使用python代码的pyqt库做一个软件，实现首先有一个登录界面，登录后做一个界面实现数据库管理系统，实现内容导入，数据库增删改查

好的，以下是一个简单的实现代码，具体的实现过程中需要根据实际需求进行调整和优化：

from PyQt5.QtWidgets import QApplication, QMainWindow, QWidget, QLabel, QLineEdit, QPushButton, QTableWidget, QTableWidgetItem, QComboBox, QFileDialog, QMessageBox
from PyQt5.QtSql import QSqlDatabase, QSqlQuery
import sys

class LoginWindow(QWidget):
    def __init__(self):
        super().__init__()
        self.setWindowTitle('登录')
        self.resize(300, 200)

        self.username_label = QLabel('用户名：', self)
        self.username_label.move(50, 50)
        self.username_edit = QLineEdit(self)
        self.username_edit.move(100, 50)

        self.password_label = QLabel('密码：', self)
        self.password_label.move(50, 100)
        self.password_edit = QLineEdit(self)
        self.password_edit.setEchoMode(QLineEdit.Password)
        self.password_edit.move(100, 100)

        self.login_button = QPushButton('登录', self)
        self.login_button.move(100, 150)
        self.login_button.clicked.connect(self.login)

    def login(self):
        username = self.username_edit.text()
        password = self.password_edit.text()

        # 进行用户名和密码验证
        if username == 'admin' and password == '123456':
            self.hide()
            self.main_window = MainWindow()
            self.main_window.show()
        else:
            QMessageBox.warning(self, '错误', '用户名或密码错误！')

class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()
        self.setWindowTitle('数据库管理系统')
        self.resize(800, 600)

        self.central_widget = QWidget(self)
        self.setCentralWidget(self.central_widget)

        self.database_label = QLabel('选择数据库：', self.central_widget)
        self.database_label.move(50, 50)
        self.database_combo = QComboBox(self.central_widget)
        self.database_combo.move(150, 50)
        self.database_combo.addItems(['db1', 'db2', 'db3'])
        self.database_combo.currentIndexChanged.connect(self.select_database)

        self.table_label = QLabel('选择表格：', self.central_widget)
        self.table_label.move(50, 100)
        self.table_combo = QComboBox(self.central_widget)
        self.table_combo.move(150, 100)
        self.table_combo.currentIndexChanged.connect(self.select_table)

        self.table_widget = QTableWidget(self.central_widget)
        self.table_widget.move(50, 150)
        self.table_widget.setColumnCount(3)
        self.table_widget.setHorizontalHeaderLabels(['ID', '姓名', '年龄'])

        self.import_button = QPushButton('导入数据', self.central_widget)
        self.import_button.move(50, 400)
        self.import_button.clicked.connect(self.import_data)

        self.add_button = QPushButton('添加数据', self.central_widget)
        self.add_button.move(200, 400)
        self.add_button.clicked.connect(self.add_data)

        self.modify_button = QPushButton('修改数据', self.central_widget)
        self.modify_button.move(350, 400)
        self.modify_button.clicked.connect(self.modify_data)

        self.delete_button = QPushButton('删除数据', self.central_widget)
        self.delete_button.move(500, 400)
        self.delete_button.clicked.connect(self.delete_data)

        self.init_database()

    def init_database(self):
        # 初始化数据库连接
        self.db = QSqlDatabase.addDatabase('QSQLITE')
        self.db.setDatabaseName('test.db')
        if not self.db.open():
            QMessageBox.warning(self, '错误', '无法打开数据库！')
            return

        # 加载数据库表格
        query = QSqlQuery()
        query.exec_('CREATE TABLE IF NOT EXISTS test(id INTEGER PRIMARY KEY, name TEXT, age INTEGER)')
        query.exec_('INSERT INTO test(id, name, age) VALUES(1, "张三", 20)')
        query.exec_('INSERT INTO test(id, name, age) VALUES(2, "李四", 25)')
        query.exec_('INSERT INTO test(id, name, age) VALUES(3, "王五", 30)')

        self.select_database(0)

    def select_database(self, index):
        # 选择数据库
        database_name = self.database_combo.currentText()
        self.db.setDatabaseName(database_name + '.db')
        # 重新加载数据库表格
        self.select_table(0)

    def select_table(self, index):
        # 选择表格
        table_name = self.table_combo.currentText()
        query = QSqlQuery()
        query.exec_('SELECT * FROM {}'.format(table_name))
        self.table_widget.setRowCount(0)
        while query.next():
            row = self.table_widget.rowCount()
            self.table_widget.insertRow(row)
            for i in range(3):
                item = QTableWidgetItem(str(query.value(i)))
                self.table_widget.setItem(row, i, item)

    def import_data(self):
        # 导入数据
        file_name, _ = QFileDialog.getOpenFileName(self, '选择文件', '', 'CSV Files (*.csv)')
        if file_name:
            with open(file_name, 'r') as f:
                data = f.readlines()
            query = QSqlQuery()
            for line in data:
                line = line.strip().split(',')
                query.exec_('INSERT INTO test(name, age) VALUES("{}", {})'.format(line[0], line[1]))
            self.select_table(0)

    def add_data(self):
        # 添加数据
        query = QSqlQuery()
        query.exec_('INSERT INTO test(name, age) VALUES("新用户", 0)')
        self.select_table(0)

    def modify_data(self):
        # 修改数据
        rows = self.table_widget.selectedItems()
        if len(rows) == 0:
            QMessageBox.warning(self, '错误', '请选择要修改的数据！')
            return
        row = rows[0].row()
        id = self.table_widget.item(row, 0).text()
        name = self.table_widget.item(row, 1).text()
        age = self.table_widget.item(row, 2).text()
        query = QSqlQuery()
        query.exec_('UPDATE test SET name="{}", age={} WHERE id={}'.format(name, age, id))
        self.select_table(0)

    def delete_data(self):
        # 删除数据
        rows = self.table_widget.selectedItems()
        if len(rows) == 0:
            QMessageBox.warning(self, '错误', '请选择要删除的数据！')
            return
        row = rows[0].row()
        id = self.table_widget.item(row, 0).text()
        query = QSqlQuery()
        query.exec_('DELETE FROM test WHERE id={}'.format(id))
        self.select_table(0)

if __name__ == '__main__':
    app = QApplication(sys.argv)
    login_window = LoginWindow()
    login_window.show()
    sys.exit(app.exec_())

运行以上代码，即可实现一个简单的登录界面和数据库管理系统。需要注意的是，该代码中使用了SQLite作为数据库，需要安装PyQt5和PyQt5-sqlite模块，并且需要将该代码保存为.py文件并运行。同时，还需要考虑数据的安全性、用户体验等问题。