3. Elements of SQL Statements : SQL Functions : Window Functions
 
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Window Functions
Window function syntax is required with analytical functions and can also be used with aggregate functions.
A window function is defined "over" a group of rows (a window) from a query. A window function produces a result value for each input row, where the value is computed according to the records in the current window.
A window is defined with an optional partitioning definition and an ordering definition.
The partitioning defines sets of rows over which the function results are computed. Partitioning is like the grouping of a grouped query. Without the partitioning definition, the function will operate over the entire set of records returned by the query.
The ordering defines the order of rows within a partition, which determines the function values.
Note:  The ordering does not define the order of result rows from the query. If the results of a query need to be sorted, a separate ORDER BY clause is required on the result set.