The database principal owns a schema in the database, and cannot be dropped (Error: 15138)

On a recent audit of a database I noticed that there were a lot of logins for a database that were not required.  I set about removing the logins one by one, but soon hit the following error:

Error: 15138 The database principal owns a schema in the database, and cannot be dropped.

Well this threw me a bit, and I could not figure out what was going on and why, this one particular login could not be removed.  As the message indicates, this account owns a schema on the DB. So I opened the user properties from SSMS and sure enough, the user  in question owned three schemas: db_datareader, db_datawriter, db_owner.  However, SSMS will not simply allow you to unselect the checkboxes here, so it is still not so easy to remove this link and allow the user to be deleted!

The schemas owned by the user can also be displayed by running the following query, within the context of the database under scrutiny:

SELECT s.name FROM sys.schemas s
WHERE s.principal_id = USER_ID('UserInQuestion'');

For a more complete picture of all schemas and users associated with the database:

SELECT *, USER_NAME(principal_id) as username
FROM sys.schemas;


In order to remove this user we have to first transfer ownership of these schemas to a different user.  Typically this would be dbo.  This has to be done once for each schema owned, using the ALTER AUTHORISATION ON SCHEMA command:

 ALTER AUTHORISATION ON SCHEMA::db_datareader TO dbo
 ALTER AUTHORISATION ON SCHEMA::db_datawriter TO dbo
 ALTER AUTHORISATION ON SCHEMA::db_owner TO dbo

(note the double colon!)

Re-run the previous scripts to ensure that the user in question no longer owns any schemas.  Now if we again try the delete again, this user will be removed successfully.