SQL SERVER - Left Join does not work. Because? [closed] - sql-server

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This works perfectly in MS Access.
Why not in MS SQL Server?
Can you help me solve it?
Here's how my query is
select *
from tblPROestoque
where idproduto = 8183
order by identrada desc
select top(1) *
from tblPROestoque
where idproduto = 8183
order by identrada desc
select *
from tblPROproduto pr
left join (select top(1) idproduto, valcusto
from tblproestoque
order by identrada desc) tmp on tmp.idproduto = pr.idproduto
where pr.idproduto = 8183

Answer 1

I suspect the problem here is your understanding of how a subquery works. We have your query:
SELECT *
FROM tblPROproduto pr
LEFT JOIN (SELECT TOP (1)
idproduto,
valcusto
FROM tblproestoque
ORDER BY identrada DESC) tmp ON tmp.idproduto = pr.idproduto
WHERE pr.idproduto = 8183;
We can separate this into 2 different parts:
SELECT TOP (1)
idproduto,
valcusto
FROM tblproestoque
ORDER BY identrada DESC;
and then:
SELECT *
FROM tblPROproduto pr
LEFT JOIN tmp ON tmp.idproduto = pr.idproduto
WHERE pr.idproduto = 8183;
This might explain to you why what you have isn't working. I'm guessing you are assuming that the ON clause on tmp is derived BEFORE the SELECT of the subquery. This isn't the case. The subquery will be derived, and then the ON. Thus the value of tmp will be whatever is rerturned in the query above.
I suspect what you want is:
SELECT *
FROM tblPROproduto pr
OUTER APPLY (SELECT TOP (1)
ca.idproduto,
ca.valcusto
FROM tblproestoque ca
WHERE ca.idproduto = pr.idproduto
ORDER BY ca.identrada DESC) tmp
WHERE pr.idproduto = 8183;
Edit: Added some sample data and explanations for the OP, to help their udnerstanding:
USE Sandbox;
GO
CREATE TABLE Product (ID int IDENTITY(1,1),
Sku varchar(10),
ProductName varchar(25));
CREATE TABLE ProductOrder (ID int IDENTITY(1,1),
ProductID int,
OrderDate date,
NumberOrdered int);
INSERT INTO dbo.Product (Sku,
ProductName)
VALUES ('65432462','Lawn Mower'),
('98742347','Helicopter'),
('89465735','BBQ');
INSERT INTO dbo.ProductOrder (ProductID,
OrderDate,
NumberOrdered)
VALUES (1,'20180101',7),
(1,'20180708',19),
(2,'20180501',12),
(3,'20180804',27);
GO
SELECT *
FROM dbo.Product;
SELECT *
FROM dbo.ProductOrder;
GO
--Use the example the OP has in their post:
SELECT *
FROM dbo.Product P
LEFT JOIN (SELECT TOP 1 *
FROM dbo.ProductOrder
ORDER BY OrderDate DESC) PO ON PO.ProductID = P.ID
WHERE P.ID = 2;
--This returns NULLs for all the latter columns.
--Why?
--Inspect the subquery:
SELECT TOP 1 *
FROM dbo.ProductOrder
ORDER BY OrderDate DESC;
--Product ID 3? 3 != 2 so the ON clause fails:
--Demonstrate
SELECT *
FROM dbo.Product P
CROSS JOIN (SELECT TOP 1 * --CROSS JOIN joins all rows (creates a cartesian product)
FROM dbo.ProductOrder
ORDER BY OrderDate DESC) PO
WHERE P.ID = 2;
--The solution, use OUTER APPLY:
SELECT *
FROM dbo.Product P
OUTER APPLY (SELECT TOP 1 *
FROM dbo.ProductOrder oa
WHERE oa.ProductID = P.ID --WHERE clause, this is like your ON
ORDER BY oa.OrderDate DESC) PO
WHERE P.ID = 2;
GO
DROP TABLE dbo.ProductOrder;
DROP TABLE dbo.Product;

Answer 2

#RafaelBueno, you basically need an outer apply. This is different in MSAcess.
See modified below:
Let me know if it works.
select *
from tblPROproduto pr
outer apply (select top(1) idproduto, valcusto
from tblproestoque tmp
where tmp.idproduto = pr.idproduto
order by identrada desc) tmp
where pr.idproduto = 8183