WITH t AS (SELECT a FROM t1 WHERE b >= 'c') SELECT * FROM t2,t WHERE t2.c=t.a;

SELECT t1.a,t1.b FROM t1,t2
  WHERE t1.a>t2.c AND
        t2.c in (WITH t as (SELECT * FROM t1 WHERE t1.a<5)
                   SELECT t2.c FROM t2,t WHERE t2.c=t.a);

WITH engineers AS (
  SELECT * FROM employees WHERE dept IN ('Development','Support')
)
SELECT * FROM engineers E1
  WHERE NOT EXISTS (SELECT 1
    FROM engineers E2
    WHERE E2.country=E1.country
    AND E2.name <> E1.name);