SQL Case Switch Default to Show Placeholder Image on Nulls - sql

Question

I am trying to get a list of products and images from two tables. When I join them and use a case switch on the HasImage column from the products table, I get this error:
Error:
Subquery returned more than 1 value.
This is not permitted when the subquery follows =, !=, <, <= , >, >=
or when the subquery is used as an expression.
When a product does not have an image, I want to replace it with a default image.
Here is the select statement:
SELECT
P.[ProductId]
,P.[ProductName]
--If HasImage is false show the default.jpg
,Case P.[HasImage]
WHEN 'True' THEN (Select I.[FileName] as ProductImage FROM [ProductImages] I
INNER JOIN [Product] P on P.ProductId = I.ProductId
WHERE I.Sequence=0)
WHEN 'False' THEN 'default.jpg'
END
FROM [Product] P
LEFT JOIN [ProductImages] I
on P.ProductId = I.ProductId
The problem is in the Case When 'True'. That's what throws the error.
Product Table
ProductId ProductName HasImage
1 Coffee Mug True
2 Pen False
3 Pencil False
Product Images Table
ProductId Sequence FileName
1 0 Mug_Image1.jpg
1 1 Mug_2.jpg
1 2 Mug_Img3.jpg
There are multiple images for ProductId=1, but I use Sequence = 0 to return only one.
The returned data I want should look like this:
ProductId ProductName ProductImage
1 Coffee Mug Mug_Image1.jpg
2 Pen default.jpg
3 Pencil default.jpg
I have tried various combinations of coalesce(NULLIF, Left Joins, and different statments, but I haven't gotten all three products to display as desired.

Answer 1

In addition to my comment below OP, this is a query I believe should have been written in the first place.
SELECT
P.[ProductId]
,P.[ProductName]
--If HasImage is false show the default.jpg
,Case P.[HasImage]
WHEN 'True'
THEN I.[FileName]
WHEN 'False'
THEN 'default.jpg'
END
FROM [Product] P
LEFT JOIN [ProductImages] I
ON P.ProductId = I.ProductId
-- Filter Sequence 0 only
-- All products will be retrieved
-- whether they have associated Image with Sequence = 0
AND I.Sequence = 0
Filtering right side of left join allows you to retain properties of left join AND join only rows of interest. If HasImage serves just to mark existance of Images and not as business rule (show/don't show image of this particular product), you might remove case in favor of simple isnull(I.FileName, 'default.jpg').
Alternatively (Sql Server 2005 and newer) you might use CROSS APPLY to retrieve images:
SELECT
P.[ProductId]
,P.[ProductName]
,I.[FileName]
FROM [Product] P
OUTER APPLY
(
SELECT CASE P.[HasImage]
WHEN 'True'
THEN ProductImages.[FileName]
WHEN 'False'
THEN 'default.jpg'
END FileName
FROM [ProductImages]
WHERE P.ProductId = ProductImages.ProductId
AND I.Sequence = 0
) I

Answer 2

Why do you want to go for a INNER JOIN?
If you are sure that when HasImage is TRUE your table will have an image then better go for a straight join.
Try this query -
SELECT P.[ProductId],
P.[ProductName],
Case P.[HasImage]
WHEN 'True' THEN
(Select
TOP 1 -- For Oracle
I.[FileName] as ProductImage FROM [ProductImages] I
WHERE P.ProductId = I.ProductId
ORDER BY I.Sequence
LIMIT 1 -- For MySQL
)
WHEN 'False' THEN 'default.jpg'
END
FROM [Product] P