Question

I’m a T-SQl and database newbie, and a little confused.
In the T-SQL book I’m reading it says that a USE statement is written to set the database context of the session. Does that mean that there can be more than one database in an instance of SQL-server and the USE statement tells SQL server which database the query will be on?

Answer 1

It’s not quite as simple as that. Yes, a server can have multiple databases, but I would caution against using a USE statement unless all of your queries are that way.

Every connection to SQL server has a “current database”. That means that any query run over that connection will use that database (though it’s possible to access information from other databases by explicitly indicating this with dot notation). The USE statement changes the current database for the connection, so if one statement expects to run in the MyDatabase database (but doesn’t specify it with a USE MyDatabase statement) and another query executes Use OtherDatabase, then the first query will fail when run.

For instance:

• Connect to SQL Server to the MyDatabase database
• Execute the following query:

select * from sometable

• Execute the next query:

  use OtherDatabase

  select * from othertable

If we try to execute the first query again it will fail (assuming that sometable doesn’t also exist in OtherDatabase) because we’ve changed the context of the connection with the USE statement. The upshot here is that the USE statement is connection-wide, not specific to an individual query.

In software development, the more common approach is to maintain a connection to the server for each database you plan to query against. That means that the query against MyDatabase will use a different connection than the one used by the query for OtherDatabase. This helps mitigate the possibility of a change in query context causing queries to fail.