WHERE clause in an SQL query - sql

Question

I THINK what is happening with this query is if there are no records in the GenericAttribute table associated with the Product, then that product is not displayed. See line below in WHERE clause: "AND GenericAttribute.KeyGroup = 'Product'"
Is there a way to reword so that that part of the WHERE is ignored if no associated record in the GenericAttribute table?
Also, looking at my ORDER BY clause, will a record from the product table still show up if it has no associated record in the Pvl_AdDates table?
Thanks!
SELECT DISTINCT Product_Category_Mapping.CategoryId, Product.Id, Product.Name, Product.ShortDescription, Pvl_AdDates.Caption, Pvl_AdDates.EventDateTime, convert(varchar(25), Pvl_AdDates.EventDateTime, 120) AS TheDate, Pvl_AdDates.DisplayOrder, Pvl_Urls.URL, [Address].FirstName, [Address].LastName, [Address].Email, [Address].Company, [Address].City, [Address].Address1, [Address].Address2, [Address].ZipPostalCode, [Address].PhoneNumber
FROM [Address]
RIGHT JOIN (GenericAttribute
RIGHT JOIN (Pvl_Urls RIGHT JOIN (Pvl_AdDates
RIGHT JOIN (Product_Category_Mapping
LEFT JOIN Product
ON Product_Category_Mapping.ProductId = Product.Id)
ON Pvl_AdDates.ProductId = Product.Id)
ON Pvl_Urls.ProductId = Product.Id)
ON GenericAttribute.EntityId = Product.Id)
ON Address.Id = convert(int, GenericAttribute.Value)
WHERE
Product_Category_Mapping.CategoryId=12
AND GenericAttribute.KeyGroup = 'Product'
AND Product.Published=1
AND Product.Deleted=0
AND Product.AvailableStartDateTimeUtc <= getdate()
AND Product.AvailableEndDateTimeUtc >= getdate()
ORDER BY
Pvl_AdDates.EventDateTime DESC,
Product.Id,
Pvl_AdDates.DisplayOrder

Answer 1

I strongly encourage you to not mix left join and right join. I have written many SQL queries and cannot think of an occasion when that was necessary.
In fact, just stick to left join.
If you want all products (or at least all products not filtered out by the where clause), then start with the products table and go from there:
FROM Products p LEFT JOIN
Product_Category_Mapping pcm
ON pcm.ProductId = p.Id LEFT JOIN
Pvl_AdDates ad
ON ad.ProductId = p.id LEFT JOIN
Pvl_Urls u
ON u.ProductId = p.id LEFT JOIN
GenericAttribute ga
ON ga.EntityId = p.id LEFT JOIN
Address a
ON a.Id = convert(int, ga.Value)
Note that I added table aliases. These make queries easier to write and to read.
I would add a caution. It looks like you are combining data along different dimensions. You are likely to get a Cartesian product of the dimension attributes for each dimension. Perhaps that is what you want or the WHERE clause takes care of the additional rows.

Answer 2

Yes put constraints (restrictions) on tables on the outer side of outer joins in the on conditions of the outer join, not in the where clause. Conditions in where clauses are not evaluated and applied until after the outer joins are evaluated, so where there is not record in the outer table, the predicate will be false and entire row will be eliminated, undoing the outer-ness. Conditions in the join are evaluated during the join, before the rows from the inner side are added back in, so the result set will still include them.
Second, formatting formatting, formatting! Stick to one direction of join (left is easier) and use Aliases for tables names!
SELECT DISTINCT m.CategoryId, p.Id,
p.Name, p.ShortDescription, d.Caption, d.EventDateTime,
convert(varchar(25), d.EventDateTime, 120) TheDate,
d.DisplayOrder, u.URL, a.FirstName, a.LastName,
a.Email, a.Company, a.City, a.Address1, a.Address2,
a.ZipPostalCode, a.PhoneNumber
FROM Product_Category_Mapping m
left join Product p on p.Id = m.ProductId
and p.Published=1
and p.Deleted=0
and p.AvailableStartDateTimeUtc <= getdate()
and p.AvailableEndDateTimeUtc >= getdate()
left join Pvl_AdDates d ON d.ProductId = p.Id
left join Pvl_Urls u ON u.ProductId = p.Id
left join GenericAttribute g ON g.EntityId = p.Id
and g.KeyGroup = 'Product'
left join [Address] a ON a.Id = convert(int, g.Value)
WHERE m.CategoryId=12
ORDER BY d.EventDateTime DESC, p.Id, d.DisplayOrder