Is there a query to set a table's engine to innodb if it's not already?

Question

I can set a (MyISAM) table's engine to InnoDB using the query:

alter table tablename engine=InnoDB

which takes some time to complete for large tables. It appears though that if I run the same query again on the same table, it takes some time again (much less but still considerable). I would expect the query to be instantaneous, since the engine is already set to innodb.

My questions are:

• Is there a single query that conditionally sets the engine, for example

  alter table tablename engine=InnoDB <if not already innodb>

• Why does the second query have such a delay? (out of curiosity mostly)