MySQL 8.0

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SELECT @@global.time_zone, @@session.time_zone;

@@global.time_zone
@@session.time_zone

SYSTEM
SYSTEM

SET @@session.time_zone='+00:00';
SELECT @@global.time_zone, @@session.time_zone;

@@global.time_zone
@@session.time_zone

SYSTEM
+00:00