T-Sql Query with dynamic (unknown) number of columns - sql

Question

We have a project where we should provide the possible to the user to add own custom columns to various tables.
Edit: these are 2 tables, not one.
**Products**
ProductId
Name
Price
Date
UserId
**ProductsCustomColumns**
ProductId
ColumnName
ColumnValue
EDIT: Please note that the dynamic columns are recorded as values and we don't know the count of these...it can be 0 or 200 or any.
Here is an example:
Now when we query the products tables we want to show all the predefined columns and after them all custom columns.
Obviously each user can have own number of columns with values and names.
SELECT *, (and the custom columns) FROM Products WHERE UserId = 3 AND ProductId = 1
Here are 2 questions:
Would that be good solution from performance point of view or there is better approach for solving the dynamic columns requirement?
How can I create a query that could read all records from ProductsCustomColumns for given userId and productId and append the records as columns to the query?
Thanks.

Answer 1

You need to write dynamic Query
DECLARE #COLUMNS VARCHAR(MAX)='', #QRY VARCHAR(MAX);
SELECT #COLUMNS = #COLUMNS +COLUMN_NAME +',' FROM
INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME='Products'
SELECT #COLUMNS =SUBSTRING (#COLUMNS,1 ,LEN(#COLUMNS)-1)
SELECT #QRY ='SELECT '+#COLUMNS + ' FROM Products WHERE UserId = 3 AND ProductId = 1'
EXEC (#QRY)
EDIT: From your Comments & Edited Question
Schema I assumed from your Question
CREATE TABLE Products (
ProductId INT,
Name VARCHAR(250),
Price DECIMAL(18,2),
DateS DATETIME,
UserId INT)
INSERT INTO Products
SELECT 1,'Oil Product', 2000, GETDATE(), 3
UNION ALL
SELECT 2,'Amway', 600, GETDATE(), 2
UNION ALL
SELECT 3,'Thermal', 5000, GETDATE(), 1
UNION ALL
SELECT 4,'Oil Product', 500, GETDATE(), 4
CREATE TABLE ProductsCustomColumns
(
ProductId INT ,
ColumnName VARCHAR(200),
ColumnValue VARCHAR(15))
INSERT INTO ProductsCustomColumns
SELECT 1, 'Licence_No', '1545'
UNION ALL
SELECT 1, 'Location ', 'Atlanta'
UNION ALL
SELECT 2, 'Qty ', '5'
UNION ALL
SELECT 3, 'Gross', '80000'
Now your Dynamic Code goes here
DECLARE #COLUMN_PCC VARCHAR(MAX)='', #PRODUCT_ID INT=1,#USER_ID INT=3, #QRY VARCHAR(MAX) ;
--preparing Custom Column Name List with comma ','
SELECT #COLUMN_PCC = #COLUMN_PCC+ [COLUMNNAME] +',' FROM ProductsCustomColumns
WHERE ProductId= #PRODUCT_ID
SELECT #COLUMN_PCC =SUBSTRING(#COLUMN_PCC,1,LEN(#COLUMN_PCC)-1)
--Preparing Dynamic Query
SELECT #QRY =' SELECT P.*, AV.* FROM Products P
INNER JOIN
(
SELECT * FROM (
SELECT * FROM ProductsCustomColumns WHERE ProductId= '+CAST(#PRODUCT_ID AS VARCHAR(50))+'
)
AS A
PIVOT
(
MAX (COLUMNVALUE)
FOR [COLUMNNAME] IN ('+#COLUMN_PCC +')
)AS PVT
)AS AV ON P.ProductId= AV.ProductId
AND P.UserId='++CAST(#USER_ID AS VARCHAR(50))+'
'
EXEC ( #QRY)
And the Result will be
+-----------+-------------+---------+-------------------------+--------+-----------+------------+----------+
| ProductId | Name | Price | DateS | UserId | ProductId | Licence_No | Location |
+-----------+-------------+---------+-------------------------+--------+-----------+------------+----------+
| 1 | Oil Product | 2000.00 | 2016-12-09 18:06:24.090 | 3 | 1 | 1545 | Atlanta |
+-----------+-------------+---------+-------------------------+--------+-----------+------------+----------+

Answer 2

You need dynamic sql no other way to do this
DECLARE #sql VARCHAR(max),
#cust_col VARCHAR(max)
SET #cust_col = (SELECT Quotename(CustomColumns) + ','
FROM ProductsCustomColumns
FOR xml path(''))
SELECT #cust_col = LEFT(#cust_col, Len(#cust_col) - 1)
SET #sql = 'SELECT *, ' + #cust_col + ' FROM Products WHERE UserId = 3 AND ProductId = 1'
EXEC (#sql)

Answer 3

In general it is a very bad idea to add custom data in additional columns of your main table. Just imagine 100 customers using this. All of them have differing table schemas and you wnat to write an update script for all of them?
It is a pain in the neck, if you have to deal with result sets where you don't know the structure in advance!
You have several choices:
Add one column of type XML. The advantage: The resultset is fix. You just need a customer specific rule, how to interpret the XML. You can solve this with an inline table valued function. Pass in the XML and get a derived table back. Call this with CROSS APPLY and you are out...
Add a new table with the customerID and Key-Value-Pairs
If the additional data is not completely different, add some of the columns to your main table as SPARSE columns