Question

我有下面的查询，它返回一定时间间隔的结果集，如1分钟，10分钟，1小时等。[以下工作例如持续10分钟。]

select MemoryUsage, TransactionTime, moduleid
from tbl_trnmemoryusage 
where TransactionTime between (now() - interval 10 minute ) and now() 
order by TransactionTime asc;

要求

如果上面的查询没有返回任何内容，那就不是该持续时间内的数据，那么返回基于＆＃34; TransactionTime＆＃34;柱

if above_query returns null
then 
select MemoryUsage, TransactionTime, moduleid
from tbl_trnmemoryusage
order by TransactionTime desc limit 1;

sqlfiddle

Answer 1

SELECT
  COALESCE(t.MemoryUsage, t2.MemoryUsage),
  COALESCE(t.TransactionTime, t2.TransactionTime), 
  COALESCE(t.moduleid, t2.moduleid)
FROM (
  SELECT *
  FROM tbl_trnmemoryusage 
  ORDER BY TransactionTime DESC
  LIMIT 1
  ) t2
LEFT JOIN (
  SELECT *
  FROM tbl_trnmemoryusage
  WHERE TransactionTime BETWEEN (NOW() - INTERVAL 10 MINUTE) and NOW() 
  ORDER BY TransactionTime ASC
) t
ON 1

Answer 2

你试过这种方式吗？

select MemoryUsage, TransactionTime, moduleid
from tbl_trnmemoryusage 
where TransactionTime >=
  IF((Select Count(*) from (
    select MemoryUsage, TransactionTime, moduleid
    from tbl_trnmemoryusage 
    where TransactionTime between (now() - interval 10 minute ) and now() 
    ) As Tbl1) > 0, 
   (now() - interval 10 minute),
     (Select Max(TransactionTime)
     from tbl_trnmemoryusage))
order by TransactionTime asc;

Answer 3

您可以将UNION ALL与SELECT语句一起使用，只有在给定范围内找不到行时才会返回一行。

select MemoryUsage, TransactionTime, moduleid
from tbl_trnmemoryusage 
where TransactionTime between (now() - interval 10 minute ) and now()
union all
select * from (
    select MemoryUsage, TransactionTime, moduleid
    from tbl_trnmemoryusage
    where not exists (
        select 1 from tbl_trnmemoryusage
        where TransactionTime between (now() - interval 10 minute ) and now()
    )
    order by TransactionTime desc limit 1
) b
order by TransactionTime asc

如果您不关心性能或结果集很小，则可以跳过NOT EXISTS条件并让UIONN DISTINCT消除重复。请注意，它还会删除数据库中已存在的所有重复项。

select MemoryUsage, TransactionTime, moduleid
from tbl_trnmemoryusage 
where TransactionTime between (now() - interval 1 minute ) and now()
union distinct
select * from (
    select MemoryUsage, TransactionTime, moduleid
    from tbl_trnmemoryusage
    order by TransactionTime desc limit 1
) b
order by TransactionTime asc

如果select查询返回null，则返回mysql中db的最新条目

3 个答案: