Maintaining Referential Integrity during Updates and Deletes

Referential integrity, as mentioned previously, is essential for preserving the consistency and reliability of the relationships between tables in your database. Updates and deletes on the primary key or foreign key columns can potentially break referential integrity if not handled carefully. Therefore, it's crucial to address the impact of these operations on related tables.

In SQL, you can use referential actions to maintain referential integrity during updates and deletes. These actions define how the changes in one table are propagated to related tables. The main referential actions are:

• CASCADE - This action automatically updates or deletes the corresponding foreign key rows in the related table when the primary key in the parent table is updated or deleted.
• SET NULL - If the primary key is updated or deleted, this action sets the corresponding foreign key values in the related table to NULL.
• SET DEFAULT - This action sets the foreign key values in the related table to their default values when the primary key in the parent table is updated or deleted.
• NO ACTION - No action is taken on the related table when the primary key is updated or deleted in the parent table. However, the SQL standard requires that the primary key update or deletion must be rejected if it violates the referential integrity constraint.

To maintain referential integrity, you can choose one of the above actions that best suits your specific data requirements. Define these actions using the FOREIGN KEY constraint definition in the CREATE TABLE or ALTER TABLE statement.

CREATE TABLE Orders (
    order_id INT PRIMARY KEY,
    customer_id INT,
    product_id INT,
    quantity INT,
    FOREIGN KEY (customer_id) REFERENCES Customers(customer_id) ON UPDATE CASCADE ON DELETE CASCADE
);

By following these guidelines, you can effectively maintain referential integrity during updates and deletes and ensure data quality and consistency in your SQL database.