postgresql Subselect Aggregate in larger query - postgresql

Question

I'm working with a gigantic dataset of individuals with demographic information and action tracking. I am trying to get the percentage of people who committed an action, which is simple, but also trying to get average ages of people who fit in a specific subgroup of the original SELECT. The CASE WHEN line works fine alone, and the subquery runs fine in it's own query but I cannot seem to get it integrated into this query as a subquery, it gives me a syntax error on the CASE WHEN statement. Here's a slightly anonymized version of the query. Any help would be VERY appreciated.
SELECT
AVG(ageagg)
FROM
(
SELECT
age AS ageagg
FROM
agetable
WHERE
age>30
AND action_taken=1) AvgAge_30Action,
COUNT(
CASE
WHEN action_taken=1
AND age> 30
THEN 1
ELSE 0 NULL) / COUNT(
CASE
WHEN age>30) AS Over_30_Action
FROM
agetable
WHERE
website_type=3

Answer 1

If I've interpreted your intent correctly, you wish to compute the following:
1) the number of people over the age of 30 that took a specific action as a percentage of the total number of people over the age of 30
2) the average age of the people over the age of 30 that took a specific action
Assuming my interpretation is correct, this query might work for you:
SELECT
100 * over_30_action / over_30_total AS percentage_of_over_30_took_action,
average_age_of_over_30_took_action
FROM (
SELECT
SUM(CASE WHEN action_taken=1 THEN 1 ELSE 0 END) AS over_30_action,
COUNT(*) AS over_30_total,
AVG(CASE WHEN action_taken=1 THEN age ELSE NULL END)
AS average_age_of_over_30_took_action
FROM agetable
WHERE website_type=3 AND age>30
) aggregated;
I created a dummy table and populated it with the following data.
postgres=# select * from agetable order by website_type, action_taken, age;
age | action_taken | website_type
-----+--------------+--------------
33 | 1 | 1
32 | 1 | 2
28 | 1 | 3
29 | 1 | 3
32 | 1 | 3
33 | 1 | 3
34 | 1 | 3
32 | 2 | 3
32 | 3 | 3
33 | 4 | 3
34 | 5 | 3
33 | 6 | 3
34 | 7 | 3
35 | 8 | 3
(14 rows)
Of the 14 rows, 4 rows (the first four in this listing) have either the wrong website_type or have age below 30. Of the ten remaining rows, you can see that 3 of them have an action_taken of 1. So, the query should determine that 30% of folks over the age of 30 took a particular action, and the average age among that particular population should be 33 (ages 32, 33, and 34). The results of the query I posted:
percentage_of_over_30_took_action | average_age_of_over_30_took_action
-----------------------------------+------------------------------------
30 | 33.0000000000000000
(1 row)
Again, all of this is predicated upon my interpretation of your intent actually being accurate. This is of course based on a highly contrived data set, but hopefully it's enough of a functional signpost to get you on the right path.