Defining a window

You can use SQL windowing extensions to configure the bounds of a window, and the partitioning and ordering of the input rows. Logically, as part of the semantics of computing the result of a query specification, partitions are created after the groups defined by the GROUP BY clause are created, but before the evaluation of the final SELECT list and the query's ORDER BY clause. So, the order of evaluation of the clauses within a SQL statement is:

  1. FROM

  2. WHERE

  3. GROUP BY

  4. HAVING

  5. WINDOW

  6. DISTINCT

  7. ORDER BY

When forming your query, the impact of the order of evaluation should be considered. For example, you cannot have a predicate on an expression referencing a window function in the same SELECT query block. However, by putting the query block in a derived table, you can specify a predicate on the derived table. The following query fails with a message indicating that the failure was the result of a predicate being specified on a window function: 



SELECT DepartmentID, Surname, StartDate, Salary,
       SUM( Salary ) OVER ( PARTITION BY DepartmentID
          ORDER BY StartDate
          RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW ) AS "Sum_Salary" 
   FROM Employees
   WHERE State IN ( 'CA', 'UT', 'NY', 'AZ' ) 
      AND DepartmentID IN ( '100', '200' )
   GROUP BY DepartmentID, Surname, StartDate, Salary
   HAVING Salary > 0 AND "Sum_Salary" > 200
   ORDER BY DepartmentID, StartDate;

Use a derived table (DT) and specify a predicate on it to achieve the results you want:



 
SELECT * FROM ( SELECT DepartmentID, Surname, StartDate, Salary,
                       SUM( Salary ) OVER ( PARTITION BY DepartmentID
                         ORDER BY StartDate
                         RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW ) AS "Sum_Salary"
                  FROM Employees
                  WHERE State IN ( 'CA', 'UT', 'NY', 'AZ' ) 
                     AND DepartmentID IN ( '100', '200' )
                  GROUP BY DepartmentID, Surname, StartDate, Salary
                  HAVING Salary > 0
                  ORDER BY DepartmentID, StartDate ) AS DT
   WHERE DT.Sum_Salary > 200;

Because window partitioning follows a GROUP BY operator, the result of any aggregate function, such as SUM, AVG, or VARIANCE, is available to the computation done for a partition. So, windows provide another opportunity to perform grouping and ordering operations in addition to a query's GROUP BY and ORDER BY clauses.

 Defining a window specification
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