QSqlQuery 类提供执行和操纵 SQL 语句的手段。 更多...
| 头： | #include <QSqlQuery> |
| CMake: |
find_package(Qt6 REQUIRED COMPONENTS Sql)
target_link_libraries(mytarget PRIVATE Qt6::Sql) |
| qmake: | QT += sql |
| enum | BatchExecutionMode { ValuesAsRows, ValuesAsColumns } |
| QSqlQuery (QSqlResult * result ) | |
| QSqlQuery (const QString & query = QString(), const QSqlDatabase & db = QSqlDatabase()) | |
| QSqlQuery (const QSqlDatabase & db ) | |
| QSqlQuery (QSqlQuery && other ) | |
| ~QSqlQuery () | |
| void | addBindValue (const QVariant & val , QSql::ParamType paramType = QSql::In) |
| int | at () const |
| void | bindValue (const QString & placeholder , const QVariant & val , QSql::ParamType paramType = QSql::In) |
| void | bindValue (int pos , const QVariant & val , QSql::ParamType paramType = QSql::In) |
| QVariant | boundValue (const QString & placeholder ) const |
| QVariant | boundValue (int pos ) const |
| QVariantList | boundValues () const |
| void | clear () |
| const QSqlDriver * | driver () const |
| bool | exec (const QString & query ) |
| bool | exec () |
| bool | execBatch (QSqlQuery::BatchExecutionMode mode = ValuesAsRows) |
| QString | executedQuery () const |
| void | finish () |
| bool | first () |
| bool | isActive () const |
| bool | isForwardOnly () const |
| bool | isNull (int field ) const |
| bool | isNull (const QString & name ) const |
| bool | isSelect () const |
| bool | isValid () const |
| bool | last () |
| QSqlError | lastError () const |
| QVariant | lastInsertId () const |
| QString | lastQuery () const |
| bool | next () |
| bool | nextResult () |
| int | numRowsAffected () const |
| QSql::NumericalPrecisionPolicy | numericalPrecisionPolicy () const |
| bool | prepare (const QString & query ) |
| bool | previous () |
| QSqlRecord | record () const |
| const QSqlResult * | result () const |
| bool | seek (int index , bool relative = false) |
| void | setForwardOnly (bool forward ) |
| void | setNumericalPrecisionPolicy (QSql::NumericalPrecisionPolicy precisionPolicy ) |
| int | size () const |
| void | swap (QSqlQuery & other ) |
| QVariant | value (int index ) const |
| QVariant | value (const QString & name ) const |
| QSqlQuery & | operator= (QSqlQuery && other ) |
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 so that
isActive
() 返回
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
() 返回
true
) before values can be retrieved.
For some databases, if an active query that is a
SELECT
statement exists when you call
commit
() 或
rollback
(), the commit or rollback will fail. See
isActive
() 了解细节。
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., 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.
例如：
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 using boundValues ().
注意： Not all SQL operations support binding values. Refer to your database system's documentation to check their availability.
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
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 SQL 数据库驱动程序 .
警告： 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.
另请参阅 QSqlDatabase , QSqlQueryModel , QSqlTableModel ，和 QVariant .
| 常量 | 值 | 描述 |
|---|---|---|
QSqlQuery::ValuesAsRows
|
0
|
- Updates multiple rows. Treats every entry in a QVariantList as a value for updating the next row. |
QSqlQuery::ValuesAsColumns
|
1
|
- Updates a single row. Treats every entry in a QVariantList as a single value of an array type. |
[explicit]
QSqlQuery::
QSqlQuery
(
QSqlResult
*
result
)
Constructs a QSqlQuery object which uses the QSqlResult result to communicate with a database.
[explicit]
QSqlQuery::
QSqlQuery
(const
QString
&
query
= QString(), const
QSqlDatabase
&
db
= QSqlDatabase())
Constructs a QSqlQuery object using the SQL query and the database db 。若 db is not specified, or is invalid, the application's default database is used. If query is not an empty string, it will be executed.
另请参阅 QSqlDatabase .
[explicit]
QSqlQuery::
QSqlQuery
(const
QSqlDatabase
&
db
)
Constructs a QSqlQuery object using the database db 。若 db is invalid, the application's default database will be used.
另请参阅 QSqlDatabase .
[since 6.2]
QSqlQuery::
QSqlQuery
(
QSqlQuery
&&
other
)
Move-constructs a QSqlQuery from other .
该函数在 Qt 6.2 引入。
销毁对象并释放任何分配资源。
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 () 调用。
To bind a NULL value, use a null
QVariant
; for example, use
QVariant(QMetaType::fromType<QString>())
if you are binding a string.
另请参阅 bindValue (), prepare (), exec (), boundValue ()，和 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.
另请参阅 previous (), next (), first (), last (), seek (), isActive ()，和 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. 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
QVariant(QMetaType::fromType<QString>())
if you are binding a string.
另请参阅 addBindValue (), prepare (), exec (), boundValue ()，和 boundValues ().
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 () 调用。
Returns the value for the placeholder .
另请参阅 boundValues (), bindValue ()，和 addBindValue ().
Returns the value for the placeholder at position pos .
[since 6.0]
QVariantList
QSqlQuery::
boundValues
() const
Returns a list of bound values.
The order of the list is in binding order, irrespective of whether named or positional binding is used.
The bound values can be examined in the following way:
QVariantList list = query.boundValues();
for (int i = 0; i < list.size(); ++i)
cout << i << ": " << list.at(i).toString().toUtf8().data() << "\n";
该函数在 Qt 6.0 引入。
另请参阅 boundValue (), bindValue ()，和 addBindValue ().
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.
Returns the database driver associated with the query.
Executes the SQL in
query
。返回
true
and sets the query state to
active
if the query was successful; otherwise returns
false
。
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.
For SQLite, the query string can contain only one statement at a time. If more than one statement is given, the function returns
false
.
范例：
QSqlQuery query;
query.exec("INSERT INTO employee (id, name, salary) "
"VALUES (1001, 'Thad Beaumont', 65000)");
另请参阅 isActive (), isValid (), next (), previous (), first (), last ()，和 seek ().
Executes a previously prepared SQL query. Returns
true
if the query executed successfully; otherwise returns
false
.
Note that the last error for this query is reset when exec() is called.
另请参阅 prepare (), bindValue (), addBindValue (), boundValue ()，和 boundValues ().
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 () 调用。
返回
true
if the query is executed successfully; otherwise returns
false
.
范例：
QSqlQuery q; q.prepare("insert into myTable values (?, ?)"); QVariantList ints; ints << 1 << 2 << 3 << 4; q.addBindValue(ints); QVariantList names; names << "Harald" << "Boris" << "Trond" << QVariant(QMetaType::fromType<QString>()); q.addBindValue(names); if (!q.execBatch()) qDebug() << q.lastError();
The example above inserts four new rows into
myTable
:
1 Harald 2 Boris 3 Trond 4 NULL
To bind NULL values, a null
QVariant
of the relevant type has to be added to the bound QVariantList; for example,
QVariant(QMetaType::fromType<QString>())
should be used if you are using strings.
注意： Every bound QVariantList must contain the same amount of variants.
注意： The type of the QVariants in a list must not change. For example, you cannot mix integer and string variants within a QVariantList.
The
mode
parameter indicates how the bound QVariantList will be interpreted. If
mode
is
ValuesAsRows
, every variant within the QVariantList will be interpreted as a value for a new row.
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
TYPE myType IS TABLE OF VARCHAR(64) INDEX BY BINARY_INTEGER;
另请参阅 prepare (), bindValue ()，和 addBindValue ().
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.
另请参阅 lastQuery ().
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.
另请参阅 prepare (), exec ()，和 isActive ().
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
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.
另请参阅 next (), previous (), last (), seek (), at (), isActive ()，和 isValid ().
返回
true
if the query is
active
. An active
QSqlQuery
is one that has been
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
() 或
clear
(), or you can delete the
QSqlQuery
实例。
注意：
Of particular interest is an active query that is a
SELECT
statement. For some databases that support transactions, an active query that is a
SELECT
statement can cause a
commit
() or a
rollback
() to fail, so before committing or rolling back, you should make your active
SELECT
statement query inactive using one of the ways listed above.
另请参阅 isSelect ().
返回
true
if you can only scroll forward through a result set; otherwise returns
false
.
另请参阅 setForwardOnly () 和 next ().
返回
true
if the query is not
active
, the query is not positioned on a valid record, there is no such
field
，或
field
is null; otherwise
false
. Note that for some drivers, isNull() will not return accurate information until after an attempt is made to retrieve data.
另请参阅 isActive (), isValid ()，和 value ().
这是重载函数。
返回
true
if there is no field with this
name
; otherwise returns isNull(int index) for the corresponding field index.
This overload is less efficient than isNull ()
返回
true
if the current query is a
SELECT
statement; otherwise returns
false
.
返回
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 the
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.
另请参阅 next (), previous (), first (), seek (), at (), isActive ()，和 isValid ().
Returns error information about the last error (if any) that occurred with this query.
另请参阅 QSqlError and QSqlDatabase::lastError ().
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.
注意：
For this function to work in PSQL, the table table must contain OIDs, which may not have been created by default. Check the
default_with_oids
configuration variable to be sure.
另请参阅 QSqlDriver::hasFeature ().
Returns the text of the current query being used, or an empty string if there is no current query text.
另请参阅 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 the 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.
另请参阅 previous (), first (), last (), seek (), at (), isActive ()，和 isValid ().
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
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
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.
另请参阅 QSqlDriver::hasFeature (), setForwardOnly (), next (), isSelect (), numRowsAffected (), isActive ()，和 lastError ().
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
, -1 is returned.
另请参阅 size () 和 QSqlDriver::hasFeature ().
Returns the current precision policy.
另请参阅 QSql::NumericalPrecisionPolicy and setNumericalPrecisionPolicy ().
Prepares the SQL query
query
for execution. Returns
true
if the query is prepared successfully; otherwise returns
false
.
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
详细描述
范例。
Portability notes: 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. When the database does not support named placeholders directly, the placeholder can only contain characters in the range [a-zA-Z0-9_].
For SQLite, the query string can contain only one statement at a time. If more than one statement is given, the function returns
false
.
范例：
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();
另请参阅 exec (), bindValue ()，和 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 the 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.
另请参阅 next (), first (), last (), seek (), at (), isActive ()，和 isValid ().
返回 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
另请参阅 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.
若 relative is false (the default), the following rules apply:
若 relative is true, the following rules apply:
另请参阅 next (), previous (), first (), last (), at (), isActive ()，和 isValid ().
Sets forward only mode to forward 。若 forward is true, only next () 和 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
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.
注意： Calling setForwardOnly after execution of the query will result in unexpected results at best, and crashes at worst.
注意： To make sure the forward-only query completed successfully, the application should check lastError () for an error not only after executing the query, but also after navigating the query results.
警告： PostgreSQL: While navigating the query results in forward-only mode, do not execute any other SQL command on the same database connection. This will cause the query results to be lost.
另请参阅 isForwardOnly (), next (), seek ()，和 QSqlResult::setForwardOnly ().
Instruct the database driver to return numerical values with a precision specified by 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 exec (QString) or prepare () in order to activate the policy.
另请参阅 QSql::NumericalPrecisionPolicy and numericalPrecisionPolicy ().
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
() 返回
false
), size() will return -1. If the query is not active (
isActive
() 返回
false
), -1 is returned.
To determine the number of rows affected by a non-
SELECT
statement, use
numRowsAffected
().
另请参阅 isActive (), numRowsAffected ()，和 QSqlDriver::hasFeature ().
[since 6.2]
void
QSqlQuery::
swap
(
QSqlQuery
&
other
)
交换 other to this object. This operation is very fast and never fails.
该函数在 Qt 6.2 引入。
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
。使用
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.
另请参阅 previous (), next (), first (), last (), seek (), isActive ()，和 isValid ().
这是重载函数。
Returns the value of the field called name in the current record. If field name does not exist an invalid variant is returned.
This overload is less efficient than value ()
[since 6.2]
QSqlQuery
&QSqlQuery::
operator=
(
QSqlQuery
&&
other
)
移动赋值 other 到此对象。
该函数在 Qt 6.2 引入。