Qt 与 SQLite 嵌入式数据库开发

Qt 与 SQLite 的结合是开发轻量级、跨平台嵌入式数据库应用的理想选择。SQLite 作为一种零配置、文件型数据库，无需独立的服务器进程，非常适合集成到 Qt 应用中。本文将深入探讨 Qt 与 SQLite 的嵌入式数据库开发，包括基础操作、高级特性、性能优化和实际应用案例。

一、SQLite 基础配置与连接

1. 驱动检查与数据库连接

#include <QSqlDatabase>
#include <QSqlError>
#include <QDebug>

void connectToSQLite() {
    // 检查 SQLite 驱动是否可用
    if (!QSqlDatabase::isDriverAvailable("QSQLITE")) {
        qDebug() << "SQLite driver not available!";
        return;
    }
    
    // 创建数据库连接
    QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE");
    db.setDatabaseName("mydatabase.db");  // 数据库文件名
    
    if (!db.open()) {
        qDebug() << "Cannot open database:" << db.lastError().text();
        return;
    }
    
    qDebug() << "SQLite database connected successfully!";
}

2. 创建表结构

void createTables(QSqlDatabase &db) {
    QSqlQuery query(db);
    
    // 创建用户表
    query.exec(R"(
        CREATE TABLE IF NOT EXISTS users (
            id INTEGER PRIMARY KEY AUTOINCREMENT,
            name TEXT NOT NULL,
            age INTEGER,
            email TEXT UNIQUE
        )
    )");
    
    if (query.lastError().isValid()) {
        qDebug() << "Error creating table:" << query.lastError().text();
    }
    
    // 创建订单表
    query.exec(R"(
        CREATE TABLE IF NOT EXISTS orders (
            id INTEGER PRIMARY KEY AUTOINCREMENT,
            user_id INTEGER NOT NULL,
            product TEXT,
            price REAL,
            order_date TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
            FOREIGN KEY (user_id) REFERENCES users(id)
        )
    )");
}

二、数据操作与查询

1. 插入数据

void insertData(QSqlDatabase &db) {
    QSqlQuery query(db);
    
    // 插入用户数据
    query.prepare("INSERT INTO users (name, age, email) VALUES (:name, :age, :email)");
    query.bindValue(":name", "Alice");
    query.bindValue(":age", 30);
    query.bindValue(":email", "alice@example.com");
    
    if (query.exec()) {
        qDebug() << "User inserted successfully!";
    } else {
        qDebug() << "Error inserting user:" << query.lastError().text();
    }
    
    // 批量插入订单数据
    db.transaction();  // 使用事务提高性能
    
    QList<QVariantList> orders = {
        {1, "Product A", 99.99},
        {1, "Product B", 49.99},
        {1, "Product C", 199.99}
    };
    
    query.prepare("INSERT INTO orders (user_id, product, price) VALUES (:user_id, :product, :price)");
    
    for (const auto &order : orders) {
        query.bindValue(":user_id", order[0]);
        query.bindValue(":product", order[1]);
        query.bindValue(":price", order[2]);
        query.exec();
    }
    
    db.commit();
}

2. 查询数据

void queryData(QSqlDatabase &db) {
    // 简单查询
    QSqlQuery query("SELECT * FROM users", db);
    
    while (query.next()) {
        int id = query.value("id").toInt();
        QString name = query.value("name").toString();
        int age = query.value("age").toInt();
        QString email = query.value("email").toString();
        
        qDebug() << "User:" << id << name << age << email;
    }
    
    // 带参数的查询
    query.prepare("SELECT * FROM orders WHERE user_id = :user_id AND price > :min_price");
    query.bindValue(":user_id", 1);
    query.bindValue(":min_price", 50.0);
    
    if (query.exec()) {
        while (query.next()) {
            QString product = query.value("product").toString();
            double price = query.value("price").toDouble();
            QDateTime date = query.value("order_date").toDateTime();
            
            qDebug() << "Order:" << product << price << date.toString("yyyy-MM-dd");
        }
    }
}

三、高级特性与优化

1. 事务处理

bool transferMoney(QSqlDatabase &db, int fromAccount, int toAccount, double amount) {
    db.transaction();
    
    try {
        QSqlQuery query(db);
        
        // 减少源账户余额
        query.prepare("UPDATE accounts SET balance = balance - :amount WHERE id = :id");
        query.bindValue(":amount", amount);
        query.bindValue(":id", fromAccount);
        
        if (!query.exec() || query.numRowsAffected() == 0) {
            throw std::runtime_error("Failed to debit from source account");
        }
        
        // 增加目标账户余额
        query.prepare("UPDATE accounts SET balance = balance + :amount WHERE id = :id");
        query.bindValue(":amount", amount);
        query.bindValue(":id", toAccount);
        
        if (!query.exec() || query.numRowsAffected() == 0) {
            throw std::runtime_error("Failed to credit to target account");
        }
        
        db.commit();
        return true;
    } catch (const std::exception &e) {
        db.rollback();
        qDebug() << "Transaction failed:" << e.what();
        return false;
    }
}

2. 索引优化

void createIndex(QSqlDatabase &db) {
    QSqlQuery query(db);
    
    // 创建索引提高查询性能
    query.exec("CREATE INDEX IF NOT EXISTS idx_orders_user_id ON orders (user_id)");
    query.exec("CREATE INDEX IF NOT EXISTS idx_orders_price ON orders (price)");
    
    // 创建复合索引
    query.exec("CREATE INDEX IF NOT EXISTS idx_users_name_age ON users (name, age)");
}

3. 外键约束

void enableForeignKeys(QSqlDatabase &db) {
    QSqlQuery query(db);
    
    // 启用外键约束（SQLite 默认禁用）
    query.exec("PRAGMA foreign_keys = ON");
    
    if (query.lastError().isValid()) {
        qDebug() << "Failed to enable foreign keys:" << query.lastError().text();
    }
}

四、SQLite 特定功能

1. 数据库加密（使用 SQLCipher）

void openEncryptedDatabase(const QString &dbPath, const QString &password) {
    QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE");
    db.setDatabaseName(dbPath);
    
    // 设置加密密钥
    db.setConnectOptions("QSQLITE_ENABLE_REGEXP=1;QSQLITE_PWD=" + password);
    
    if (!db.open()) {
        qDebug() << "Cannot open encrypted database:" << db.lastError().text();
        return;
    }
    
    qDebug() << "Encrypted database opened successfully!";
}

2. 全文搜索

void createFtsTable(QSqlDatabase &db) {
    QSqlQuery query(db);
    
    // 创建 FTS5 全文搜索表
    query.exec(R"(
        CREATE VIRTUAL TABLE IF NOT EXISTS notes USING fts5(
            title,
            content,
            tags
        )
    )");
    
    // 插入测试数据
    query.prepare("INSERT INTO notes (title, content, tags) VALUES (:title, :content, :tags)");
    query.bindValue(":title", "Qt Development");
    query.bindValue(":content", "Learn Qt with SQLite for embedded applications");
    query.bindValue(":tags", "Qt,SQLite,Embedded");
    query.exec();
}

void searchFtsTable(QSqlDatabase &db, const QString &keyword) {
    QSqlQuery query(db);
    query.prepare("SELECT * FROM notes WHERE content MATCH :keyword");
    query.bindValue(":keyword", keyword);
    
    if (query.exec()) {
        while (query.next()) {
            QString title = query.value("title").toString();
            QString content = query.value("content").toString();
            
            qDebug() << "Search result:" << title << content.left(50) + "...";
        }
    }
}

五、性能优化策略

1. 批量操作优化

void batchInsertOptimized(QSqlDatabase &db, const QList<QVariantList> &data) {
    db.transaction();
    
    QSqlQuery query(db);
    query.prepare("INSERT INTO large_table (col1, col2, col3) VALUES (:val1, :val2, :val3)");
    
    // 使用预处理语句和事务进行批量插入
    for (const auto &row : data) {
        query.bindValue(":val1", row[0]);
        query.bindValue(":val2", row[1]);
        query.bindValue(":val3", row[2]);
        query.exec();
    }
    
    db.commit();
}

2. 内存管理优化

void optimizeMemoryUsage(QSqlDatabase &db) {
    QSqlQuery query(db);
    
    // 设置页面大小
    query.exec("PRAGMA page_size = 4096");
    
    // 设置缓存大小（以页面为单位）
    query.exec("PRAGMA cache_size = -2000");  // 负值表示 KiB，此处为 2000 KiB
    
    // 优化写入性能
    query.exec("PRAGMA synchronous = NORMAL");
    
    // 启用 WAL 模式
    query.exec("PRAGMA journal_mode = WAL");
}

六、错误处理与恢复

1. 错误处理机制

bool executeSafely(QSqlQuery &query, const QString &sql, const QVariantMap &bindValues = {}) {
    query.prepare(sql);
    
    // 绑定参数
    for (auto it = bindValues.begin(); it != bindValues.end(); ++it) {
        query.bindValue(it.key(), it.value());
    }
    
    if (!query.exec()) {
        qDebug() << "SQL execution error:" << query.lastError().text();
        qDebug() << "Failed query:" << sql;
        return false;
    }
    
    return true;
}

2. 数据库恢复

bool backupDatabase(const QString &sourceDb, const QString &targetDb) {
    QSqlDatabase srcDb = QSqlDatabase::addDatabase("QSQLITE", "sourceConnection");
    srcDb.setDatabaseName(sourceDb);
    
    if (!srcDb.open()) {
        qDebug() << "Cannot open source database:" << srcDb.lastError().text();
        return false;
    }
    
    QSqlDatabase destDb = QSqlDatabase::addDatabase("QSQLITE", "targetConnection");
    destDb.setDatabaseName(targetDb);
    
    if (!destDb.open()) {
        qDebug() << "Cannot open target database:" << destDb.lastError().text();
        srcDb.close();
        return false;
    }
    
    // 使用 SQLite 的备份 API
    QSqlQuery query(srcDb);
    query.exec("BACKUP TO '" + targetDb + "'");
    
    srcDb.close();
    destDb.close();
    
    return true;
}

七、ORM 框架集成

1. 简单 ORM 实现

class Note {
public:
    int id;
    QString title;
    QString content;
    QDateTime created;
    
    // 从记录创建对象
    static Note fromRecord(const QSqlRecord &record) {
        Note note;
        note.id = record.value("id").toInt();
        note.title = record.value("title").toString();
        note.content = record.value("content").toString();
        note.created = record.value("created").toDateTime();
        return note;
    }
    
    // 保存到数据库
    bool save(QSqlDatabase &db) {
        QSqlQuery query(db);
        
        if (id <= 0) {
            // 插入新记录
            query.prepare("INSERT INTO notes (title, content, created) VALUES (:title, :content, :created)");
            query.bindValue(":title", title);
            query.bindValue(":content", content);
            query.bindValue(":created", created.isValid() ? created : QDateTime::currentDateTime());
            
            if (query.exec()) {
                id = query.lastInsertId().toInt();
                return true;
            }
        } else {
            // 更新现有记录
            query.prepare("UPDATE notes SET title = :title, content = :content WHERE id = :id");
            query.bindValue(":title", title);
            query.bindValue(":content", content);
            query.bindValue(":id", id);
            return query.exec();
        }
        
        return false;
    }
};

八、实际应用案例

1. 嵌入式设备数据记录

class DataLogger {
public:
    DataLogger(const QString &dbPath) {
        db = QSqlDatabase::addDatabase("QSQLITE");
        db.setDatabaseName(dbPath);
        
        if (!db.open()) {
            qDebug() << "Failed to open database:" << db.lastError().text();
        } else {
            createTables();
        }
    }
    
    void logSensorData(const QString &sensorId, double value) {
        if (!db.isOpen()) return;
        
        QSqlQuery query(db);
        query.prepare("INSERT INTO sensor_data (sensor_id, value, timestamp) VALUES (:sensor_id, :value, :timestamp)");
        query.bindValue(":sensor_id", sensorId);
        query.bindValue(":value", value);
        query.bindValue(":timestamp", QDateTime::currentDateTime());
        query.exec();
    }
    
private:
    QSqlDatabase db;
    
    void createTables() {
        QSqlQuery query(db);
        query.exec(R"(
            CREATE TABLE IF NOT EXISTS sensor_data (
                id INTEGER PRIMARY KEY AUTOINCREMENT,
                sensor_id TEXT NOT NULL,
                value REAL,
                timestamp TIMESTAMP DEFAULT CURRENT_TIMESTAMP
            )
        )");
    }
};

九、总结

Qt 与 SQLite 的结合为嵌入式数据库应用开发提供了强大而灵活的解决方案。SQLite 的轻量级特性与 Qt 的跨平台能力相得益彰，使开发者能够轻松构建高效、可靠的数据驱动应用。通过合理使用事务、索引、全文搜索等高级特性，并采用适当的性能优化策略，可以充分发挥 SQLite 在嵌入式环境中的优势。