Joining onto NULL if Record does not exist SQL - sql

Question

I have a query that selects customers from a table CustomerDetails, and left joins onto another table (CustomerActivity) to get their last login time and finally left joins onto another table (OpenOrderDetails) to get their last open order (if applicable). I also have a big WHERE clause filtering this data
A customer can only have one record in the OpenOrderDetails table at anytime. My query looks like the following:
SELECT CD.*, H.LastCustomerLoginTime, OD.OrderFiledDate, OD.OrderCompletedDate
FROM CustomerDetails CD
LEFT JOIN CustomerActivity H ON H.CustomerID = CD.CustomerID
LEFT JOIN OpenOrderDetails OD ON CD.CustomerID = OD.CustomerID
WHERE CD.OrderStatus IN (1,2,3)
AND (CustomerType = 1 or (CustomerType = 3 and CustomerActive IN (1,2)))
AND (OD.OrderFiledDate IS NULL OR CD.TimeStamp >= OD.OrderFiledDate)
AND (OD.OrderCompletedDate IS NULL OR CD.TimeStamp <= OD.OrderCompletedDate)
My issue is that this query only returns customer records that have a record in the OpenOrderDetails table. How do I return every customer, and OrderFiledDate/OrderCompletedDate if present, and NULL if a record for that customer does not exist in the OpenOrderDetails table?

Answer 1

When doing a LEFT JOIN, referencing ANY of the right side table columns in the WHERE clause will turn it into an INNER JOIN. To get around this, simply remove any and all predicates that reference "right side" columns from the WHERE clause and move them to the LEFT JOIN condition.
Something along these lines...
SELECT
CD.*,
H.LastCustomerLoginTime,
OD.OrderFiledDate,
OD.OrderCompletedDate
FROM
CustomerDetails CD
LEFT JOIN CustomerActivity H
ON H.CustomerID = CD.CustomerID
LEFT JOIN OpenOrderDetails OD
ON CD.CustomerID = DLECS.CustomerID
AND ( OD.OrderFiledDate IS NULL
OR CD.TimeStamp >= OD.OrderFiledDate
)
AND ( OD.OrderCompletedDate IS NULL
OR CD.TimeStamp <= OD.OrderCompletedDate
)
WHERE
CD.OrderStatus IN ( 1, 2, 3 )
AND (
CustomerType = 1
OR
(
CustomerType = 3
AND CustomerActive IN ( 1, 2 )
)
);

Answer 2

Just move some OpenOrderDetails conditions to join clause
SELECT CD.*, H.LastCustomerLoginTime, OD.OrderFiledDate, OD.OrderCompletedDate
FROM CustomerDetails CD
LEFT JOIN CustomerActivity H ON H.CustomerID = CD.CustomerID
LEFT JOIN OpenOrderDetails OD ON CD.CustomerID = DLECS.CustomerID
AND (OD.OrderFiledDate IS NULL OR CD.TimeStamp >= OD.OrderFiledDate)
AND (OD.OrderCompletedDate IS NULL OR CD.TimeStamp <= OD.OrderCompletedDate)
WHERE CD.OrderStatus IN (1,2,3)
AND (CustomerType = 1 or (CustomerType = 3 and CustomerActive IN (1,2)))