问题
When I want to select from table Y in database X I can use
select * from [X].[dbo].[Y]
or
USE X
select * from [Y]
Is there any reason to prefer one over the other?
回答1:
dbo
Using dbo as the owner of all the database objects can simplify managing the objects. You will always have a dbo user in the database. Users in the database will be able to access any object owned by dbo without specifying the owner as long as the user has appropriate permission.
USE X
When a SQL Server login connects to SQL Server, the login is automatically connected to its default database and acquires the security context of a database user. If no database user has been created for the SQL Server login, the login connects as guest. If the database user does not have CONNECT permission on the database, the USE statement will fail. If no default database has been assigned to the login, its default database will be set to master.
Understanding the Difference between Owners and Schemas in SQL Server
USE (Transact-SQL)
回答2:
USE X will change the context to X and all the following statements will execute under the context X.
But X.dbo.Y will access the object Y without changing the current context.
Eg: Let us consider there is two databases DB1 and DB2. DB1 contains table T1 & T2 and DB2 contains tables U1 & U2.
Now,
USE DB1 -- here context set to DB1
select * from T1 -- works fine
select * from U1 -- gives error, because U1 is not in current context
select * from DB2.dbo.U1 -- works fine, because it access the context DB2 from current content context DB1
select * from T2 -- works fine
USE DB2 -- here context changed to DB2
select * from U2 -- works fine
select * from T1 -- gives error, because T1 is not in current context
select * from DB1.dbo.T1 -- works fine, because it access the context DB1 from current content context DB2
回答3:
I'd tend to use [server].[database].[schema].[table] in instances where a script may query mutliple tables from multiple databases.
The USE [database] would typically be used in scenarios where all statements were to apply to the same database and you needed to make sure they were applied to the correct database. Have you ever connected to a server and run a script only to find you ran it on the master database?
回答4:
by using first Query you can perform that selection from other databases.In the same window you can have the selection for other data beses also.
But By using second selection, From the same window you can have selection for that(USE X) databse only.
回答5:
Sometimes you want the schema and database to be dictated by the login, and in this case you should simply use the object name. That's one reason to not fully qualify them.
来源：https://stackoverflow.com/questions/21844807/using-use-keyword-vs-full-table-name-in-t-sql