Index on multiple columns for SQL performance
Since clustered index records the table and arranges the data according to the index key, SQL Server allows only one clustered index per table. This results in amazing performance and the worry arises when you need to order the data by more than one column. Since we can’t use clustered index for multiple columns, a unclustered index is used to gain an increase in performance. We can specify a key value for every unclustered index, plus the value obtained from cluster index; which acts as a pointer to the actual values when we need their data. If a unclustered index is keyed on the values needed for a given task then query engine will never be in need of visiting the actual rows. After learning everything it needs from the index and scope then it will visit the qualifying rows.