SQL Server database restricts single-user use and release of single-user use

A database in single-user (Single_user) mode can have only one connection at a time. Databases in restricted User (Restricted_user) mode can only accept connections that are considered "eligible" users--these users belong to the dbcreator or sysadmin server role, or are members of the db_owner role for that database.
The database is in multi-user mode (Multiple_user) By default, which means that all users with a valid user name in a database can connect to the database.

It is generally necessary to limit the database to single-user mode during the database maintenance phase. You can use the following statement to make changes. Set to single user mode, the database is flagged as "single user"

ALTER database name SET single_user;

Sometimes when a database restore error occurs, the database is also set to single-user mode, resulting in the application cannot connect to the database, in this case the user access mode to more users "multi_user" to restore access.

ALTER database name SET multi_user;

SQL Server database restricts single-user use and release of single-user use