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sql 导航函数 lag
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返回上一行中的 value_expression 值。更改 offset 值會改變所返回的上一行；默認值是 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 divisionUNION 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;
namefinish_timedivisionpreceding_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 |
下一個示例使用可選的 offset 參數
WITH finishers AS(SELECT 'Sophia Liu' as name,TIMESTAMP '2016-10-18 2:51:45' as finish_time,'F30-34' as divisionUNION 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;
namefinish_timedivisiontwo_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 |
以下示例使用默認值替換 NULL 值
WITH finishers AS(SELECT 'Sophia Liu' as name,TIMESTAMP '2016-10-18 2:51:45' as finish_time,'F30-34' as divisionUNION 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;
namefinish_timedivisiontwo_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 |
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