C++ 使用 Qt 从 SQLite 中选择
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Select from SQLite with Qt
提问by vas3k
I try to deal with SQLite database on Qt 4.5.3 on Linux. I've already created the databsae.
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。
Then, I try to perform select on Qt:
然后，我尝试在 Qt 上执行选择：
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();
But debug console says:
但是调试控制台说：
("lessons", "weeklessons", "weeks")
-1
true
false
Why it's select nothing? What I have doing wrong?
为什么什么都不选？我做错了什么？
回答by Leiaz
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
如果查询定位在有效记录上，则 isValid() 方法返回 true，但在调用 exec() 后，它不是：您必须首先移动到有效记录，例如使用 query.first() 或查询。下一个（）。请参阅 Qt 文档：http: //doc.qt.io/archives/4.6/qsqlquery.html
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().
size() 返回 -1 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）。您可以检查行是否返回并使用循环和 query.next() 查找大小。
Depending on what you want to do with the result of your select, you could also use QSqlQueryModel instead of QSqlQuery.
根据您想对选择的结果做什么，您还可以使用 QSqlQueryModel 而不是 QSqlQuery。
回答by matt_man22
In your "QSqlQuery query;" declaration, you have to specify the database connection, e.g. "QSqlQuery query(db)"
在您的“QSqlQuery 查询”中；声明，您必须指定数据库连接，例如“QSqlQuery query(db)”
回答by Ton van den Heuvel
Given that your program reports that the query is invalid, have a look at the error message as follows:
鉴于您的程序报告查询无效，请查看错误消息，如下所示：
QDebug() << query.lastError().text();
This should help you in debugging the problem.
这应该可以帮助您调试问题。
回答by poke
Without knowing much about Qt, I was asking myself one question: How does the QSqlQueryknow which connection to use. Looking that up in the manual revealed that there are basically two possibilities to execute a query:
在不太了解 Qt 的情况下，我问自己一个问题：如何QSqlQuery知道使用哪个连接。在手册中查找发现执行查询基本上有两种可能性：
query = db.exec("select * from lessons");
Using the database connection, and executingfrom there.QSqlQuery query( db );
Constructing the query using the existing databaseand then executing the query:query.exec("select * from lessons");
query = db.exec("select * from lessons");
使用数据库连接，并从那里执行。QSqlQuery query( db );
使用现有数据库构建查询，然后执行查询：query.exec("select * from lessons");
edit:After reading a bit more, it seems that default connections are supported, so your example should work…
编辑：阅读更多内容后，似乎支持默认连接，因此您的示例应该可以工作...
Could you try passing the query as the constructor's value instead? Maybe it works then.
您可以尝试将查询作为构造函数的值传递吗？也许它会起作用。
QSqlQuery query("select * from lessons");
