The main sources of second-hand data are

This situation includes enterprise internal information, government information, professional databases, libraries, museums and other institutions, network data and so on.

1. Internal information: internal sales data, customer data, financial data, etc.

2. Government information: government reports, laws and regulations, etc.

3. Specialized databases: economic databases and industry databases.

4. Libraries, museums and other institutions: These institutions will collect some valuable books, documents and other materials.

5. Network data: social media, forums, blogs, comments and other network channels.