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Connecting to Databases

# SQL Programming

This overview assumes that you have at least a basic knowledge of SQL. You should be able to understand simple `SELECT`, `INSERT`, `UPDATE`, and `DELETE` statements. Although the QSqlTableModel class provides an interface to database browsing and editing that does not require a knowledge of SQL, a basic understanding of SQL is highly recommended. A standard text covering SQL databases is An Introduction to Database Systems (7th Ed.) by C. J. Date, ISBN 0201385902.

## Topics:

- Database Classes
- Connecting to Databases
  - SQL Database Drivers
- Executing SQL Statements
  - Data Types for Qt-supported Database Systems
- Using the SQL Model Classes
- Presenting Data in a Table View
- Creating Data-Aware Forms

## Database Classes

These classes provide access to SQL databases.

QSql | Contains miscellaneous identifiers used throughout the Qt SQL library
QSqlDatabase | Represents a connection to a database
QSqlDriver | Abstract base class for accessing specific SQL databases
QSqlDriverCreator | Template class that provides a SQL driver factory for a specific driver type
QSqlDriverCreatorBase | The base class for SQL driver factories
QSqlError | SQL database error information
QSqlField | Manipulates the fields in SQL database tables and views
QSqlIndex | Functions to manipulate and describe database indexes
QSqlQuery | Means of executing and manipulating SQL statements
QSqlQueryModel | Read-only data model for SQL result sets
QSqlRecord | Encapsulates a database record
QSqlRelationalTableModel | Editable data model for a single database table, with foreign key support
QSqlResult | Abstract interface for accessing data from specific SQL databases
QSqlTableModel | Editable data model for a single database table

The SQL classes are divided into three layers:

### Driver Layer

This comprises the classes QSqlDriver, QSqlDriverCreator<T>, QSqlDriverCreatorBase, QSqlDriverPlugin, and QSqlResult.

This layer provides the low-level bridge between the specific databases and the SQL API layer. See SQL Database Drivers for more information.

### SQL API Layer

These classes provide access to databases. Connections are made using the QSqlDatabase class. Database interaction is achieved by using the QSqlQuery class. In addition to QSqlDatabase and QSqlQuery, the SQL API layer is supported by QSqlError, QSqlField, QSqlIndex, and QSqlRecord.

### User Interface Layer

These classes link the data from a database to data-aware widgets. They include QSqlQueryModel, QSqlTableModel, and QSqlRelationalTableModel. These classes are designed to work with Qt's model/view framework.

Note that to use any of these classes, a QCoreApplication object must have been instantiated first.