LEFT JOIN two tables with multiple AND conditions - sql

Question

I need help getting a sql statement correctly by joining two tables.
My goal is to return the number of purchases between certain purchase dates for given products where customer_id is null. The foreign key for table Purchases is prospect_id corresponding to id in Prospect
In separate SQL statements, I will have this:
SELECT COUNT (id) FROM Purchases
WHERE (purchasedate BETWEEN '5/1/18' AND '12/31/18')
AND (product = 'Scooter')
SELECT id
from Prospect
where customer_id is null
So, I am coming up with a query like this:
SELECT COUNT (id)
FROM Purchases
LEFT JOIN Prospect
ON Prospect.id = Purchases.prospect_id
AND (Purchases.purchasedate BETWEEN '5/1/18' AND '12/31/18')
AND Purchases.product = 'Scooter'
AND Prospect.customer_id is null;
but then I am getting ERROR: column reference "id" is ambiguous.

Answer 1

Use count(*):
SELECT COUNT(*)
FROM Purchases pu LEFT JOIN
Prospect pr
ON pr.id = pu.prospect_id AND
pr.customer_id is null
WHERE pu.purchasedate >= '2018-05-01' AND
pu.purchasedate < '2019-01-01' AND
pu.product = 'Scooter';
I made a few changes to the query.
First, the conditions on purchase are in the where clause rather than the on clause. Presumably, you actually want these to be filters.
Second, the dates use a proper format, YYYY-MM-DD.
I've also replaced the between with explicit comparisons. This means that the code works even when the "date" column has a time component.
Finally, I also introduced table aliases.

Answer 2

the reason for your error is that you did not define what table the "id" field you want to count is coming from.
SELECT
COUNT(PURCHASES.ID) AS PURCHASE_COUNT
FROM
Purchases
LEFT JOIN Prospect
ON Prospect.id = Purchases.prospect_id
AND (Purchases.purchasedate BETWEEN '5/1/18' AND '12/31/18')
AND Purchases.product = 'Scooter'
AND Prospect.customer_id is null;