Let me first describe the tables I'm working with:

table1 with these columns:
user_id cat_id more_data even_more_data

table
cat_id

table1 may have one row for every cat_id in table2 which may be as many
as 20

I want to make sure there is a row in table1 for every cat_id, such that
every user_id has 20 rows in table.

I'm trying to use an insert with a select to accomplish this. Is this
even possible? Here is my query all comments appreciated.

INSERT INTO table1
(cat_id)
SELECT cat_id
FROM table2
WHERE not exists
(select cat_id from table1
where table1.user_id = 9999
)
where table1.user_id = 9999

The way I read this like this:
insert into table1's cat_id column all cat_id's that exist in table2 but
not in table2 for a given user_id. But this query fails with a not
properly ended error.