Microsoft SQL Server Express does not have an official "portable" version from Microsoft. By design, SQL Server is a database engine that requires a formal installation to register system services, manage security accounts, and hook into the Windows OS.
For developers, the combination of LocalDB + a USB-batched attach/detach workflow is the gold standard. It respects Windows security, prevents data corruption, and requires only a one-time installation of a lightweight Microsoft tool on each host machine. ms sql server express portable
, but LocalDB gives you: