Question

I have the following code below and I’m trying to get an AVG for the 3 columns.

 SELECT 
    (SUM(score) * .3) As score_a,
    (SUM(score) * .6) As score_b,
    (SUM(score) * .8) As score_c

     --now I want to get the AVG of the above scores
    AVG(score_a + score_b + score_c) As avg_score
 FROM score_table

but this does not work. The error I’m getting is “Invalid column name score_a”.
I’m using SQL Server 2008

Answer 1

AVG() is an aggregate function that takes values from multiple rows and gives their average.

You’re trying to average 3 columns.

I your specific case, it can be done using algebra.

SELECT 
  (SUM(score) * .3)                       As score_a,
  (SUM(score) * .6)                       As score_b,
  (SUM(score) * .8)                       As score_c,
  (SUM(score) * ((0.3 + 0.6 + .8) / 3.0)) As score_avg
FROM
  score_table

In a more generalised case, you’re limited by the fact that you can’t reference a column you just defined in another column…

SELECT
  a + 1      AS inc_a,
  inc_a * 2  AS this_is_invalid
FROM
  your_table

You’d either need to repeat yourself, or use a sub query…

Repetition

SELECT 
  (SUM(score) * .3)                                                 As score_a,
  (SUM(score) * .6)                                                 As score_b,
  (SUM(score) * .8)                                                 As score_c,
  ((SUM(score) * .3) + (SUM(score) * .3) + (SUM(score) * .3)) / 3.0 As score_avg
FROM
  score_table

Sub Query

SELECT
  score_a,
  score_b,
  score_c,
  (score_a + score_b + score_c) / 3.0 AS score_avg
FROM
(
  SELECT 
    (SUM(score) * .3)                                               As score_a,
    (SUM(score) * .6)                                               As score_b,
    (SUM(score) * .8)                                               As score_c
  FROM
    score_table
)
  AS data