MySQL questions

[Billy Ng]

Sorry guys, I can't get any help from the mysql newsgroup.  That's why I
post it here.

Question 1
I have a freq field that contains '"15:0" MINUTE_SECOND', why can't I do
this?

UPDATE table SET set next_update=DATE_ADD(NOW(), INTERVAL table.freq);

Question2
How can I INSERT a record into the table if the table does not have any
record with a provided id in the id field?  Do I need to join a SELECT
statement?

Question3
Can I query from a query in MySQL?

Thanks in advance!

Billy Ng