You could use COALESCE just as well on INSERT. Something like:

INSERT INTO table (id,type) VALUES (
 id,
 COALESCE(?,defaulttypevalue)
)
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Well, in general if a field is given the NULL value (which will be inserted if the given value for a placeholder is undef), then the database decides what to do with it. Usually it will just be marked as NULL in the database (as in: not having any value). Sometimes you can specify a default value when you create the table. Sometimes something special happens (e.g. with a TIMESTAMP field in MySQL).

Liz