How to change timezone?

-> Determine your timezone
# cd /usr/share/zoneinfo
# tzselect
e.g.
Europe/Istanbul

-> Remove current localtime soft link
# rm /etc/localtime 

-> Define your localtime soft link as new
# ln -s /usr/share/zoneinfo/Europe/Istanbul /etc/localtime

-> Check your latest timezone setting
# timedatectl

How to install/configure/enable NTP service?

-> Install ntp service
# yum install ntp

-> Once NTP server pool zones are taking
e.g. check from https://www.pool.ntp.org/zone/europe

-> Add specific pool zones into config file if they are not existed
# vi /etc/ntp.conf
server 0.europe.pool.ntp.org

-> Enable service
# systemctl enable ntpd

-> Run service
# systemctl start ntpd 

-> Lastly, check the service and the settings

# timedatectl 

SQL PLUS system parameters usage (set/show)

To show parameter in detail
SQL> SHOW DEFINE;

To escape from substitution variable (&):
SQL> SET DEFINE OFF;

To enable DBMS_OUTPUT:
SQL> SET SERVEROUTPUT ON;

To see old and new values of variables:
SQL> SET VERIFY ON;

To calculate total execution time of SQL:
SQL> SET TIMING ON;

To print time on the SQL terminal:
SQL> SET TIME ON;

To see table columns name and types for a table:
SQL> DESC hr.employees@XE;

To see currently logged-in user:
SQL> SHOW user;