I am reading the MySQL documentation on this page: http://dev.mysql.com/doc/refman/5.1/en/set-statement.html

It often uses "@@", but does not define what "@@" means.

Another example is in variable names:

mysql> select @@hostname;
+------------+
| @@hostname |
+------------+
| server1    |
+------------+
1 row in set (0.00 sec)

mysql> select @hostname;
+-----------+
| @hostname |
+-----------+
| NULL      |
+-----------+
1 row in set (0.00 sec)

What is @ vs @@?

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