Ms Sql Server Express Portable -

Zero configuration on the host machine; no registry keys or leftover files. 3. SQLite (The "True" Portable)

If you find tutorials suggesting User Instances today, they are obsolete. Do not use them for new projects. ms sql server express portable

If you need SQL Server features without the overhead of a full service, is the closest official solution. Zero configuration on the host machine; no registry

Use LocalDB with a one-line PowerShell installer script. Script: Zero configuration on the host machine

If you need a truly portable environment that doesn't mess with the host OS, use Docker.