System Variables vs Local Variables in MySQL

The local variable has the scope for only a set of statements or block of statement. Whenever a set of statements or block of statement has completed then local variable goes out of scope.

For Example

Local variable can be used in stored procedure, function etc. It is used with the DECLARE keyword.

The syntax is as follows to local variables.

DECLARE yourVariableName dataType;

The global variables or system variables has the scope across connections until server restart. It is set using GLOBAL keyword. The syntax is as follows −

SET GLOBAL max_connections=value;
OR
SET @@global.max_connections=value;

If you know port number then you can use system variable @@port using select statement. The query is as follows −

mysql> select @@port;

Output

The following is the output −

+--------+
| @@port |
+--------+
| 3306 |
+--------+
1 row in set (0.00 sec)