In one of my recent projects, I needed to combine multiple row data values into a single one-liner. No idea about performance, but I leveraged the FOR XML PATH for my example.. Thanks to this link for the tip.

USE AdventureWorks 
GO 
 
SELECT 
    CustomerID, 
    SalesOrderIDs = REPLACE( 
        ( 
            SELECT 
                SalesOrderID AS [data()] 
            FROM 
                Sales.SalesOrderHeader soh 
            WHERE 
                soh.CustomerID = c.CustomerID 
            ORDER BY 
                SalesOrderID 
            FOR XML PATH ('') 
        ), ' ', ',') 
FROM 
    Sales.Customer c 
ORDER BY 
    CustomerID