BOL citation:

FOR ATTACH [ WITH < attach_database_option > ]

Specifies that the database is created by attaching an existing set of operating system files. There must be a <filespec> entry that specifies the primary file. The only other <filespec> entries required are those for any files that have a different path from when the database was first created or last attached. A <filespec> entry must be specified for these files.

FOR ATTACH requires the following:

All data files (MDF and NDF) must be available.

If multiple log files exist, they must all be available.

[highlight=#ffff11] If a read/write database has a single log file that is currently unavailable, and if the database was shut down with no users or open transactions before the attach operation, FOR ATTACH automatically rebuilds the log file and updates the primary file. [/highlight]In contrast, for a read-only database, the log cannot be rebuilt because the primary file cannot be updated. Therefore, when you attach a read-only database with a log that is unavailable, you must provide the log files, or the files in the FOR ATTACH clause.