MYSQL set a default database - mysql

Question

When I connect my MYSQL , I have to choose database to use everytime using :
use mysql_crashcourse;
I noticed the DATABASE() returns null when I check it right after I connect to MYSQL
mysql> SELECT DATABASE();
+------------+
| DATABASE() |
+------------+
| NULL |
+------------+
My question is : how can I set a default MYSQL database ,so that I don't need to provide database info each time I connect or login to MYSQL ?

Answer 1

If you log-in directly from cli.
Create a client session on my.cnf and restart mysql service.
Note every root user logged from localhost will have the default database.
Example:
root#ergesttstsrv:~# cat /etc/mysql/my.cnf
[client]
host=localhost
user=root
password=
database=gesti
!includedir /etc/mysql/conf.d/
!includedir /etc/mysql/mysql.conf.d/
root#ergesttstsrv:~# service mysql restart
root#ergesttstsrv:~# mysql -u root -p
Enter password:
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 9
Server version: 8.0.25 MySQL Community Server - GPL
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Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql> select database();
+------------+
| database() |
+------------+
| gesti |
+------------+
1 row in set (0.00 sec)

Answer 2

If you do not select anything else in the query, the return value is null. If you want to get anything else, you can do for example:
mysql> SELECT 1 IS NULL, 1 IS NOT "something";