**Course Title:** SQLite Mastery: Lightweight Database Management **Section Title:** Transactions and Data Integrity **Topic:** Error handling and ensuring data integrity with constraints ### 1. Introduction to Constraints in SQLite In the previous topics, we've explored how to manage transactions and maintain data integrity using ACID properties. However, SQLite also provides another essential feature to ensure data integrity: constraints. A constraint is a rule applied to a table to ensure that the data inserted or updated meets specific criteria. In this topic, we'll delve into error handling and data integrity using constraints in SQLite. ### 2. Types of Constraints in SQLite SQLite supports several types of constraints: * **Primary Key (PK) constraint**: Ensures that each row in a table has a unique identifier. We've already discussed primary keys in the [Creating and Managing SQLite Databases and Tables](#) topic. * **Foreign Key (FK) constraint**: Ensures that a relationship between two tables is correctly established. We've already discussed foreign keys in the [Creating and Managing SQLite Databases and Tables](#) topic. * **Unique (U) constraint**: Ensures that each value in a column is unique. * **Not Null (NN) constraint**: Ensures that a column cannot contain null values. * **Check (C) constraint**: Ensures that a column satisfies a specific condition. To demonstrate the application of these constraints, let's create a simple table with various constraints: ```sql CREATE TABLE Employees ( EmployeeID INTEGER PRIMARY KEY, FirstName TEXT NOT NULL, LastName TEXT NOT NULL, Email TEXT UNIQUE, Department TEXT CHECK (Department IN ('Sales', 'Marketing', 'IT')), JobTitle TEXT NOT NULL ); ``` ### 3. Error Handling with Constraints When a constraint is applied to a table, any attempt to insert or update data violating that constraint will result in an error. In SQLite, errors are typically handled using the `INSERT OR` or `UPDATE OR` clauses. These clauses allow you to specify what action to take when a constraint is violated. * **INSERT OR ABORT**: If an insert operation fails due to a constraint violation, it will roll back the entire operation. * **INSERT OR ROLLBACK**: If an insert operation fails due to a constraint violation, it will roll back the entire operation. * **INSERT OR IGNORE**: If an insert operation fails due to a constraint violation, it will ignore the failing row and continue with the other rows. * **INSERT OR REPLACE**: If an insert operation fails due to a constraint violation, it will replace the constrained row with the new row. ```sql INSERT OR IGNORE INTO Employees (EmployeeID, FirstName, LastName, Email, Department, JobTitle) VALUES (1, 'John', 'Doe', 'john@example.com', 'Sales', 'Sales Representative'); ``` ### 4. Constraint Triggers SQLite also supports constraint triggers, which allow you to execute a custom action when a constraint is triggered. You can create triggers using the `CREATE TRIGGER` statement. ```sql CREATE TRIGGER EmployeeTrigger BEFORE INSERT ON Employees FOR EACH ROW WHEN NEW.Department NOT IN ('Sales', 'Marketing', 'IT') BEGIN SELECT RAISE (ABORT, 'Invalid department'); END; ``` ### 5. Practical Example: Implementing Error Handling with Constraints To illustrate error handling with constraints, let's create a realistic scenario. Assume we're building a database application that manages customer information. We want to ensure that the email addresses entered are valid and unique. ```sql CREATE TABLE Customers ( CustomerID INTEGER PRIMARY KEY, FirstName TEXT NOT NULL, LastName TEXT NOT NULL, Email TEXT UNIQUE ); CREATE TRIGGER CustomerEmailTrigger BEFORE INSERT ON Customers FOR EACH ROW WHEN NEW.Email NOT LIKE '%@%.%' BEGIN SELECT RAISE (ABORT, 'Invalid email address'); END; INSERT INTO Customers (CustomerID, FirstName, LastName, Email) VALUES (1, 'John', 'Doe', 'john@example.com'); ``` ### Key Concepts and Takeaways In this topic, we've explored error handling and ensuring data integrity using constraints in SQLite. Key takeaways include: * **Constraint types**: SQLite supports various constraint types, including Primary Key (PK), Foreign Key (FK), Unique (U), Not Null (NN), and Check (C) constraints. * **Error handling**: SQLite provides several methods for handling errors that occur when constraints are violated, including `INSERT OR` and `UPDATE OR` clauses. * **Constraint triggers**: SQLite allows you to create custom actions using constraint triggers. Now that you've finished this topic, you should have a better understanding of error handling and data integrity using constraints in SQLite. Practice implementing constraints in your own projects to reinforce your learning. If you have any questions or need further clarification on any of the concepts covered in this topic, please leave a comment below. For further reading, you can check the official SQLite documentation: * [SQLite Constraints](https://www.sqlite.org/lang_createtable.html#constraints) * [SQLite Triggers](https://www.sqlite.org/lang_createtrigger.html) We'll cover the next topic: **Introduction to indexing and its impact on performance** in the [Indexing and Performance Optimization](#) section.