DECLARE @MissingNumbers 
TABLE   (n INTEGER NOT NULL);

INSERT  @MissingNumbers (n)
VALUES  (1),(2),(4),(5),(7),(8),(11),(12),(13),(15),(17),(19),(20);

SELECT  N.n
FROM    dbo.Numbers((SELECT MAX(n) FROM @MissingNumbers)) N
EXCEPT  SELECT n FROM @MissingNumbers;

DECLARE @MissingNumbers TABLE (N INT)
Declare @vMax int

INSERT INTO @MissingNumbers
VALUES (19),(20)

Set @vMax = (Select MAX(N) from @MissingNumbers)

;with wcte as (
Select Top(@vMax) ROW_NUMBER() over(order by a.Object_id,a.Column_ID) NAll
from sys.columns a, sys.columns b
) Select NAll from wcte a
Left outer Join @MissingNumbers b on b.N = a.Nall
where b.N is Null

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You can check my BLOG here

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You can check my BLOG here

DROP TABLE #MyTest
GO
--===== Create and populate a 2,000,000 row test table.
     -- This first SELECT creates a range of 1 to 1,000,000 unique numbers starting at 10,000,001 
 SELECT TOP 1000000
        MyID       = ISNULL(CAST(ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) + 10000000 AS BIGINT),0)
   INTO #MyTest
   FROM Master.sys.All_Columns ac1,
        Master.sys.All_Columns ac2
     -- This second SELECT creates a range of 1 to 1,000,000 unique numbers starting at 82,011,000,000,001
  UNION ALL
 SELECT TOP 1000000
        MyID       = ISNULL(CAST(ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) + 82011000000000 AS BIGINT),0)
   FROM Master.sys.All_Columns ac1,
        Master.sys.All_Columns ac2

--===== Create the quintessential Primary Key
     -- Takes about 3 seconds to execute.
  ALTER TABLE #MyTest
        ADD PRIMARY KEY CLUSTERED (MyID)

--===== Delete some know rows to demo the gap detection code
     -- This deletes 50 rows spaced 2000 apart in the given range
     -- to demo small gaps
 DELETE #MyTest
  WHERE MyID BETWEEN 82011000400001 AND 82011000500000
    AND MyID %2000 = 0

     -- This deletes 100,000 rows in a given range to demo large gaps
 DELETE #MyTest
  WHERE MyID BETWEEN 82011000600001 AND 82011000700000

--===== Find the "gap ranges" --This takes 3 seconds on my 8 year old machine
     -- Finds trailing edge of "islands" and then computes the gaps
     -- This assumes that gaps include any whole number greater than 0
 SELECT GapStart = (SELECT ISNULL(MAX(lo.MyID),0)+1
                      FROM #MyTest lo 
                     WHERE lo.MyID < hi.MyID),
        GapEnd = hi.MyID - 1  
   FROM #MyTest hi
  WHERE hi.MyID NOT IN (SELECT MyID + 1 FROM #MyTest)