Converting multiple rows into one in SQL Server - sql-server

Question

I have 2 tables:
Product:
ProductId | Name | Description
----------+-------+-------------------------------------
1 | shirt | this is description for shirt
2 | pent | this is description for pent
ProductOverride:
ProductOverrideId | ColumnId | Value | ProductId
------------------+-----------+------------------------+-----------
1 | 1 | overridden name | 1
2 | 2 | overridden description | 1
where ColumnId is column_id from sys.columns.
I want to select all the products with the following requirement:
if product name or product description is overridden in ProductOverride table, get the overridden value of name/description, otherwise get the name/description value from the product table.
Sample output:
ProductId | Name | Description
----------+-----------------+---------------------------
1 | overridden name | overridden description
2 | pent | this is description for pent
I have the following query which returns the exact result.
DECLARE #productNameColumnId INT = 1;
DECLARE #productDescriptionColumnId INT = 2;
WITH OverriddenProductNameCTE ([Value], [ProductId]) AS
(
SELECT
temp.[Value], temp.ProductId
FROM
ProductOverride temp
WHERE
temp.ColumnId = #productNameColumnId
), OverriddenProductDescriptionCTE ([Value], [ProductId]) AS
(
SELECT
temp.[Value], temp.ProductId
FROM
ProductOverride temp
WHERE
temp.ColumnId = #productDescriptionColumnId
)
SELECT
p.ProductId,
CASE
WHEN EXISTS(SELECT [Value]
FROM OverriddenProductNameCTE opnc
WHERE opnc.ProductId = p.ProductId)
THEN (SELECT [Value]
FROM OverriddenProductNameCTE opnc
WHERE opnc.ProductId = p.ProductId)
ELSE p.[Name]
END AS [Name],
CASE
WHEN EXISTS(SELECT [Value]
FROM OverriddenProductDescriptionCTE opdc
WHERE opdc.ProductId = p.ProductId)
THEN (SELECT [Value]
FROM OverriddenProductDescriptionCTE opdc
WHERE opdc.ProductId = p.ProductId)
ELSE p.[Description]
END AS [Description]
FROM
product p
but in the CASE statements, I have the following repetitive code:
SELECT [Value]
FROM OverriddenProductNameCTE opnc
WHERE opnc.ProductId = p.ProductId
which means if the CASE statement's first condition is true DBMS will execute the same query again in the THEN part.
I want to improve this query both in terms of simplifying the query and in terms of processing.
Also if there is any advantage of using CTEs in this situation?

Answer 1

If it's only 2 columns I think the simplest thing you can do is left join twice with coalesce:
SELECT p.ProductId
,COALESCE(poN.Value, p.Name) As Name
,COALESCE(poD.Value, p.Description) As Description
FROM Product p
LEFT JOIN ProductOverride poN ON p.ProductId = poN.ProductId AND poN.ColumnId = 1
LEFT JOIN ProductOverride poD ON p.ProductId = poD.ProductId AND poD.ColumnId = 2
If it's more columns I would suggest pivoting the ProductOverride table and left join to that - Like this (a complete example):
Create and populate sample tables (Please save us this step in your future questions)
CREATE TABLE Product
(
ProductId int,
Name varchar(100),
Description varchar(100),
price int null
);
INSERT INTO Product VALUES
(1, 'shirt', 'Description for shirts', 1),
(2, 'Pants', 'Description for pants', 4),
(3, 'Socks', 'Description for socks', 5)
CREATE TABLE ProductOverride
(
ProductOverrideId int,
ColumnId int,
Value varchar(100),
ProductId int
);
INSERT INTO ProductOverride VALUES
(1,1,'product 1 name',1),
(2,2,'product 1 desc',1),
(3,3,'7',1),
(4,1,'pants name',2),
--Note: no pants description in the override tabl
(6,3,'8',2);
-- Note: no socks at all in override table
The query:
SELECT p.ProductId
,COALESCE(override.[1], p.Name) As Name
,COALESCE(override.[2], p.Description) As Description
,COALESCE(CAST(override.[3] as int), p.Price) As Price
FROM Product p
LEFT JOIN
(
SELECT *
FROM
(
SELECT ProductId, Value, ColumnId -- Columns To use for pivot
FROM ProductOverride
) ColumnsToPivot
PIVOT (
max (Value)
for ColumnId in ([1], [2], [3]) -- Values in ColumnId column to make the column names
) as pivotedData
) as override ON p.ProductId = override.ProductId
Results:
ProductId Name Description Price
1 product 1 name product 1 desc 7
2 pants name Description for pants 8
3 Socks Description for socks 5
You can see a live demo on rextester.