Database migration issues – Error #1046 No database selected

99.99% chance that you configure wordpress with wrong database details.

make sure you are using correct database details in wp-config.php file

/** The name of the database for WordPress */
define('DB_NAME', 'database_name_here');

/** MySQL database username */
define('DB_USER', 'username_here');

/** MySQL database password */
define('DB_PASSWORD', 'password_here');

/** MySQL hostname */
define('DB_HOST', 'localhost');

some possibilities are listed here based on my experience:

1. May be you didn’t use any prefix with database name or user name, which are added by default when we create them on online server.

2. May be using wrong server details. mostly we use localhost but sometimes
   database server may be use from outside.

3. May be user not assign to database.

Hope this helps you. All the best 😉