To link users across different authentication providers (email/password and Google SSO) in Next.js, you can implement a strategy to associate the same user account with both login methods. Here’s a basic outline of the approach:

1. Identify the user by email: When a user logs in via Google SSO, you can check if an account with the same email exists in your database (from email/password registration).
2. Link the accounts: If the user is already registered via email/password, update the existing record to link the Google SSO account, typically by storing the Google ID or other unique identifier in the user record.
3. Use a session or JWT: After successful login, use the session or JWT token to track which authentication method was used, and ensure that the user is authenticated properly across both login methods.

Strategy:

• Database model: Ensure your user model supports multiple authentication methods (e.g., a field for googleId, email, and passwordHash).
• Authentication flow: When a user signs in with Google, check if an account with their email exists. If it does, associate the Google login with their existing account; if not, create a new user.
• Libraries: Use a library like NextAuth.js that supports multiple authentication providers and can help handle account linking and management.