**Course Title:** Qt 6 Application Development with C++ **Section Title:** Integrating Databases with Qt **Topic:** Using QSqlQuery to interact with databases. ### Overview In the previous topic, we introduced the QSqlDatabase class, which allows you to connect to SQL databases in Qt. However, simply connecting to a database is not enough; you need to interact with the database to store and retrieve data. This is where QSqlQuery comes in. In this topic, we will explore the QSqlQuery class and how you can use it to interact with your databases. ### What is QSqlQuery? QSqlQuery is a class in the Qt SQL module that allows you to execute SQL queries on a database. It supports various database drivers, including MySQL, PostgreSQL, Oracle, and SQLite. You can use QSqlQuery to execute both read-only and read-write queries. ### Creating a QSqlQuery Object To create a QSqlQuery object, you need to pass a QSqlDatabase object to its constructor. This tells QSqlQuery which database to connect to and execute queries on. Here is an example: ```cpp QSqlDatabase db = QSqlDatabase::database(); QSqlQuery query(db); ``` ### Executing Queries To execute a query, you can use the `exec()` function. This function takes a QString argument containing the SQL query you want to execute. Here is an example of how to execute a SELECT query: ```cpp QString queryStr = "SELECT * FROM mytable"; QSqlQuery query(db); query.exec(queryStr); ``` If the query is successful, the `isValid()` function will return true. You can then use the `nextInt()` or `nextString()` functions to retrieve the results. ### Example Use Case Here is an example of how you can use QSqlQuery to retrieve data from a database: ```cpp QSqlDatabase db = QSqlDatabase::database(); QSqlQuery query(db); QString queryStr = "SELECT name, age FROM mytable"; query.exec(queryStr); while (query.next()) { QString name = query.value(0).toString(); int age = query.value(1).toInt(); qDebug() << "Name: " << name << ", Age: " << age; } ``` ### Parameter Binding To prevent SQL injection attacks, it is recommended that you use parameter binding instead of directly inserting data into your SQL query strings. Here is an example of how you can use parameter binding with QSqlQuery: ```cpp QString queryStr = "SELECT * FROM mytable WHERE name = :name AND age = :age"; QSqlQuery query(db); query.bindValue(":name", "John"); query.bindValue(":age", 25); query.exec(queryStr); ``` ### Batch Execution If you need to execute multiple similar queries, you can use batch execution. Batch execution can improve performance by reducing the number of database connections. Here is an example of how you can use batch execution with QSqlQuery: ```cpp QString queryStr = "INSERT INTO mytable (name, age) VALUES (:name, :age)"; QSqlQuery query(db); query.prepare(queryStr); query.bindValue(":name", "Alice"); query.bindValue(":age", 20); query.execBatch(); query.bindValue(":name", "Bob"); query.bindValue(":age", 25); query.execBatch(); ``` ### Key Concepts * QSqlQuery allows you to execute SQL queries on a database. * To execute a query, use the `exec()` function. * Use parameter binding to prevent SQL injection attacks. * Batch execution can improve performance by reducing database connections. ### Conclusion In this topic, we explored the QSqlQuery class and how you can use it to interact with your databases. We covered how to create a QSqlQuery object, execute queries, and use parameter binding and batch execution. You can practice what you learned by executing queries on a test database. ### Practice Exercise Create a test database with a table called "mytable" and columns called "name" and "age". Then, use QSqlQuery to execute a SELECT query that retrieves all data from the table. Display the results using `qDebug()`. ### External Links * [Qt Documentation: QSqlQuery](https://doc.qt.io/qt-6/qsqlquery.html) * [Qt Documentation: QSqlDatabase](https://doc.qt.io/qt-6/qsqldatabase.html) Do you have any questions? Please leave a comment below.