Why can't we specify group functions in Oracle in the where clause?
AVG and other aggregate functions work on sets of data.  The WHERE cause does not have access to the entire set, only to data for the row it is operating on.  If you created your own AVG function (as a normal function and not a custom aggregate function) it would only be passed one ID value when called from the WHERE clause not the entire set of ID values.

Mezmo's solution will give you your expected results, but if you want to avoid two full table scans (assuming no indexes) you can use a windowing function like this:

SELECT * FROM (SELECT AVG(id) OVER () avgid, t.* FROM tab1 t) WHERE id &gt; avgid;