Nested Query to CTE in 4 Steps

Before

SELECT a.x FROM
  (SELECT b.x FROM
    (SELECT x, y FROM t WHERE y > 0) b
   JOIN u ON b.id = u.id) a
WHERE a.x > 10

Aliases a, b, c. The interviewer stopped reading at line 12.

After

WITH filtered AS (
  SELECT x, id FROM t WHERE y > 0
),
joined AS (
  SELECT f.x FROM filtered f JOIN u ON f.id = u.id
)
SELECT x FROM joined WHERE x > 10;

The 4 steps

1. Name each level by what it IS, not a/b/c.
2. One CTE per business step: filter first, aggregate second, join last.
3. Keep the final SELECT flat and obvious.
4. Test each CTE standalone — debugging becomes binary search.

Interviewers grade "could I maintain this at 2am," not cleverness. Practice refactors at Topfolio Practice.

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