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How to use it
Name the table, add its columns and tick the options for each. The preview shows the table as MySQL would describe it, and the files below update as you type.
Questions
What is db_schema.xml?
It is the declarative schema file Magento 2.3 and later use to create and change database tables, in place of install and upgrade scripts.
Do I need db_schema_whitelist.json?
Yes, if you want Magento to be able to drop or change columns later. The tool writes one, but regenerate it with bin/magento setup:db-declaration:generate-whitelist after you install the module, because Magento shortens constraint names longer than 64 characters.
Is the SQL preview what Magento runs?
It is an approximation to help you check the structure. Magento builds its own statements from db_schema.xml.