Syntax:
USE db_name

The USE statement tells MySQL to use the named database as the default
(current) database for subsequent statements. This statement requires
some privilege for the database or some object within it.

The named 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

The database name must be specified on a single line. Newlines in
database names are not supported.

URL: https://dev.mysql.com/doc/refman/5.7/en/use.html

