SQL window function

In the SQL database query language, window functions allow access to data in the records right before and after the current record.[1][2][3][4] A window function defines a frame or window of rows with a given length around the current row, and performs a calculation across the set of data in the window.[5][6]

      NAME |
------------
      Aaron| <-- Preceding (unbounded)
     Andrew|
     Amelia|
      James|
       Jill|
     Johnny| <-- 1st preceding row
    Michael| <-- Current row
       Nick| <-- 1st following row
    Ophelia|
       Zach| <-- Following (unbounded)

In the above table, the next query extracts for each row the values of a window with one preceding and one following row:

 SELECT
  LAG(name, 1) 
    OVER(ORDER BY name) "prev",
  name, 
  LEAD(name, 1) 
    OVER(ORDER BY name) "next"
 FROM people
 ORDER BY name

The result query contains the following values:

|     PREV |     NAME |     NEXT |
|----------|----------|----------|
|    (null)|     Aaron|    Andrew|
|     Aaron|    Andrew|    Amelia|
|    Andrew|    Amelia|     James|
|    Amelia|     James|      Jill|
|     James|      Jill|    Johnny|
|      Jill|    Johnny|   Michael|
|    Johnny|   Michael|      Nick|
|   Michael|      Nick|   Ophelia|
|      Nick|   Ophelia|      Zach|
|   Ophelia|      Zach|    (null)|

References
  1. Leis, Viktor; Kundhikanjana, Kan; Kemper, Alfons; Neumann, Thomas (June 2015). "Efficient Processing of Window Functions in Analytical SQL Queries". Proc. VLDB Endow. 8 (10): 1058–1069. doi:10.14778/2794367.2794375. ISSN 2150-8097.
  2. Cao, Yu; Chan, Chee-Yong; Li, Jie; Tan, Kian-Lee (July 2012). "Optimization of Analytic Window Functions". Proc. VLDB Endow. 5 (11): 1244–1255. arXiv:1208.0086. doi:10.14778/2350229.2350243. ISSN 2150-8097.
  3. "Probably the Coolest SQL Feature: Window Functions". Java, SQL and jOOQ. 2013-11-03. Retrieved 2017-09-26.
  4. "Window Functions in SQL - Simple Talk". Simple Talk. 2013-10-31. Retrieved 2017-09-26.
  5. "SQL Window Functions Introduction". Apache Drill.
  6. "PostgreSQL: Documentation: Window Functions". www.postgresql.org. Retrieved 2020-04-04.


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