13.8.4 USE Syntax

The USE db_name statement tell MySQL to use the db_name database as the default(current) database for subsequent statements. The database remains the default until the end of the session or another USE statement is issued:

USE db1;
SELECT COUNT(*) FROM mytable; # selects from db1.mytable
USE db2;
SELECT COUNT(*) FROM mytable; # selects from db2.mytable

Making a particular database the default by means of the USE statement does not preclude you from accessing tables in other databases. The following example accesses the author table from the db1 database and the editor table from the db2 database:

USE db1;
SELECT author\_name, editor\_name FROM author, db2.editor 
    WHERE author.editor\_id = db2.editor.editor\_id;