sql 导航函数 lag

返回上一行中的 value_expression 值。更改 offset 值会改变所返回的上一行；默认值是 1，表示窗口框架中的上一行。

WITH finishers AS
 (SELECT 'Sophia Liu' as name,
  TIMESTAMP '2016-10-18 2:51:45' as finish_time,
  'F30-34' as division
  UNION ALL SELECT 'Lisa Stelzner', TIMESTAMP '2016-10-18 2:54:11', 'F35-39'
  UNION ALL SELECT 'Nikki Leith', TIMESTAMP '2016-10-18 2:59:01', 'F30-34'
  UNION ALL SELECT 'Lauren Matthews', TIMESTAMP '2016-10-18 3:01:17', 'F35-39'
  UNION ALL SELECT 'Desiree Berry', TIMESTAMP '2016-10-18 3:05:42', 'F35-39'
  UNION ALL SELECT 'Suzy Slane', TIMESTAMP '2016-10-18 3:06:24', 'F35-39'
  UNION ALL SELECT 'Jen Edwards', TIMESTAMP '2016-10-18 3:06:36', 'F30-34'
  UNION ALL SELECT 'Meghan Lederer', TIMESTAMP '2016-10-18 3:07:41', 'F30-34'
  UNION ALL SELECT 'Carly Forte', TIMESTAMP '2016-10-18 3:08:58', 'F25-29'
  UNION ALL SELECT 'Lauren Reasoner', TIMESTAMP '2016-10-18 3:10:14', 'F30-34')
SELECT name,
  finish_time,
  division,
  LAG(name)
    OVER (PARTITION BY division ORDER BY finish_time ASC) AS preceding_runner
FROM finishers;

name	finish_time	division	preceding_runner
Carly Forte	03:08:58	F25-29	NULL
Sophia Liu	02:51:45	F30-34	NULL
Nikki Leith	02:59:01	F30-34	Sophia Liu
Jen Edwards	03:06:36	F30-34	Nikki Leith
Meghan Lederer	03:07:41	F30-34	Jen Edwards
Lauren Reasoner	03:10:14	F30-34	Meghan Lederer
Lisa Stelzner	02:54:11	F35-39	NULL
Lauren Matthews	03:01:17	F35-39	Lisa Stelzner
Desiree Berry	03:05:42	F35-39	Lauren Matthews
Suzy Slane	03:06:24	F35-39	Desiree Berry

下一个示例使用可选的 offset 参数

WITH finishers AS
 (SELECT 'Sophia Liu' as name,
  TIMESTAMP '2016-10-18 2:51:45' as finish_time,
  'F30-34' as division
  UNION ALL SELECT 'Lisa Stelzner', TIMESTAMP '2016-10-18 2:54:11', 'F35-39'
  UNION ALL SELECT 'Nikki Leith', TIMESTAMP '2016-10-18 2:59:01', 'F30-34'
  UNION ALL SELECT 'Lauren Matthews', TIMESTAMP '2016-10-18 3:01:17', 'F35-39'
  UNION ALL SELECT 'Desiree Berry', TIMESTAMP '2016-10-18 3:05:42', 'F35-39'
  UNION ALL SELECT 'Suzy Slane', TIMESTAMP '2016-10-18 3:06:24', 'F35-39'
  UNION ALL SELECT 'Jen Edwards', TIMESTAMP '2016-10-18 3:06:36', 'F30-34'
  UNION ALL SELECT 'Meghan Lederer', TIMESTAMP '2016-10-18 3:07:41', 'F30-34'
  UNION ALL SELECT 'Carly Forte', TIMESTAMP '2016-10-18 3:08:58', 'F25-29'
  UNION ALL SELECT 'Lauren Reasoner', TIMESTAMP '2016-10-18 3:10:14', 'F30-34')
SELECT name,
  finish_time,
  division,
  LAG(name, 2)
    OVER (PARTITION BY division ORDER BY finish_time ASC) AS two_runners_ahead
FROM finishers;

name	finish_time	division	two_runners_ahead
Carly Forte	03:08:58	F25-29	NULL
Sophia Liu	02:51:45	F30-34	NULL
Nikki Leith	02:59:01	F30-34	NULL
Jen Edwards	03:06:36	F30-34	Sophia Liu
Meghan Lederer	03:07:41	F30-34	Nikki Leith
Lauren Reasoner	03:10:14	F30-34	Jen Edwards
Lisa Stelzner	02:54:11	F35-39	NULL
Lauren Matthews	03:01:17	F35-39	NULL
Desiree Berry	03:05:42	F35-39	Lisa Stelzner
Suzy Slane	03:06:24	F35-39	Lauren Matthews

以下示例使用默认值替换 NULL 值

WITH finishers AS
 (SELECT 'Sophia Liu' as name,
  TIMESTAMP '2016-10-18 2:51:45' as finish_time,
  'F30-34' as division
  UNION ALL SELECT 'Lisa Stelzner', TIMESTAMP '2016-10-18 2:54:11', 'F35-39'
  UNION ALL SELECT 'Nikki Leith', TIMESTAMP '2016-10-18 2:59:01', 'F30-34'
  UNION ALL SELECT 'Lauren Matthews', TIMESTAMP '2016-10-18 3:01:17', 'F35-39'
  UNION ALL SELECT 'Desiree Berry', TIMESTAMP '2016-10-18 3:05:42', 'F35-39'
  UNION ALL SELECT 'Suzy Slane', TIMESTAMP '2016-10-18 3:06:24', 'F35-39'
  UNION ALL SELECT 'Jen Edwards', TIMESTAMP '2016-10-18 3:06:36', 'F30-34'
  UNION ALL SELECT 'Meghan Lederer', TIMESTAMP '2016-10-18 3:07:41', 'F30-34'
  UNION ALL SELECT 'Carly Forte', TIMESTAMP '2016-10-18 3:08:58', 'F25-29'
  UNION ALL SELECT 'Lauren Reasoner', TIMESTAMP '2016-10-18 3:10:14', 'F30-34')
SELECT name,
  finish_time,
  division,
  LAG(name, 2, 'Nobody')
    OVER (PARTITION BY division ORDER BY finish_time ASC) AS two_runners_ahead
FROM finishers;

name	finish_time	division	two_runners_ahead
Carly Forte	03:08:58	F25-29	Nobody
Sophia Liu	02:51:45	F30-34	Nobody
Nikki Leith	02:59:01	F30-34	Nobody
Jen Edwards	03:06:36	F30-34	Sophia Liu
Meghan Lederer	03:07:41	F30-34	Nikki Leith
Lauren Reasoner	03:10:14	F30-34	Jen Edwards
Lisa Stelzner	02:54:11	F35-39	Nobody
Lauren Matthews	03:01:17	F35-39	Nobody
Desiree Berry	03:05:42	F35-39	Lisa Stelzner
Suzy Slane	03:06:24	F35-39	Lauren Matthews