Genbrug aliaseret felt i SQL SELECT-sætning

En workaroud ville være at bruge en underforespørgsel:

SELECT
  FieldA,
  FieldA + 20 AS FieldB
FROM (
  SELECT 
    (CASE WHEN ...) AS FieldA
  FROM Tbl
) t

For at forbedre læsbarheden kan du også bruge en CTE :

WITH t AS (
  SELECT 
    (CASE WHEN ...) AS FieldA
  FROM Tbl
)
SELECT
  FieldA,
  FieldA + 20 AS FieldB
FROM
  t