Understanding window functions

A window function operates over a set of rows called a window and returns a column that is included with the other selected columns. The window becomes a partition on the table and allows the rows to retain their details while providing an aggregate. That is what makes the window function different from aggregates, even though, in some cases, it is evaluating an aggregate.

Here is a partial list of the window functions:

• Ranking:
  • ROW_NUMBER
  • RANK
  • DENSE_RANK
  • NTILE
  • PERCENT_RANK
  • CUME_DISTLEAD
  • FIRST_VALUE
  • LAST_VALUE
  • COUNT
  • PERCENTILE_CONT
  • PERCENTILE_DIST
• Value:
  • LAG
• Aggregate:
  • AVG
  • SUM
  • MAX/MIN

Next, we will look at the structure of the window functions and the required clauses to create them.

Using a window function in a query

To use a window function in a query, you just include it as a column in the query definition. Every window function requires an OVER clause and optionally can include a PARTITION BY clause and an ORDER BY clause, which is...