我是 sql server 的新手。有没有办法可以比较前 2 行的时间？我的 id 是唯一的，所以如果我添加另一个时间，我想要相同的时间而不改变我的查询值。如果我的问题不清楚，我想知道最后两行的时差。
【问题讨论】：
lead/lag()。如果您只是想学习，那么其他方法可能更合适。
标签： sql sql-server timestamp
您可能会想出以下解决方案的十几种变体。如果你能理解它们，你就会在这个过程中学到很多 SQL。
-- scalar subqueries
select datediff(second, max("time"), (select max("time") from T)) as difference
from T
where "time" < (select max("time") from T)
-- correlated subquery
select datediff(
second,
(select t2."time" from T as t2 where t2."time" < t."time"),
t."time") as difference
from T as t
where t.id = (select max(id) from T)
-- self outer joins
select datediff(second, max(t2."time"), max(t."time")) as difference
from T as t left outer join T as t2 on t2."time" < t."time" -- by time
select datediff(second, max(t2."time"), max(t."time")) as difference
from T as t left outer join T as t2 on t2.id = t.id - 1 -- by id
-- cross apply
select datediff(second, max(prev."time"), max(t."time")) as difference
from T as t cross apply
(select max("time") from T as t2 where t2."time" < t."time") as prev("time")
-- cte with case expression
with max_time("time") as (select max("time") from T)
select min(case when t."time" <> max_time."time"
then datediff(second, t."time", max_time."time") end) as difference
from T as t cross apply max_time
-- derived table and analytic functions
select difference
from (
select
row_number() over (order by "time" desc) as rn,
datediff(second, lag("time") over (order by "time"), "time") as difference
from T
) T2
where rn = 1
【讨论】：
您可以像比较数字一样直接比较日期。
SELECT id FROM table WHERE time >= '2016-07-15'
要获得两次之间的差异，请使用 DATEDIFF
http://www.techonthenet.com/sql_server/functions/datediff.php
【讨论】：