Window Function
Categories of widow function 1. Aggregated fun : avg, sum , max, min , count 2. Ranking func : Rank, dense_rank, row_number 3. Analytical func : Lead, Lag, first_value, last_value Lag : Return previous row and depends on offset LAG ( expression [, offset [, default] ] ) OVER ( [ query_partition_clause ] order_by_clause ) Lead : Return next row and depends on offset. LEAD( expression [, offset [, default] ] ) OVER ( [ query_partition_clause ] order_by_clause )