MySQL columns with DEFAULT NULL – stylistic choice, or is it?

As documented under Data Type Default Values:

If the column can take NULL as a value, the column is defined with an explicit DEFAULT NULL clause.

(I think they meant implicit, not explicit).

Moreover, as documented under CREATE TABLE Syntax:

If neither NULL nor NOT NULL is specified, the column is treated as though NULL had been specified.

Therefore, in MySQL the following column definitions are all identical:

columnname type
columnname type NULL
columnname type DEFAULT NULL
columnname type NULL DEFAULT NULL

The choice of which to use is a balance between being explicit, and being concise. Depending on the circumstances, I might use any of the above.