QSqlField 类

QSqlField 类操纵 SQL 数据库表和视图中的字段。更多...

头：	#include <QSqlField>
CMake:	find_package(Qt6 REQUIRED COMPONENTS Sql) target_link_libraries(mytarget PRIVATE Qt6::Sql)
qmake:	QT += sql

公共类型

enum	RequiredStatus { Required, Optional, Unknown }

公共函数

	QSqlField (const QString & fieldName = QString(), QMetaType type = QMetaType(), const QString & table = QString())
	QSqlField (const QSqlField & other )
	~QSqlField ()
void	clear ()
QVariant	defaultValue () const
bool	isAutoValue () const
bool	isGenerated () const
bool	isNull () const
bool	isReadOnly () const
bool	isValid () const
int	length () const
QMetaType	metaType () const
QString	name () const
int	precision () const
QSqlField::RequiredStatus	requiredStatus () const
void	setAutoValue (bool autoVal )
void	setDefaultValue (const QVariant & value )
void	setGenerated (bool gen )
void	setLength (int fieldLength )
void	setMetaType (QMetaType type )
void	setName (const QString & name )
void	setPrecision (int precision )
void	setReadOnly (bool readOnly )
void	setRequired (bool required )
void	setRequiredStatus (QSqlField::RequiredStatus required )
void	setTableName (const QString & table )
void	setValue (const QVariant & value )
QString	tableName () const
QVariant	value () const
bool	operator!= (const QSqlField & other ) const
QSqlField &	operator= (const QSqlField & other )
bool	operator== (const QSqlField & other ) const

详细描述

QSqlField represents the characteristics of a single column in a database table or view, such as the data type and column name. A field also contains the value of the database column, which can be viewed or changed.

Field data values are stored as QVariants. Using an incompatible type is not permitted. For example:

    QSqlField field("age", QMetaType::fromType<int>());
    field.setValue(QPixmap());  // WRONG

However, the field will attempt to cast certain data types to the field data type where possible:

    QSqlField field("age", QMetaType::fromType<int>());
    field.setValue(QString("123"));  // casts QString to int

QSqlField objects are rarely created explicitly in application code. They are usually accessed indirectly through QSqlRecord s that already contain a list of fields. For example:

    QSqlQuery query;
    ...
    QSqlRecord record = query.record();
    QSqlField field = record.field("country");

A QSqlField object can provide some meta-data about the field, for example, its name (), variant type(), length (), precision (), defaultValue (), typeID(), and its requiredStatus (), isGenerated () 和 isReadOnly (). The field's data can be checked to see if it isNull ()，及其 value () retrieved. When editing the data can be set with setValue () or set to NULL with clear ().

另请参阅 QSqlRecord .

成员类型文档编制

enum QSqlField:: RequiredStatus

Specifies whether the field is required or optional.

常量	值	描述
`QSqlField::Required`	`1`	The field must be specified when inserting records.
`QSqlField::Optional`	`0`	The fields doesn't have to be specified when inserting records.
`QSqlField::Unknown`	`-1`	The database driver couldn't determine whether the field is required or optional.

另请参阅 requiredStatus ().

成员函数文档编制

`[explicit, since 6.0]` QSqlField:: QSqlField (const QString & fieldName = QString(), QMetaType type = QMetaType(), const QString & table = QString())

这是重载函数。

Constructs an empty field called fieldName 类型 type in table .

该函数在 Qt 6.0 引入。

另请参阅 setRequiredStatus (), setLength (), setPrecision (), setDefaultValue (), setGenerated ()，和 setReadOnly ().

QSqlField:: QSqlField (const QSqlField & other )

构造副本为 other .

QSqlField:: ~QSqlField ()

销毁对象并释放任何分配资源。

void QSqlField:: clear ()

Clears the value of the field and sets it to NULL. If the field is read-only, nothing happens.

另请参阅 setValue (), isReadOnly ()，和 requiredStatus ().

QVariant QSqlField:: defaultValue () const

Returns the field's default value (which may be NULL).

另请参阅 setDefaultValue (), metaType (), requiredStatus (), length (), precision ()，和 isGenerated ().

bool QSqlField:: isAutoValue () const

返回 true if the value is auto-generated by the database, for example auto-increment primary key values.

注意： When using the ODBC driver, due to limitations in the ODBC API, the isAutoValue() field is only populated in a QSqlField resulting from a QSqlRecord obtained by executing a SELECT query. It is false 在 QSqlField resulting from a QSqlRecord returned from QSqlDatabase::record () 或 QSqlDatabase::primaryIndex ().

另请参阅 setAutoValue ().

bool QSqlField:: isGenerated () const

返回 true if the field is generated; otherwise returns false.

另请参阅 setGenerated (), metaType (), requiredStatus (), length (), precision ()，和 defaultValue ().

bool QSqlField:: isNull () const

返回 true if the field's value is NULL; otherwise returns false.

另请参阅 value ().

bool QSqlField:: isReadOnly () const

返回 true if the field's value is read-only; otherwise returns false.

另请参阅 setReadOnly (), metaType (), requiredStatus (), length (), precision (), defaultValue ()，和 isGenerated ().

bool QSqlField:: isValid () const

返回 true if the field's variant type is valid; otherwise returns false .

int QSqlField:: length () const

Returns the field's length.

If the returned value is negative, it means that the information is not available from the database.

另请参阅 setLength (), metaType (), requiredStatus (), precision (), defaultValue ()，和 isGenerated ().

QMetaType QSqlField:: metaType () const

Returns the field's type as stored in the database. Note that the actual value might have a different type, Numerical values that are too large to store in a long int or double are usually stored as strings to prevent precision loss.

另请参阅 setMetaType ().

QString QSqlField:: name () const

Returns the name of the field.

另请参阅 setName ().

int QSqlField:: precision () const

Returns the field's precision; this is only meaningful for numeric types.

If the returned value is negative, it means that the information is not available from the database.

另请参阅 setPrecision (), metaType (), requiredStatus (), length (), defaultValue ()，和 isGenerated ().

QSqlField::RequiredStatus QSqlField:: requiredStatus () const

返回 true if this is a required field; otherwise returns false . An INSERT will fail if a required field does not have a value.

另请参阅 setRequiredStatus (), metaType (), length (), precision (), defaultValue ()，和 isGenerated ().

void QSqlField:: setAutoValue ( bool autoVal )

Marks the field as an auto-generated value if autoVal 为 true。

另请参阅 isAutoValue ().

void QSqlField:: setDefaultValue (const QVariant & value )

Sets the default value used for this field to value .

另请参阅 defaultValue (), value (), setMetaType (), setRequiredStatus (), setLength (), setPrecision (), setGenerated ()，和 setReadOnly ().

void QSqlField:: setGenerated ( bool gen )

Sets the generated state. If gen is false, no SQL will be generated for this field; otherwise, Qt classes such as QSqlQueryModel and QSqlTableModel will generate SQL for this field.

另请参阅 isGenerated (), setMetaType (), setRequiredStatus (), setLength (), setPrecision (), setDefaultValue ()，和 setReadOnly ().

void QSqlField:: setLength ( int fieldLength )

Sets the field's length to fieldLength . For strings this is the maximum number of characters the string can hold; the meaning varies for other types.

另请参阅 length (), setMetaType (), setRequiredStatus (), setPrecision (), setDefaultValue (), setGenerated ()，和 setReadOnly ().

void QSqlField:: setMetaType ( QMetaType type )

Set's the field's variant type to type .

另请参阅 metaType (), setRequiredStatus (), setLength (), setPrecision (), setDefaultValue (), setGenerated ()，和 setReadOnly ().

void QSqlField:: setName (const QString & name )

Sets the name of the field to name .

另请参阅 name ().

void QSqlField:: setPrecision ( int precision )

Sets the field's precision . This only affects numeric fields.

另请参阅 precision (), setMetaType (), setRequiredStatus (), setLength (), setDefaultValue (), setGenerated ()，和 setReadOnly ().

void QSqlField:: setReadOnly ( bool readOnly )

Sets the read only flag of the field's value to readOnly . A read-only field cannot have its value set with setValue () and cannot be cleared to NULL with clear ().

另请参阅 isReadOnly ().

void QSqlField:: setRequired ( bool required )

Sets the required status of this field to Required if required is true; otherwise sets it to 可选 .

另请参阅 setRequiredStatus () 和 requiredStatus ().

void QSqlField:: setRequiredStatus ( QSqlField::RequiredStatus required )

Sets the required status of this field to required .

另请参阅 requiredStatus (), setMetaType (), setLength (), setPrecision (), setDefaultValue (), setGenerated ()，和 setReadOnly ().

void QSqlField:: setTableName (const QString & table )

设置 tableName of the field to table .

另请参阅 tableName ().

void QSqlField:: setValue (const QVariant & value )

Sets the value of the field to value . If the field is read-only ( isReadOnly () 返回 true ), nothing happens.

If the data type of value differs from the field's current data type, an attempt is made to cast it to the proper type. This preserves the data type of the field in the case of assignment, e.g. a QString to an integer data type.

To set the value to NULL, use clear ().

另请参阅 value (), isReadOnly ()，和 defaultValue ().

QString QSqlField:: tableName () const

Returns the tableName of the field.

注意： When using the QPSQL driver, due to limitations in the libpq library, the tableName() field is not populated in a QSqlField resulting from a QSqlRecord obtained by QSqlQuery::record () of a forward-only query.

另请参阅 setTableName ().

QVariant QSqlField:: value () const

Returns the value of the field as a QVariant .

使用 isNull () to check if the field's value is NULL.

另请参阅 setValue ().

bool QSqlField:: operator!= (const QSqlField & other ) const

返回 true if the field is unequal to other ；否则返回 false。

QSqlField &QSqlField:: operator= (const QSqlField & other )

Sets the field equal to other .

bool QSqlField:: operator== (const QSqlField & other ) const

返回 true if the field is equal to other ；否则返回 false。