How do I select all columns in q?

By listing nothing at all in the select clause:

q)table: ([] x: 5 10 15 20; y: 1 2 3 4)
q)table
x  y
----
5  1
10 2
15 3
20 4
q)select from table where x > 10
x  y
----
15 3
20 4
q)

or SQL via s):

q)s)select * from table where x > 10
x y
—-
15 3
20 4
q)

See also: $QHOME/s.k