Syntax
------ 
DEFAULT(col_name)
 
Description
----------- 
Returns the default value for a table column. If the column
has no default value, NULL is returned.
For integer columns using AUTO_INCREMENT, 0 is returned.
 
When using DEFAULT as a value to set in an INSERT or UPDATE
statement, you can use the bare keyword DEFAULT without the
parentheses and argument to
refer to the column in context. You can only use DEFAULT as
a bare keyword if you are using it
alone without a surrounding expression or function.
 
Examples
-------- 
Select only non-default values for a column:
 
SELECT i FROM t WHERE i != DEFAULT(i);
 
Update values to be one greater than the default value:
 
UPDATE t SET i = DEFAULT(i)+1 WHERE i 

URL: https://mariadb.com/kb/en/default/