The reasons why QT fails to connect to the database and common solutions

use qDebug() << db.lastError().text(); Look at the cause of the error

① First, check your database name , user name , Host name , password , Are these port numbers set correctly

For example, if I write the user name wrong, the following error will appear

"Access denied for user 'rot'@'localhost' (using password: YES) QMYSQL: Unable to connect"

The database connection code is as follows

	db.setDatabaseName("qtqq");// Database name （ Keep consistent with your settings ）
	db.setHostName("localhost");// Host name 
	db.setUserName("root");// user name 
	db.setPassword("sqhxxxx");// Password and us Mysql Your account and password are the same 
	db.setPort(3306);// Set the port number 

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② Look at your Navicat Is it open and connected （ I often forget ）

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③ If the problem is the lack of drivers, you have to add some files
The problem is displayed as ：QMYSQL driver not loaded

resolvent ：
mysql Of the installation file lib In the catalog libmysql.dll and libmysqld.dll Copied to the Qt5.5 Medium mingw Of bin Under the document .

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If I know the reasons for other types of connection failures later, I will add , Now make do with it