Re-use aliased field in SQL SELECT statement

Question

I\'d like to achieve something like this:

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

Assuming that by \"...\" I\

Answer 1

A workaroud would be to use a sub-query:

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

---

To improve readability you could also use a CTE:

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

Answer 2

When I have complicated logic to compute a "virtual" column value from other column values in a table I generally create a single-table view of the original table with all the original columns plus the computed values as well. Then I do other SELECTs against the view. That allows me:

1. To name my computed columns.

2. To keep the logic for the computations in one place instead of scattered through various queries in the application.