How do you decide the order of columns in a composite index?

Column order should reflect the workload. Important factors include equality predicates, range predicates, join columns, required ordering, grouping, selectivity, and whether the query needs to use a useful leftmost prefix.

For example, if a frequent query filters by CustomerId and then a date range, an index on (CustomerId, OrderDate) may provide an efficient access path.

The simplistic rule 'put the most selective column first' is not always correct because sorting and predicate structure can be more important.