组复制的状态变量只有一个，目前在8.0.17这个版本上还是存在的，未来会被废弃掉，我们在单主的模式下，可以用来查看哪个节点是读写节点。
mysql: [Warning] Using a password on the command line interface can be insecure.
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Server version: 8.0.17 MySQL Community Server - GPL
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Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql> show status like 'group_replication_primary_member';
+----------------------------------+--------------------------------------+
| Variable_name | Value |
+----------------------------------+--------------------------------------+
| group_replication_primary_member | 1fb894d8-e752-11e9-a0a1-00505687032a |
+----------------------------------+--------------------------------------+
1 row in set (0.00 sec)
group_replication_primary_member
Shows the primary member's UUID when the group is operating in single-primary mode. If the group is operating in multi-primary mode, shows an empty string.
The group_replication_primary_member status variable has been deprecated and is scheduled to be removed in a future version.
