If we have a composite non-cluster index, the order of columns is important. For example, if I create an index on columns A,B,C and I query database filtering based on A,B,C columns, this index will help my query to run faster but if I want to query on B, C columns and not A, this index will not be used by SQL because the index has been created first on column A and if column A is not in the filtering criteria then SQL cannot use this index. If I query on A, B columns and not C because the starting column exists in the where clause then SQL can partially use the index.