Show the default value for a variable

In MySQL 5.7 you can use performance_schema to get the variables.

Before modifying or set you can select the variable to see the default value then modify.

Method #1

SELECT 
    VARIABLE_VALUE
FROM
    performance_schema.global_variables
WHERE
    VARIABLE_NAME = 'max_connections';

Output #1

| VARIABLE_VALUE |
| :------------- |
| 151            |

Method #2

If you are not sure the exact name of variable use like, it can used used also in above query too.

SHOW GLOBAL VARIABLES LIKE 'max_connect%';

Output #2

Variable_name      | Value
:----------------- | :----
max_connect_errors | 100  
max_connections    | 151  

Method #3

SELECT @@GLOBAL.max_connections;

Output #3

| @@GLOBAL.max_connections |
| -----------------------: |
|                      151 |

Refer here db-fiddle

Note: If you need to have a history kind of thing then you need to create a table to store those values before changing.

P.S. There is one more type of variables session. By replacing global to session those variables can be changed but it will affect only to the current session.

credits: @scaisedge, @richard

Could be selecting from information_schema.GLOBAL_STATUS

select VARIABLE_VALUE 
from information_schema.GLOBAL_STATUS 
where VARIABLE_NAME = 'max_connections';

This might not be ideal but if it were me trying to solve the problem I'd create my own table with all of the initial values and reference that when I needed.

CREATE TABLE 
   default_variables
SELECT 
   * 
FROM 
   information_schema.GLOBAL_STATUS;

This would require you to spin up a new DB installation that is the same version as the one you're currently using, but I think you could use this method to solve your problem.

Simply export the data and import it wherever you need it.

I don't currently see any built-in way to query the defaults.