使用 Qt 从 SQLite 中选择 [英] Select from SQLite with Qt

问题描述

我尝试在 Linux 上的 Qt 4.5.3 上处理 SQLite 数据库.我已经创建了数据库.

I try to deal with SQLite database on Qt 4.5.3 on Linux. I've already created the databsae.

然后，我尝试在 Qt 上执行选择:

Then, I try to perform select on Qt:

db = QSqlDatabase::addDatabase("QSQLITE");
db.setDatabaseName(filename); // Here is FULL path to the database. I've checked it twice :)
bool ok = db.open();
qDebug() << db.tables();

QSqlQuery query;
query.exec("select * from lessons");
qDebug() << query.size();
qDebug() << query.isSelect();
qDebug() << query.isValid();

但是调试控制台说:

("lessons", "weeklessons", "weeks") 
-1 
true 
false 

为什么什么都不选?我做错了什么?

Why it's select nothing? What I have doing wrong?

推荐答案

如果查询定位在有效记录上，isValid() 方法返回 true，但在调用 exec() 后，它不是:您必须移动首先到有效记录，例如使用 query.first() 或 query.next().请参阅 Qt 文档:http://doc.qt.io/archives/4.6/qsqlquery.html

The isValid() method returns true if the query is positionned on a valid record, but after calling exec(), it isn't : you have to move to a valid record first, for example with query.first() or query.next(). See Qt docs : http://doc.qt.io/archives/4.6/qsqlquery.html

返回 -1 的 size() 并不意味着没有结果:SQLite 是无法直接获得查询大小的数据库之一(查看 QSqlDriver::hasFeature() 的文档).您可以检查是否返回了行并使用循环和 query.next() 查找大小.

The size() returning -1 doesn't mean there is no result : SQLite is one of the databases for which the size of the query is not directly available (look in the documentation for QSqlDriver::hasFeature()). You can check that rows are returned and find the size with a loop and query.next().

根据您想对选择的结果做什么，您也可以使用 QSqlQueryModel 而不是 QSqlQuery.

Depending on what you want to do with the result of your select, you could also use QSqlQueryModel instead of QSqlQuery.

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