These functions allow the programmer to move forward, backward or arbitrarily through the records returned by the query. If you only need to move forward through the results, e.g. using next() or using seek() with a positive offset, you can use setForwardOnly() and save a significant amount of memory overhead. Once an active query is positioned on a valid record, data can be retrieved using value(). All data is transferred from the SQL backend using QVariants.
To access the data returned by a query, use value(int). Each field in the data returned by a SELECT statement is accessed by passing the field's position in the statement, starting from 0. This makes using SELECT * queries inadvisable because the order of the fields returned is indeterminate.
For the sake of efficiency, there are no functions to access a field by name (unless you use prepared queries with names, as explained below). To convert a field name into an index, use record().indexOf(), for example:
QSqlQuery supports prepared query execution and the binding of parameter values to placeholders. Some databases don't support these features, so for those, Qt emulates the required functionality. For example, the Oracle and ODBC drivers have proper prepared query support, and Qt makes use of it; but for databases that don't have this support, Qt implements the feature itself, e.g. by replacing placeholders with actual values when a query is executed. Use numRowsAffected() to find out how many rows were affected by a non-SELECT query, and size() to find how many were retrieved by a SELECT.
You can retrieve the values of all the fields in a single variable (a map) using boundValues().
Approaches to Binding Values
Below we present the same example using each of the four different binding approaches, as well as one example of binding values to a stored procedure.
Named binding using named placeholders:
QSqlQuery query;
query.prepare("INSERT INTO person (id, forename, surname) "
"VALUES (:id, :forename, :surname)");
query.bindValue(":id", 1001);
query.bindValue(":forename", "Bart");
query.bindValue(":surname", "Simpson");
query.exec();
Positional binding using named placeholders:
QSqlQuery query;
query.prepare("INSERT INTO person (id, forename, surname) "
"VALUES (:id, :forename, :surname)");
query.bindValue(0, 1001);
query.bindValue(1, "Bart");
query.bindValue(2, "Simpson");
query.exec();
Binding values using positional placeholders (version 1):
QSqlQuery query;
query.prepare("INSERT INTO person (id, forename, surname) "
"VALUES (?, ?, ?)");
query.bindValue(0, 1001);
query.bindValue(1, "Bart");
query.bindValue(2, "Simpson");
query.exec();
Binding values using positional placeholders (version 2):
QSqlQuery query;
query.prepare("INSERT INTO person (id, forename, surname) "
"VALUES (?, ?, ?)");
query.addBindValue(1001);
query.addBindValue("Bart");
query.addBindValue("Simpson");
query.exec();
Binding values to a stored procedure:
This code calls a stored procedure called AsciiToInt(), passing it a character through its in parameter, and taking its result in the out parameter.
QSqlQuery query;
query.prepare("CALL AsciiToInt(?, ?)");
query.bindValue(0, "A");
query.bindValue(1, 0, QSql::Out);
query.exec();
int i = query.boundValue(1).toInt(); // i is 65
See also QSqlDatabase, QSqlQueryModel, QSqlTableModel, and QVariant.
Member Function Documentation
QSqlQuery::QSqlQuery ( QSqlResult * result )
Constructs a QSqlQuery object which uses the QSqlResult result to communicate with a database.
QSqlQuery::QSqlQuery ( const QString & query = QString(), QSqlDatabase db = QSqlDatabase() )
Constructs a QSqlQuery object using the SQL query and the database db. If db is not specified, the application's default database is used. If query is not an empty string, it will be executed.
See also QSqlDatabase.
QSqlQuery::QSqlQuery ( QSqlDatabase db )
Constructs a QSqlQuery object using the database db.
See also QSqlDatabase.
QSqlQuery::QSqlQuery ( const QSqlQuery & other )
Constructs a copy of other.
QSqlQuery::~QSqlQuery ()
Destroys the object and frees any allocated resources.
void QSqlQuery::addBindValue ( const QVariant & val, QSql::ParamType paramType = QSql::In )
Adds the value val to the list of values when using positional value binding. The order of the addBindValue() calls determines which placeholder a value will be bound to in the prepared query. If paramType is QSql::Out or QSql::InOut, the placeholder will be overwritten with data from the database after the exec() call.
See also bindValue(), prepare(), exec(), boundValue(), and boundValues().
int QSqlQuery::at () const
Returns the current internal position of the query. The first record is at position zero. If the position is invalid, the function returns QSql::BeforeFirstRow or QSql::AfterLastRow, which are special negative values.
See also previous(), next(), first(), last(), seek(), isActive(), and isValid().
void QSqlQuery::bindValue ( const QString & placeholder, const QVariant & val, QSql::ParamType paramType = QSql::In )
Set the placeholder placeholder to be bound to value val in the prepared statement. Note that the placeholder mark (e.g :) must be included when specifying the placeholder name. If paramType is QSql::Out or QSql::InOut, the placeholder will be overwritten with data from the database after the exec() call.
See also addBindValue(), prepare(), exec(), boundValue(), and boundValues().
void QSqlQuery::bindValue ( int pos, const QVariant & val, QSql::ParamType paramType = QSql::In )
This is an overloaded member function, provided for convenience.
Set the placeholder in position pos to be bound to value val in the prepared statement. Field numbering starts at 0. If paramType is QSql::Out or QSql::InOut, the placeholder will be overwritten with data from the database after the exec() call.
QVariant QSqlQuery::boundValue ( const QString & placeholder ) const
Returns the value for the placeholder.
See also boundValues(), bindValue(), and addBindValue().
QVariant QSqlQuery::boundValue ( int pos ) const
This is an overloaded member function, provided for convenience.
Returns the value for the placeholder at position pos.
Returns a map of the bound values.
With named binding, the bound values can be examined in the following ways:
QMapIterator<QString, QVariant> i(query.boundValues());
while (i.hasNext()) {
i.next();
cout << i.key().toAscii().data() << ": "
<< i.value().toString().toAscii().data() << endl;
}
With positional binding, the code becomes:
QList<QVariant> list = query.boundValues().values();
for (int i = 0; i < list.size(); ++i)
cout << i << ": " << list.at(i).toString().toAscii().data() << endl;
See also boundValue(), bindValue(), and addBindValue().
void QSqlQuery::clear ()
Clears the result set and releases any resources held by the query. You should rarely if ever need to call this function.
const QSqlDriver * QSqlQuery::driver () const
Returns the database driver associated with the query.
bool QSqlQuery::exec ( const QString & query )
Executes the SQL in query. Returns true and sets the query state to active if the query was successful; otherwise returns false. The query string must use syntax appropriate for the SQL database being queried (for example, standard SQL).
After the query is executed, the query is positioned on an invalid record and must be navigated to a valid record before data values can be retrieved (for example, using next()).
Note that the last error for this query is reset when exec() is called.
Example:
QSqlQuery query;
query.prepare("INSERT INTO person (id, forename, surname) "
"VALUES (:id, :forename, :surname)");
query.bindValue(":id", 1001);
query.bindValue(":forename", "Bart");
query.bindValue(":surname", "Simpson");
query.exec();
See also isActive(), isValid(), next(), previous(), first(), last(), and seek().
bool QSqlQuery::exec ()
This is an overloaded member function, provided for convenience.
Executes a previously prepared SQL query. Returns true if the query executed successfully; otherwise returns false.
See also prepare(), bindValue(), addBindValue(), boundValue(), and boundValues().
QString QSqlQuery::executedQuery () const
Returns the last query that was executed.
In most cases this function returns the same string as lastQuery(). If a prepared query with placeholders is executed on a DBMS that does not support it, the preparation of this query is emulated. The placeholders in the original query are replaced with their bound values to form a new query. This function returns the modified query. It is mostly useful for debugging purposes.
See also lastQuery().
bool QSqlQuery::first ()
Retrieves the first record in the result, if available, and positions the query on the retrieved record. Note that the result must be in an active state and isSelect() must return true before calling this function or it will do nothing and return false. Returns true if successful. If unsuccessful the query position is set to an invalid position and false is returned.
See also next(), previous(), last(), seek(), at(), isActive(), and isValid().
bool QSqlQuery::isActive () const
Returns true if the query is currently active; otherwise returns false.
bool QSqlQuery::isForwardOnly () const
Returns true if you can only scroll forward through a result set; otherwise returns false.
See also setForwardOnly() and next().
bool QSqlQuery::isNull ( int field ) const
Returns true if the query is active and positioned on a valid record and the field is NULL; otherwise returns false. Note that for some drivers, isNull() will not return accurate information until after an attempt is made to retrieve data.
See also isActive(), isValid(), and value().
bool QSqlQuery::isSelect () const
Returns true if the current query is a SELECT statement; otherwise returns false.
bool QSqlQuery::isValid () const
Returns true if the query is currently positioned on a valid record; otherwise returns false.
bool QSqlQuery::last ()
Retrieves the last record in the result, if available, and positions the query on the retrieved record. Note that the result must be in an active state and isSelect() must return true before calling this function or it will do nothing and return false. Returns true if successful. If unsuccessful the query position is set to an invalid position and false is returned.
See also next(), previous(), first(), seek(), at(), isActive(), and isValid().
QSqlError QSqlQuery::lastError () const
Returns error information about the last error (if any) that occurred with this query.
See also QSqlError and QSqlDatabase::lastError().
QVariant QSqlQuery::lastInsertId () const
Returns the object ID of the most recent inserted row if the database supports it. An invalid QVariant will be returned if the query did not insert any value or if the database does not report the id back. If more than one row was touched by the insert, the behavior is undefined.
See also QSqlDriver::hasFeature().
QString QSqlQuery::lastQuery () const
Returns the text of the current query being used, or an empty string if there is no current query text.
See also executedQuery().
bool QSqlQuery::next ()
Retrieves the next record in the result, if available, and positions the query on the retrieved record. Note that the result must be in an active state and isSelect() must return true before calling this function or it will do nothing and return false.
The following rules apply:
- If the result is currently located before the first record, e.g. immediately after a query is executed, an attempt is made to retrieve the first record.
- If the result is currently located after the last record, there is no change and false is returned.
- If the result is located somewhere in the middle, an attempt is made to retrieve the next record.
If the record could not be retrieved, the result is positioned after the last record and false is returned. If the record is successfully retrieved, true is returned.
See also previous(), first(), last(), seek(), at(), isActive(), and isValid().
int QSqlQuery::numRowsAffected () const
Returns the number of rows affected by the result's SQL statement, or -1 if it cannot be determined. Note that for SELECT statements, the value is undefined; use size() instead. If the query is not active (isActive() returns false), -1 is returned.
See also size() and QSqlDriver::hasFeature().
bool QSqlQuery::prepare ( const QString & query )
Prepares the SQL query query for execution. The query may contain placeholders for binding values. Both Oracle style colon-name (e.g., :surname), and ODBC style (?) placeholders are supported; but they cannot be mixed in the same query. See the Detailed Description for examples.
Portability note: Some databases choose to delay preparing a query until it is executed the first time. In this case, preparing a syntactically wrong query succeeds, but every consecutive exec() will fail.
See also exec(), bindValue(), and addBindValue().
bool QSqlQuery::previous ()
Retrieves the previous record in the result, if available, and positions the query on the retrieved record. Note that the result must be in an active state and isSelect() must return true before calling this function or it will do nothing and return false.
The following rules apply:
- If the result is currently located before the first record, there is no change and false is returned.
- If the result is currently located after the last record, an attempt is made to retrieve the last record.
- If the result is somewhere in the middle, an attempt is made to retrieve the previous record.
If the record could not be retrieved, the result is positioned before the first record and false is returned. If the record is successfully retrieved, true is returned.
See also next(), first(), last(), seek(), at(), isActive(), and isValid().
QSqlRecord QSqlQuery::record () const
Returns a QSqlRecord containing the field information for the current query. If the query points to a valid row (isValid() returns true), the record is populated with the row's values. An empty record is returned when there is no active query (isActive() returns false).
To retrieve values from a query, value() should be used since its index-based lookup is faster.
In the following example, a SELECT * FROM query is executed. Since the order of the columns is not defined, QSqlRecord::indexOf() is used to obtain the index of a column.
QSqlQuery q("select * from employees");
QSqlRecord rec = q.record();
qDebug() << "Number of columns: " << rec.count();
int nameCol = rec.indexOf("name"); // index of the field "name"
while (q.next())
qDebug() << q.value(nameCol).toString(); // output all names
See also value().
const QSqlResult * QSqlQuery::result () const
Returns the result associated with the query.
bool QSqlQuery::seek ( int index, bool relative = false )
Retrieves the record at position index, if available, and positions the query on the retrieved record. The first record is at position 0. Note that the query must be in an active state and isSelect() must return true before calling this function.
If relative is false (the default), the following rules apply:
- If index is negative, the result is positioned before the first record and false is returned.
- Otherwise, an attempt is made to move to the record at position index. If the record at position index could not be retrieved, the result is positioned after the last record and false is returned. If the record is successfully retrieved, true is returned.
If relative is true, the following rules apply:
- If the result is currently positioned before the first record or on the first record, and index is negative, there is no change, and false is returned.
- If the result is currently located after the last record, and index is positive, there is no change, and false is returned.
- If the result is currently located somewhere in the middle, and the relative offset index moves the result below zero, the result is positioned before the first record and false is returned.
- Otherwise, an attempt is made to move to the record index records ahead of the current record (or index records behind the current record if index is negative). If the record at offset index could not be retrieved, the result is positioned after the last record if index >= 0, (or before the first record if index is negative), and false is returned. If the record is successfully retrieved, true is returned.
See also next(), previous(), first(), last(), at(), isActive(), and isValid().
void QSqlQuery::setForwardOnly ( bool forward )
Sets forward only mode to forward. If forward is true, only next() and seek() with positive values, are allowed for navigating the results. Forward only mode needs far less memory since results do not need to be cached.
Forward only mode is off by default.
See also isForwardOnly(), next(), and seek().
int QSqlQuery::size () const
Returns the size of the result (number of rows returned), or -1 if the size cannot be determined or if the database does not support reporting information about query sizes. Note that for non-SELECT statements (isSelect() returns false), size() will return -1. If the query is not active (isActive() returns false), -1 is returned.
To determine the number of rows affected by a non-SELECT statement, use numRowsAffected().
See also isActive(), numRowsAffected(), and QSqlDriver::hasFeature().
QVariant QSqlQuery::value ( int index ) const
Returns the value of field index in the current record.
The fields are numbered from left to right using the text of the SELECT statement, e.g. in
SELECT forename, surname FROM people;
field 0 is forename and field 1 is surname. Using SELECT * is not recommended because the order of the fields in the query is undefined.
An invalid QVariant is returned if field index does not exist, if the query is inactive, or if the query is positioned on an invalid record.
See also previous(), next(), first(), last(), seek(), isActive(), and isValid().
QSqlQuery & QSqlQuery::operator= ( const QSqlQuery & other )
Assigns other to this object.
