SQL query records within a range of boundaries and max/min outside the range

Question

I have the following three simple T-SQL queries. First one is to get records within a range of boundaries (DATETIME type):

 SELECT value, timestamp 
 FROM myTable
 WHERE timestamp BETWEEN @startDT AND @endDT

the second one is to get the closest record to @startDT (DATETIME type)

 SELECT TOP 1
   value, timestamp
 FROM myTable
 WHERE timestamp > @startDT
 ORDER BY timestamp DESC

and the last one is to get the closest record after @endDT:

 SELECT TOP 1
   value, timestamp
 FROM myTable
 WHERE timestamp < @endDT
 ORDER BY timestamp ASC

I would like to get all the records of above three queries as one group of records. I tried to use UNION, but it seems that sub-queries within UNION does not allow ORDER BY clause. Is there efficient way to get my result?

 . .  * | * * * * * | * . . .
      start        end

The above graph simply shows the records of *s as my required records, and |...| is the boundaries.

By the way, the amount of data in myTable is huge. My understanding UNION is not an efficient way to get data from UNIONs. Any efficient way to get data without UNION?

Answer 1

As you wish, without UNION.

MySQL (TESTED)

SELECT 
     dv1.timestamp, dv1.values
FROM 
     myTable AS dv1
WHERE 
    dv1.timestamp 
    BETWEEN (
           SELECT dv2.timestamp 
           FROM myTable AS dv2
           WHERE dv2.timestamp < '@START_DATE' 
           ORDER BY dv2.timestamp DESC 
           LIMIT 1
           )
    AND ( SELECT dv3.timestamp 
          FROM myTable AS dv3
          WHERE dv3.timestamp > '@END_DATE' 
          ORDER BY dv3.timestamp ASC 
          LIMIT 1
        )

EDIT Sorry, I forgot to notice about T-SQL.

T-SQL (NOT TESTED)

SELECT 
     dv1.timestamp, dv1.values
FROM 
     myTable AS dv1
WHERE 
    dv1.timestamp 
    BETWEEN (
           SELECT TOP 1 dv2.timestamp 
           FROM myTable AS dv2
           WHERE dv2.timestamp >  @START_DATE 
           ORDER BY dv2.timestamp DESC
           )
    AND ( SELECT TOP 1 dv3.timestamp 
          FROM myTable AS dv3
          WHERE dv3.timestamp <  @END_DATE 
          ORDER BY dv3.timestamp ASC
        )

Note If the result is not right, you could just exchange the sub queries (i.e. operators, and ASC/DESC).

Think out of the box :)

Answer 2

U can use max/min to get value u need. Order by +top 1 isnt best way to get max value, what i can see in ur querys. To sort n items its O(n to power 2), getting max should be only O(n)