Try this:
echo "$a" | awk -F"CREATE.*TABLE" '{print $2}'
as (uid, cid, mid)
Not sure if regex in Field Separator do work.
Using variable Field Separators:
sep=two
echo "_one_two_three_four" | awk -v var=$sep '{split($0,a,"_"var);print a[2]}'
_three_four
sep=three
echo "_one_two_three_four" | awk -v var=$sep '{split($0,a,"_"var);print a[2]}'
_four