SELECT a, b
FROM (
  SELECT a, MAX(b) AS b FROM t GROUP BY a
) AS x;

WITH x AS (SELECT a, MAX(b) AS b FROM t GROUP BY a)
SELECT a, b FROM x;

WITH
  t1 AS (SELECT a, MAX(b) AS b FROM x GROUP BY a),
  t2 AS (SELECT a, AVG(d) AS d FROM y GROUP BY a)
SELECT t1.*, t2.*
FROM t1
JOIN t2 ON t1.a = t2.a;

WITH
  x AS (SELECT a FROM t),
  y AS (SELECT a AS b FROM x),
  z AS (SELECT b AS c FROM y)
SELECT c FROM z;

WITH RECURSIVE t(n) AS (
    VALUES (1)
    UNION ALL
    SELECT n + 1 FROM t WHERE n < 4
)
SELECT sum(n) FROM t;