CASE AND WHEN SQL - sql

Question

I have transactional data of customers' purchase. I tried to select customer_id from the last 1 month and calculate recency as the average day customers come to purchase (AVG(gap))
SELECT
customer_id,
(
CASE WHEN day::DATE<= '2015-05-01'::DATE AND day::DATE > '2015-05-01'::DATE - INTERVAL '1 month'
THEN
(
SELECT
AVG(gap)
FROM
(
SELECT
customer_id,
( day- LAG(day) OVER ( PARTITION BY customer_id ORDER BY day ) ) AS gap
FROM
baskets
JOIN
basket_lines
USING
( basket_id )
GROUP BY 1
) a
) b
ELSE 0
) AS A
FROM
baskets
JOIN
basket_lines
USING
(basket_id)
GROUP BY
1;
However, I have an error like `
ERROR: syntax error at or near "b"
LINE 45: GROUP BY 1)a)b ELSE 0) AS A
^
Does it mean I can not use subquery after THEN statement?

Answer 1

A subquery in the THEN clause does not take an alias. Also, you must end your CASE expression with END:
SELECT
customer_id,
(CASE WHEN day::DATE<= '2015-05-01'::DATE AND
day::DATE > '2015-05-01'::DATE - INTERVAL '1 month'
THEN
(SELECT AVG(gap) FROM (
SELECT customer_id,
(day- LAG(day) OVER (PARTITION BY customer_id ORDER BY day)) as gap
FROM baskets
JOIN basket_lines
USING (basket_id)
GROUP BY 1) a) ELSE 0 END) AS A
FROM baskets
JOIN basket_lines
USING (basket_id)
GROUP BY 1;
But you have a correlated subquery in your select statement. This is probably not optimal, and we can likely rewrite your query using a join.
I propose the following refactor:
WITH cte AS (
SELECT
customer_id,
(day- LAG(day) OVER (PARTITION BY customer_id ORDER BY day)) as gap
FROM baskets
INNER JOIN basket_lines
USING (basket_id)
WHERE day::DATE<= '2015-05-01'::DATE AND
day::DATE > '2015-05-01'::DATE - INTERVAL '1 month'
)
SELECT
customer_id,
AVG(gap) AS cust_avg
FROM cte
GROUP BY
customer_id;