Query returns no records - sql

Question

Hello i am making schema for purchasing orders these orders can be ordered by certain user and then received by another user.
so i created the below schema.
Schema
The issue is when UserID column in PurchaseOrders and Deliveries tables has different values the query returns no records.
Query
SELECT
dbo.Users.FirstName,
dbo.Users.LastName,
dbo.PurchaseOrders.PurchaseOrderDate,
dbo.Deliveries.ExpectedDeliveryDate,
dbo.Deliveries.ActualDeliveryDate
FROM dbo.PurchaseOrders
INNER JOIN dbo.Users
ON dbo.PurchaseOrders.UserID = dbo.Users.UserID
INNER JOIN dbo.Deliveries
ON dbo.PurchaseOrders.PurchaseOrderID = dbo.Deliveries.PurchaseOrderID
AND dbo.Users.UserID = dbo.PurchaseOrders.UserID
AND dbo.Users.UserID = dbo.Deliveries.UserID

Answer 1

You need two different joins to Users. You also need to learn to use table aliases:
SELECT pu.FirstName as purchase_FirstName, pu.LastName as purchase_LastName,
du.FirstName as delivery_FirstName, du.LastName as delivery_LastName,
po.PurchaseOrderDate,
d.ExpectedDeliveryDate, d.ActualDeliveryDate
FROM dbo.PurchaseOrders po JOIN
dbo.Deliveries d
ON po.PurchaseOrderID = d.PurchaseOrderID JOIN
dbo.Users pu
ON p.UserID = pu.UserID JOIN
dbo.Users du
ON d.UserId = du.UserId;
Your query returns no records because of this condition:
AND dbo.Users.UserID = dbo.PurchaseOrders.UserID
AND dbo.Users.UserID = dbo.Deliveries.UserID
This obviously means that dbo.PurchaseOrders.UserID = dbo.Deliveries.UserID. So, if this is not true, then no records match the condition.

Answer 2

When you use Inner Join, if the data doesnt match, you will not get any records. You need to use LEFT JOIN and also the second join doesnt need condition with user id. Try the below query.
SELECT
dbo.Users.FirstName,
dbo.Users.LastName,
dbo.PurchaseOrders.PurchaseOrderDate,
dbo.Deliveries.ExpectedDeliveryDate,
dbo.Deliveries.ActualDeliveryDate
FROM dbo.PurchaseOrders
INNER JOIN dbo.Users
ON dbo.PurchaseOrders.UserID = dbo.Users.UserID
LEFT JOIN dbo.Deliveries
ON dbo.PurchaseOrders.PurchaseOrderID = dbo.Deliveries.PurchaseOrderID