WHMCS uses a MySQL® database to store all of your business data, including client and financial records. For successful installation, updates, and operation, your system must meet the minimum system requirements for your WHMCS version. Troubleshoot and resolve Argument 1 passed must be an instance of PDOStatement errors. Troubleshoot and resolve Base table or view not found errors. Troubleshoot and resolve Column 'group_id' cannot be null errors. Troubleshoot and resolve MySQL Error Code 1142 - Command denied to user errors. Troubleshoot and resolve Could not connect to database errors. Troubleshoot and resolve issues with the cron job becoming stuck while executing the system queue. We strongly recommend backing up your WHMCS installation's database regularly. If a catastrophic system failure occurs, restoring a recent database backup can ensure the safety and security of your data. You can monitor and manage your WHMCS installation's database, including optimizing database tables and manually downloading database backups, from within the WHMCS Admin Area. Troubleshoot and resolve InvalidArgumentException: Query string must not include a URI fragment errors. Troubleshoot and resolve Oops! InvalidDomain: The domain is invalid errors while managing domains in the Admin Area. Troubleshoot and resolve Down for Maintenance errors. Troubleshoot and resolve Duplicate entry '0' for key 'PRIMARY' errors. Troubleshoot and resolve 1062 Duplicate entry for key tblconfiguration.PRIMARY errors. Troubleshoot and resolve Field xxx doesn't have a default value errors. Models offer a code-driven method for interacting with the database. Many WHMCS database tables are available as modeled classes. Troubleshoot and resolve MySQL server has gone away errors. Troubleshoot and resolve Order Not Found...Exiting... errors. Troubleshoot and resolve General error: 3988 Conversion from collation utf8mb3_unicode_ci into latin1_swedish_ci impossible for parameter errors.