path: root/src/sql/kernel/qsqlquery.cpp

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#include "qsqlquery.h"

//#define QT_DEBUG_SQL

#include "qdebug.h"
#include "qelapsedtimer.h"
#include "qatomic.h"
#include "qsqlrecord.h"
#include "qsqlresult.h"
#include "qsqldriver.h"
#include "qsqldatabase.h"
#include "private/qsqlnulldriver_p.h"
#include "qvector.h"
#include "qmap.h"

QT_BEGIN_NAMESPACE

class QSqlQueryPrivate
{
public:
    QSqlQueryPrivate(QSqlResult* result);
    ~QSqlQueryPrivate();
    QAtomicInt ref;
    QSqlResult* sqlResult;

    static QSqlQueryPrivate* shared_null();
};

Q_GLOBAL_STATIC_WITH_ARGS(QSqlQueryPrivate, nullQueryPrivate, (0))
Q_GLOBAL_STATIC(QSqlNullDriver, nullDriver)
Q_GLOBAL_STATIC_WITH_ARGS(QSqlNullResult, nullResult, (nullDriver()))

QSqlQueryPrivate* QSqlQueryPrivate::shared_null()
{
    QSqlQueryPrivate *null = nullQueryPrivate();
    null->ref.ref();
    return null;
}

/*!
\internal
*/
QSqlQueryPrivate::QSqlQueryPrivate(QSqlResult* result)
    : ref(1), sqlResult(result)
{
    if (!sqlResult)
        sqlResult = nullResult();
}

QSqlQueryPrivate::~QSqlQueryPrivate()
{
    QSqlResult *nr = nullResult();
    if (!nr || sqlResult == nr)
        return;
    delete sqlResult;
}

/*!
    \class QSqlQuery
    \brief The QSqlQuery class provides a means of executing and
    manipulating SQL statements.

    \ingroup database
    \ingroup shared

    \inmodule QtSql

    QSqlQuery encapsulates the functionality involved in creating,
    navigating and retrieving data from SQL queries which are
    executed on a \l QSqlDatabase. It can be used to execute DML
    (data manipulation language) statements, such as \c SELECT, \c
    INSERT, \c UPDATE and \c DELETE, as well as DDL (data definition
    language) statements, such as \c{CREATE} \c{TABLE}. It can also
    be used to execute database-specific commands which are not
    standard SQL (e.g. \c{SET DATESTYLE=ISO} for PostgreSQL).

    Successfully executed SQL statements set the query's state to
    active so that isActive() returns \c true. Otherwise the query's
    state is set to inactive. In either case, when executing a new SQL
    statement, the query is positioned on an invalid record. An active
    query must be navigated to a valid record (so that isValid()
    returns \c true) before values can be retrieved.

    For some databases, if an active query that is a \c{SELECT}
    statement exists when you call \l{QSqlDatabase::}{commit()} or
    \l{QSqlDatabase::}{rollback()}, the commit or rollback will
    fail. See isActive() for details.

    \target QSqlQuery examples

    Navigating records is performed with the following functions:

    \list
    \li next()
    \li previous()
    \li first()
    \li last()
    \li seek()
    \endlist

    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., by using
    next()), you can use setForwardOnly(), which will save a
    significant amount of memory overhead and improve performance on
    some databases. 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.

    For example:

    \snippet sqldatabase/sqldatabase.cpp 7

    To access the data returned by a query, use value(int). Each
    field in the data returned by a \c SELECT statement is accessed
    by passing the field's position in the statement, starting from
    0. This makes using \c{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().\l{QSqlRecord::indexOf()}{indexOf()}, for example:

    \snippet sqldatabase/sqldatabase.cpp 8

    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-\c SELECT query, and size() to find how
    many were retrieved by a \c SELECT.

    Oracle databases identify placeholders by using a colon-name
    syntax, e.g \c{:name}. ODBC simply uses \c ? characters. Qt
    supports both syntaxes, with the restriction that you can't mix
    them in the same query.

    You can retrieve the values of all the fields in a single variable
    (a map) using boundValues().

    \section1 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.

    \b{Named binding using named placeholders:}

    \snippet sqldatabase/sqldatabase.cpp 9

    \b{Positional binding using named placeholders:}

    \snippet sqldatabase/sqldatabase.cpp 10

    \b{Binding values using positional placeholders (version 1):}

    \snippet sqldatabase/sqldatabase.cpp 11

    \b{Binding values using positional placeholders (version 2):}

    \snippet sqldatabase/sqldatabase.cpp 12

    \b{Binding values to a stored procedure:}

    This code calls a stored procedure called \c AsciiToInt(), passing
    it a character through its in parameter, and taking its result in
    the out parameter.

    \snippet sqldatabase/sqldatabase.cpp 13

    Note that unbound parameters will retain their values.

    Stored procedures that uses the return statement to return values,
    or return multiple result sets, are not fully supported. For specific
    details see \l{SQL Database Drivers}.

    \warning You must load the SQL driver and open the connection before a
    QSqlQuery is created. Also, the connection must remain open while the
    query exists; otherwise, the behavior of QSqlQuery is undefined.

    \sa QSqlDatabase, QSqlQueryModel, QSqlTableModel, QVariant
*/

/*!
    Constructs a QSqlQuery object which uses the QSqlResult \a result
    to communicate with a database.
*/

QSqlQuery::QSqlQuery(QSqlResult *result)
{
    d = new QSqlQueryPrivate(result);
}

/*!
    Destroys the object and frees any allocated resources.
*/

QSqlQuery::~QSqlQuery()
{
    if (!d->ref.deref())
        delete d;
}

/*!
    Constructs a copy of \a other.
*/

QSqlQuery::QSqlQuery(const QSqlQuery& other)
{
    d = other.d;
    d->ref.ref();
}

/*!
    \internal
*/
static void qInit(QSqlQuery *q, const QString& query, QSqlDatabase db)
{
    QSqlDatabase database = db;
    if (!database.isValid())
        database = QSqlDatabase::database(QLatin1String(QSqlDatabase::defaultConnection), false);
    if (database.isValid()) {
        *q = QSqlQuery(database.driver()->createResult());
    }
    if (!query.isEmpty())
        q->exec(query);
}

/*!
    Constructs a QSqlQuery object using the SQL \a query and the
    database \a db. If \a db is not specified, or is invalid, the application's
    default database is used. If \a query is not an empty string, it
    will be executed.

    \sa QSqlDatabase
*/
QSqlQuery::QSqlQuery(const QString& query, QSqlDatabase db)
{
    d = QSqlQueryPrivate::shared_null();
    qInit(this, query, db);
}

/*!
    Constructs a QSqlQuery object using the database \a db.
    If \a db is invalid, the application's default database will be used.

    \sa QSqlDatabase
*/

QSqlQuery::QSqlQuery(QSqlDatabase db)
{
    d = QSqlQueryPrivate::shared_null();
    qInit(this, QString(), db);
}


/*!
    Assigns \a other to this object.
*/

QSqlQuery& QSqlQuery::operator=(const QSqlQuery& other)
{
    qAtomicAssign(d, other.d);
    return *this;
}

/*!
    Returns \c true if the query is not \l{isActive()}{active},
    the query is not positioned on a valid record,
    there is no such \a field, or the \a field is null; otherwise \c false.
    Note that for some drivers, isNull() will not return accurate
    information until after an attempt is made to retrieve data.

    \sa isActive(), isValid(), value()
*/

bool QSqlQuery::isNull(int field) const
{
    return !d->sqlResult->isActive()
             || !d->sqlResult->isValid()
             || d->sqlResult->isNull(field);
}

/*!
    \overload

    Returns \c true if there is no field with this \a name; otherwise
    returns isNull(int index) for the corresponding field index.

    This overload is less efficient than \l{QSqlQuery::}{isNull()}
*/

bool QSqlQuery::isNull(const QString &name) const
{
    int index = d->sqlResult->record().indexOf(name);
    if (index > -1)
        return isNull(index);
    qWarning("QSqlQuery::isNull: unknown field name '%s'", qPrintable(name));
    return true;
}

/*!

  Executes the SQL in \a query. Returns \c true and sets the query state
  to \l{isActive()}{active} if the query was successful; otherwise
  returns \c false. The \a 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 \e
  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.

  For SQLite, the query string can contain only one statement at a time.
  If more than one statement is given, the function returns \c false.

  Example:

  \snippet sqldatabase/sqldatabase.cpp 34

  \sa isActive(), isValid(), next(), previous(), first(), last(),
  seek()
*/

bool QSqlQuery::exec(const QString& query)
{
#ifdef QT_DEBUG_SQL
    QElapsedTimer t;
    t.start();
#endif
    if (d->ref.load() != 1) {
        bool fo = isForwardOnly();
        *this = QSqlQuery(driver()->createResult());
        d->sqlResult->setNumericalPrecisionPolicy(d->sqlResult->numericalPrecisionPolicy());
        setForwardOnly(fo);
    } else {
        d->sqlResult->clear();
        d->sqlResult->setActive(false);
        d->sqlResult->setLastError(QSqlError());
        d->sqlResult->setAt(QSql::BeforeFirstRow);
        d->sqlResult->setNumericalPrecisionPolicy(d->sqlResult->numericalPrecisionPolicy());
    }
    d->sqlResult->setQuery(query.trimmed());
    if (!driver()->isOpen() || driver()->isOpenError()) {
        qWarning("QSqlQuery::exec: database not open");
        return false;
    }
    if (query.isEmpty()) {
        qWarning("QSqlQuery::exec: empty query");
        return false;
    }

    bool retval = d->sqlResult->reset(query);
#ifdef QT_DEBUG_SQL
    qDebug().nospace() << "Executed query (" << t.elapsed() << "ms, " << d->sqlResult->size()
                       << " results, " << d->sqlResult->numRowsAffected()
                       << " affected): " << d->sqlResult->lastQuery();
#endif
    return retval;
}

/*!
    Returns the value of field \a index in the current record.

    The fields are numbered from left to right using the text of the
    \c SELECT statement, e.g. in

    \snippet code/src_sql_kernel_qsqlquery.cpp 0

    field 0 is \c forename and field 1 is \c
    surname. Using \c{SELECT *} is not recommended because the order
    of the fields in the query is undefined.

    An invalid QVariant is returned if field \a index does not
    exist, if the query is inactive, or if the query is positioned on
    an invalid record.

    \sa previous(), next(), first(), last(), seek(), isActive(), isValid()
*/

QVariant QSqlQuery::value(int index) const
{
    if (isActive() && isValid() && (index > -1))
        return d->sqlResult->data(index);
    qWarning("QSqlQuery::value: not positioned on a valid record");
    return QVariant();
}

/*!
    \overload

    Returns the value of the field called \a name in the current record.
    If field \a name does not exist an invalid variant is returned.

    This overload is less efficient than \l{QSqlQuery::}{value()}
*/

QVariant QSqlQuery::value(const QString& name) const
{
    int index = d->sqlResult->record().indexOf(name);
    if (index > -1)
        return value(index);
    qWarning("QSqlQuery::value: unknown field name '%s'", qPrintable(name));
    return QVariant();
}

/*!
    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.

    \sa previous(), next(), first(), last(), seek(), isActive(), isValid()
*/

int QSqlQuery::at() const
{
    return d->sqlResult->at();
}

/*!
    Returns the text of the current query being used, or an empty
    string if there is no current query text.

    \sa executedQuery()
*/

QString QSqlQuery::lastQuery() const
{
    return d->sqlResult->lastQuery();
}

/*!
    Returns the database driver associated with the query.
*/

const QSqlDriver *QSqlQuery::driver() const
{
    return d->sqlResult->driver();
}

/*!
    Returns the result associated with the query.
*/

const QSqlResult* QSqlQuery::result() const
{
    return d->sqlResult;
}

/*!
  Retrieves the record at position \a 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 \l{isActive()}
  {active} state and isSelect() must return true before calling this
  function.

  If \a relative is false (the default), the following rules apply:

  \list

  \li If \a index is negative, the result is positioned before the
  first record and false is returned.

  \li Otherwise, an attempt is made to move to the record at position
  \a index. If the record at position \a 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.

  \endlist

  If \a relative is true, the following rules apply:

  \list

  \li If the result is currently positioned before the first record and:
  \list
  \li \a index is negative or zero, there is no change, and false is
  returned.
  \li \a index is positive, an attempt is made to position the result
  at absolute position \a index - 1, following the sames rule for non
  relative seek, above.
  \endlist

  \li If the result is currently positioned after the last record and:
  \list
  \li \a index is positive or zero, there is no change, and false is
  returned.
  \li \a index is negative, an attempt is made to position the result
  at \a index + 1 relative position from last record, following the
  rule below.
  \endlist

  \li If the result is currently located somewhere in the middle, and
  the relative offset \a index moves the result below zero, the result
  is positioned before the first record and false is returned.

  \li Otherwise, an attempt is made to move to the record \a index
  records ahead of the current record (or \a index records behind the
  current record if \a index is negative). If the record at offset \a
  index could not be retrieved, the result is positioned after the
  last record if \a index >= 0, (or before the first record if \a
  index is negative), and false is returned. If the record is
  successfully retrieved, true is returned.

  \endlist

  \sa next(), previous(), first(), last(), at(), isActive(), isValid()
*/
bool QSqlQuery::seek(int index, bool relative)
{
    if (!isSelect() || !isActive())
        return false;
    int actualIdx;
    if (!relative) { // arbitrary seek
        if (index < 0) {
            d->sqlResult->setAt(QSql::BeforeFirstRow);
            return false;
        }
        actualIdx = index;
    } else {
        switch (at()) { // relative seek
        case QSql::BeforeFirstRow:
            if (index > 0)
                actualIdx = index - 1;
            else {
                return false;
            }
            break;
        case QSql::AfterLastRow:
            if (index < 0) {
                d->sqlResult->fetchLast();
                actualIdx = at() + index + 1;
            } else {
                return false;
            }
            break;
        default:
            if ((at() + index) < 0) {
                d->sqlResult->setAt(QSql::BeforeFirstRow);
                return false;
            }
            actualIdx = at() + index;
            break;
        }
    }
    // let drivers optimize
    if (isForwardOnly() && actualIdx < at()) {
        qWarning("QSqlQuery::seek: cannot seek backwards in a forward only query");
        return false;
    }
    if (actualIdx == (at() + 1) && at() != QSql::BeforeFirstRow) {
        if (!d->sqlResult->fetchNext()) {
            d->sqlResult->setAt(QSql::AfterLastRow);
            return false;
        }
        return true;
    }
    if (actualIdx == (at() - 1)) {
        if (!d->sqlResult->fetchPrevious()) {
            d->sqlResult->setAt(QSql::BeforeFirstRow);
            return false;
        }
        return true;
    }
    if (!d->sqlResult->fetch(actualIdx)) {
        d->sqlResult->setAt(QSql::AfterLastRow);
        return false;
    }
    return true;
}

/*!

  Retrieves the next record in the result, if available, and positions
  the query on the retrieved record. Note that the result must be in
  the \l{isActive()}{active} state and isSelect() must return true
  before calling this function or it will do nothing and return false.

  The following rules apply:

  \list

  \li 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.

  \li If the result is currently located after the last record, there
  is no change and false is returned.

  \li If the result is located somewhere in the middle, an attempt is
  made to retrieve the next record.

  \endlist

  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.

  \sa previous(), first(), last(), seek(), at(), isActive(), isValid()
*/
bool QSqlQuery::next()
{
    if (!isSelect() || !isActive())
        return false;
    bool b = false;
    switch (at()) {
    case QSql::BeforeFirstRow:
        b = d->sqlResult->fetchFirst();
        return b;
    case QSql::AfterLastRow:
        return false;
    default:
        if (!d->sqlResult->fetchNext()) {
            d->sqlResult->setAt(QSql::AfterLastRow);
            return false;
        }
        return true;
    }
}

/*!

  Retrieves the previous record in the result, if available, and
  positions the query on the retrieved record. Note that the result
  must be in the \l{isActive()}{active} state and isSelect() must
  return true before calling this function or it will do nothing and
  return false.

  The following rules apply:

  \list

  \li If the result is currently located before the first record, there
  is no change and false is returned.

  \li If the result is currently located after the last record, an
  attempt is made to retrieve the last record.

  \li If the result is somewhere in the middle, an attempt is made to
  retrieve the previous record.

  \endlist

  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.

  \sa next(), first(), last(), seek(), at(), isActive(), isValid()
*/
bool QSqlQuery::previous()
{
    if (!isSelect() || !isActive())
        return false;
    if (isForwardOnly()) {
        qWarning("QSqlQuery::seek: cannot seek backwards in a forward only query");
        return false;
    }

    bool b = false;
    switch (at()) {
    case QSql::BeforeFirstRow:
        return false;
    case QSql::AfterLastRow:
        b = d->sqlResult->fetchLast();
        return b;
    default:
        if (!d->sqlResult->fetchPrevious()) {
            d->sqlResult->setAt(QSql::BeforeFirstRow);
            return false;
        }
        return true;
    }
}

/*!
  Retrieves the first record in the result, if available, and
  positions the query on the retrieved record. Note that the result
  must be in the \l{isActive()}{active} state and isSelect() must
  return true before calling this function or it will do nothing and
  return false.  Returns \c true if successful. If unsuccessful the query
  position is set to an invalid position and false is returned.

  \sa next(), previous(), last(), seek(), at(), isActive(), isValid()
 */
bool QSqlQuery::first()
{
    if (!isSelect() || !isActive())
        return false;
    if (isForwardOnly() && at() > QSql::BeforeFirstRow) {
        qWarning("QSqlQuery::seek: cannot seek backwards in a forward only query");
        return false;
    }
    bool b = false;
    b = d->sqlResult->fetchFirst();
    return b;
}

/*!

  Retrieves the last record in the result, if available, and positions
  the query on the retrieved record. Note that the result must be in
  the \l{isActive()}{active} state and isSelect() must return true
  before calling this function or it will do nothing and return false.
  Returns \c true if successful. If unsuccessful the query position is
  set to an invalid position and false is returned.

  \sa next(), previous(), first(), seek(), at(), isActive(), isValid()
*/

bool QSqlQuery::last()
{
    if (!isSelect() || !isActive())
        return false;
    bool b = false;
    b = d->sqlResult->fetchLast();
    return b;
}

/*!
  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-\c SELECT
  statements (isSelect() returns \c false), size() will return -1. If the
  query is not active (isActive() returns \c false), -1 is returned.

  To determine the number of rows affected by a non-\c SELECT
  statement, use numRowsAffected().

  \sa isActive(), numRowsAffected(), QSqlDriver::hasFeature()
*/
int QSqlQuery::size() const
{
    if (isActive() && d->sqlResult->driver()->hasFeature(QSqlDriver::QuerySize))
        return d->sqlResult->size();
    return -1;
}

/*!
  Returns the number of rows affected by the result's SQL statement,
  or -1 if it cannot be determined. Note that for \c SELECT
  statements, the value is undefined; use size() instead. If the query
  is not \l{isActive()}{active}, -1 is returned.

  \sa size(), QSqlDriver::hasFeature()
*/

int QSqlQuery::numRowsAffected() const
{
    if (isActive())
        return d->sqlResult->numRowsAffected();
    return -1;
}

/*!
  Returns error information about the last error (if any) that
  occurred with this query.

  \sa QSqlError, QSqlDatabase::lastError()
*/

QSqlError QSqlQuery::lastError() const
{
    return d->sqlResult->lastError();
}

/*!
  Returns \c true if the query is currently positioned on a valid
  record; otherwise returns \c false.
*/

bool QSqlQuery::isValid() const
{
    return d->sqlResult->isValid();
}

/*!

  Returns \c true if the query is \e{active}. An active QSqlQuery is one
  that has been \l{QSqlQuery::exec()} {exec()'d} successfully but not
  yet finished with.  When you are finished with an active query, you
  can make the query inactive by calling finish() or clear(), or
  you can delete the QSqlQuery instance.

  \note Of particular interest is an active query that is a \c{SELECT}
  statement. For some databases that support transactions, an active
  query that is a \c{SELECT} statement can cause a \l{QSqlDatabase::}
  {commit()} or a \l{QSqlDatabase::} {rollback()} to fail, so before
  committing or rolling back, you should make your active \c{SELECT}
  statement query inactive using one of the ways listed above.

  \sa isSelect()
 */
bool QSqlQuery::isActive() const
{
    return d->sqlResult->isActive();
}

/*!
  Returns \c true if the current query is a \c SELECT statement;
  otherwise returns \c false.
*/

bool QSqlQuery::isSelect() const
{
    return d->sqlResult->isSelect();
}

/*!
  Returns \c true if you can only scroll forward through a result set;
  otherwise returns \c false.

  \sa setForwardOnly(), next()
*/
bool QSqlQuery::isForwardOnly() const
{
    return d->sqlResult->isForwardOnly();
}

/*!
  Sets forward only mode to \a forward. If \a forward is true, only
  next() and seek() with positive values, are allowed for navigating
  the results.

  Forward only mode can be (depending on the driver) more memory
  efficient since results do not need to be cached. It will also
  improve performance on some databases. For this to be true, you must
  call \c setForwardOnly() before the query is prepared or executed.
  Note that the constructor that takes a query and a database may
  execute the query.

  Forward only mode is off by default.

  Setting forward only to false is a suggestion to the database engine,
  which has the final say on whether a result set is forward only or
  scrollable. isForwardOnly() will always return the correct status of
  the result set.

  \note Calling setForwardOnly after execution of the query will result
  in unexpected results at best, and crashes at worst.

  \sa isForwardOnly(), next(), seek(), QSqlResult::setForwardOnly()
*/
void QSqlQuery::setForwardOnly(bool forward)
{
    d->sqlResult->setForwardOnly(forward);
}

/*!
  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 \c{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.

  \snippet code/src_sql_kernel_qsqlquery.cpp 1

  \sa value()
*/
QSqlRecord QSqlQuery::record() const
{
    QSqlRecord rec = d->sqlResult->record();

    if (isValid()) {
        for (int i = 0; i < rec.count(); ++i)
            rec.setValue(i, value(i));
    }
    return rec;
}

/*!
  Clears the result set and releases any resources held by the
  query. Sets the query state to inactive. You should rarely if ever
  need to call this function.
*/
void QSqlQuery::clear()
{
    *this = QSqlQuery(driver()->createResult());
}

/*!
  Prepares the SQL query \a query for execution. Returns \c true if the
  query is prepared successfully; otherwise returns \c false.

  The query may contain placeholders for binding values. Both Oracle
  style colon-name (e.g., \c{:surname}), and ODBC style (\c{?})
  placeholders are supported; but they cannot be mixed in the same
  query. See the \l{QSqlQuery examples}{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.

  For SQLite, the query string can contain only one statement at a time.
  If more than one statement is given, the function returns \c false.

  Example:

  \snippet sqldatabase/sqldatabase.cpp 9

  \sa exec(), bindValue(), addBindValue()
*/
bool QSqlQuery::prepare(const QString& query)
{
    if (d->ref.load() != 1) {
        bool fo = isForwardOnly();
        *this = QSqlQuery(driver()->createResult());
        setForwardOnly(fo);
        d->sqlResult->setNumericalPrecisionPolicy(d->sqlResult->numericalPrecisionPolicy());
    } else {
        d->sqlResult->setActive(false);
        d->sqlResult->setLastError(QSqlError());
        d->sqlResult->setAt(QSql::BeforeFirstRow);
        d->sqlResult->setNumericalPrecisionPolicy(d->sqlResult->numericalPrecisionPolicy());
    }
    if (!driver()) {
        qWarning("QSqlQuery::prepare: no driver");
        return false;
    }
    if (!driver()->isOpen() || driver()->isOpenError()) {
        qWarning("QSqlQuery::prepare: database not open");
        return false;
    }
    if (query.isEmpty()) {
        qWarning("QSqlQuery::prepare: empty query");
        return false;
    }
#ifdef QT_DEBUG_SQL
    qDebug("\n QSqlQuery::prepare: %s", query.toLocal8Bit().constData());
#endif
    return d->sqlResult->savePrepare(query);
}

/*!
  Executes a previously prepared SQL query. Returns \c true if the query
  executed successfully; otherwise returns \c false.

  Note that the last error for this query is reset when exec() is
  called.

  \sa prepare(), bindValue(), addBindValue(), boundValue(), boundValues()
*/
bool QSqlQuery::exec()
{
#ifdef QT_DEBUG_SQL
    QElapsedTimer t;
    t.start();
#endif
    d->sqlResult->resetBindCount();

    if (d->sqlResult->lastError().isValid())
        d->sqlResult->setLastError(QSqlError());

    bool retval = d->sqlResult->exec();
#ifdef QT_DEBUG_SQL
    qDebug().nospace() << "Executed prepared query (" << t.elapsed() << "ms, "
                       << d->sqlResult->size() << " results, " << d->sqlResult->numRowsAffected()
                       << " affected): " << d->sqlResult->lastQuery();
#endif
    return retval;
}

/*! \enum QSqlQuery::BatchExecutionMode

    \value ValuesAsRows - Updates multiple rows. Treats every entry in a QVariantList as a value for updating the next row.
    \value ValuesAsColumns - Updates a single row. Treats every entry in a QVariantList as a single value of an array type.
*/

/*!
    \since 4.2

  Executes a previously prepared SQL query in a batch. All the bound
  parameters have to be lists of variants. If the database doesn't
  support batch executions, the driver will simulate it using
  conventional exec() calls.

  Returns \c true if the query is executed successfully; otherwise
  returns \c false.

  Example:

  \snippet code/src_sql_kernel_qsqlquery.cpp 2

  The example above inserts four new rows into \c myTable:

  \snippet code/src_sql_kernel_qsqlquery.cpp 3

  To bind NULL values, a null QVariant of the relevant type has to be
  added to the bound QVariantList; for example, \c
  {QVariant(QVariant::String)} should be used if you are using
  strings.

  \note Every bound QVariantList must contain the same amount of
  variants.

  \note The type of the QVariants in a list must not change. For
  example, you cannot mix integer and string variants within a
  QVariantList.

  The \a mode parameter indicates how the bound QVariantList will be
  interpreted.  If \a mode is \c ValuesAsRows, every variant within
  the QVariantList will be interpreted as a value for a new row. \c
  ValuesAsColumns is a special case for the Oracle driver. In this
  mode, every entry within a QVariantList will be interpreted as
  array-value for an IN or OUT value within a stored procedure.  Note
  that this will only work if the IN or OUT value is a table-type
  consisting of only one column of a basic type, for example \c{TYPE
  myType IS TABLE OF VARCHAR(64) INDEX BY BINARY_INTEGER;}

  \sa prepare(), bindValue(), addBindValue()
*/
bool QSqlQuery::execBatch(BatchExecutionMode mode)
{
    d->sqlResult->resetBindCount();
    return d->sqlResult->execBatch(mode == ValuesAsColumns);
}

/*!
  Set the placeholder \a placeholder to be bound to value \a val in
  the prepared statement. Note that the placeholder mark (e.g \c{:})
  must be included when specifying the placeholder name. If \a
  paramType is QSql::Out or QSql::InOut, the placeholder will be
  overwritten with data from the database after the exec() call.
  In this case, sufficient space must be pre-allocated to store
  the result into.

  To bind a NULL value, use a null QVariant; for example, use
  \c {QVariant(QVariant::String)} if you are binding a string.

  \sa addBindValue(), prepare(), exec(), boundValue(), boundValues()
*/
void QSqlQuery::bindValue(const QString& placeholder, const QVariant& val,
                          QSql::ParamType paramType
)
{
    d->sqlResult->bindValue(placeholder, val, paramType);
}

/*!
  Set the placeholder in position \a pos to be bound to value \a val
  in the prepared statement. Field numbering starts at 0. If \a
  paramType is QSql::Out or QSql::InOut, the placeholder will be
  overwritten with data from the database after the exec() call.
*/
void QSqlQuery::bindValue(int pos, const QVariant& val, QSql::ParamType paramType)
{
    d->sqlResult->bindValue(pos, val, paramType);
}

/*!
  Adds the value \a 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 \a paramType is QSql::Out or QSql::InOut, the placeholder will be
  overwritten with data from the database after the exec() call.

  To bind a NULL value, use a null QVariant; for example, use \c
  {QVariant(QVariant::String)} if you are binding a string.

  \sa bindValue(), prepare(), exec(), boundValue(), boundValues()
*/
void QSqlQuery::addBindValue(const QVariant& val, QSql::ParamType paramType)
{
    d->sqlResult->addBindValue(val, paramType);
}

/*!
  Returns the value for the \a placeholder.

  \sa boundValues(), bindValue(), addBindValue()
*/
QVariant QSqlQuery::boundValue(const QString& placeholder) const
{
    return d->sqlResult->boundValue(placeholder);
}

/*!
  Returns the value for the placeholder at position \a pos.
*/
QVariant QSqlQuery::boundValue(int pos) const
{
    return d->sqlResult->boundValue(pos);
}

/*!
  Returns a map of the bound values.

  With named binding, the bound values can be examined in the
  following ways:

  \snippet sqldatabase/sqldatabase.cpp 14

  With positional binding, the code becomes:

  \snippet sqldatabase/sqldatabase.cpp 15

  \sa boundValue(), bindValue(), addBindValue()
*/
QMap<QString,QVariant> QSqlQuery::boundValues() const
{
    QMap<QString,QVariant> map;

    const QVector<QVariant> values(d->sqlResult->boundValues());
    for (int i = 0; i < values.count(); ++i)
        map[d->sqlResult->boundValueName(i)] = values.at(i);
    return map;
}

/*!
  Returns the last query that was successfully 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.

  \sa lastQuery()
*/
QString QSqlQuery::executedQuery() const
{
    return d->sqlResult->executedQuery();
}

/*!
  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.

  For MySQL databases the row's auto-increment field will be returned.

  \note For this function to work in PSQL, the table table must
  contain OIDs, which may not have been created by default.  Check the
  \c default_with_oids configuration variable to be sure.

  \sa QSqlDriver::hasFeature()
*/
QVariant QSqlQuery::lastInsertId() const
{
    return d->sqlResult->lastInsertId();
}

/*!

  Instruct the database driver to return numerical values with a
  precision specified by \a precisionPolicy.

  The Oracle driver, for example, can retrieve numerical values as
  strings to prevent the loss of precision. If high precision doesn't
  matter, use this method to increase execution speed by bypassing
  string conversions.

  Note: Drivers that don't support fetching numerical values with low
  precision will ignore the precision policy. You can use
  QSqlDriver::hasFeature() to find out whether a driver supports this
  feature.

  Note: Setting the precision policy doesn't affect the currently
  active query. Call \l{exec()}{exec(QString)} or prepare() in order
  to activate the policy.

  \sa QSql::NumericalPrecisionPolicy, numericalPrecisionPolicy()
*/
void QSqlQuery::setNumericalPrecisionPolicy(QSql::NumericalPrecisionPolicy precisionPolicy)
{
    d->sqlResult->setNumericalPrecisionPolicy(precisionPolicy);
}

/*!
  Returns the current precision policy.

  \sa QSql::NumericalPrecisionPolicy, setNumericalPrecisionPolicy()
*/
QSql::NumericalPrecisionPolicy QSqlQuery::numericalPrecisionPolicy() const
{
    return d->sqlResult->numericalPrecisionPolicy();
}

/*!
  \since 4.3.2

  Instruct the database driver that no more data will be fetched from
  this query until it is re-executed. There is normally no need to
  call this function, but it may be helpful in order to free resources
  such as locks or cursors if you intend to re-use the query at a
  later time.

  Sets the query to inactive. Bound values retain their values.

  \sa prepare(), exec(), isActive()
*/
void QSqlQuery::finish()
{
    if (isActive()) {
        d->sqlResult->setLastError(QSqlError());
        d->sqlResult->setAt(QSql::BeforeFirstRow);
        d->sqlResult->detachFromResultSet();
        d->sqlResult->setActive(false);
    }
}

/*!
  \since 4.4

  Discards the current result set and navigates to the next if available.

  Some databases are capable of returning multiple result sets for
  stored procedures or SQL batches (a query strings that contains
  multiple statements). If multiple result sets are available after
  executing a query this function can be used to navigate to the next
  result set(s).

  If a new result set is available this function will return true.
  The query will be repositioned on an \e invalid record in the new
  result set and must be navigated to a valid record before data
  values can be retrieved. If a new result set isn't available the
  function returns \c false and the query is set to inactive. In any
  case the old result set will be discarded.

  When one of the statements is a non-select statement a count of
  affected rows may be available instead of a result set.

  Note that some databases, i.e. Microsoft SQL Server, requires
  non-scrollable cursors when working with multiple result sets.  Some
  databases may execute all statements at once while others may delay
  the execution until the result set is actually accessed, and some
  databases may have restrictions on which statements are allowed to
  be used in a SQL batch.

  \sa QSqlDriver::hasFeature(), setForwardOnly(), next(), isSelect(),
      numRowsAffected(), isActive(), lastError()
*/
bool QSqlQuery::nextResult()
{
    if (isActive())
        return d->sqlResult->nextResult();
    return false;
}

QT_END_NAMESPACE