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The QSqlQuery class provides a means of executing and manipulating SQL statements. More...
#include <QSqlQuery>
Inherited by Q3SqlCursor.
The QSqlQuery class provides a means of executing and manipulating SQL statements.
QSqlQuery encapsulates the functionality involved in creating, navigating and retrieving data from SQL queries which are executed on a QSqlDatabase. It can be used to execute DML (data manipulation language) statements, such as SELECT, INSERT, UPDATE and DELETE, as well as DDL (data definition language) statements, such as CREATE TABLE. It can also be used to execute database-specific commands which are not standard SQL (e.g. SET DATESTYLE=ISO for PostgreSQL).
Successfully executed SQL statements set the query's state to active; isActive() then returns 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 true) before values can be retrieved.
Navigating records is performed with the following functions:
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.
For example:
QSqlQuery query("SELECT country FROM artist");
while (query.next()) {
QString country = query.value(0).toString();
doSomething(country);
}
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 query("SELECT * FROM artist");
int fieldNo = query.record().indexOf("country");
while (query.next()) {
QString country = query.value(fieldNo).toString();
doSomething(country);
}
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.
Oracle databases identify placeholders by using a colon-name syntax, e.g :name. ODBC simply uses ? 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().
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.
Constructs a QSqlQuery object which uses the QSqlResult result to communicate with a database.
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.
Constructs a QSqlQuery object using the database db.
See also QSqlDatabase.
Constructs a copy of other.
Destroys the object and frees any allocated resources.
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().
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().
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().
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
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.
Returns the value for the placeholder.
See also boundValues(), bindValue(), and addBindValue().
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
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().
Clears the result set and releases any resources held by the query. You should rarely if ever need to call this function.
Returns the database driver associated with the 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().
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
Executes a previously prepared SQL query. Returns true if the query executed successfully; otherwise returns false.
See also prepare(), bindValue(), addBindValue(), boundValue(), and boundValues().
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().
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().
Returns true if the query is currently active; otherwise returns false.
Returns true if you can only scroll forward through a result set; otherwise returns false.
See also setForwardOnly() and next().
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().
Returns true if the current query is a SELECT statement; otherwise returns false.
Returns true if the query is currently positioned on a valid record; otherwise returns false.
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().
Returns error information about the last error (if any) that occurred.
See also QSqlError.
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().
Returns the text of the current query being used, or an empty string if there is no current query text.
See also executedQuery().
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 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().
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().
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.
See also exec(), bindValue(), and addBindValue().
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 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().
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().
Returns the result associated with the query.
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 relative is true, the following rules apply:
See also next(), previous(), first(), last(), at(), isActive(), and isValid().
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().
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().
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().
Assigns other to this object.
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