DECLARE @MissingNumbers TABLE (N INT)

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

;WITH Tally(N) AS
(
  SELECT number N FROM master.dbo.spt_values WHERE Type = 'P' AND number > 0
)
SELECT
  T.N 
FROM
  Tally T
LEFT JOIN
  @MissingNumbers MN ON MN.N = T.N
WHERE
  MN.N IS NULL
  AND T.N <= (SELECT MAX(N) FROM @MissingNumbers)

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

declare @list  varchar(50),
        @Delimiter char(1);
set @list = '1,2,4,5,7,8,11,12,13,15,17,19,20';
set @Delimiter = ',';

-- first, need to break down into separate items.
-- See Jeff Moden's article The "Numbers" or "Tally" Table: What it is and how it replaces a loop.
-- at http://www.sqlservercentral.com/articles/T-SQL/62867/ for how a tally table can split strings apart.
WITH Tens      (N) AS (SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL
                       SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 ),
     Thousands (N) AS (SELECT 1 FROM Tens t1 CROSS JOIN Tens t2 CROSS JOIN Tens t3),
     Millions  (N) AS (select 1 FROM Thousands t1 CROSS JOIN Thousands t2),
     Tally     (N) AS (SELECT ROW_NUMBER() OVER (ORDER BY (SELECT 0)) FROM Millions),
     ItemSplit (ItemOrder, Item) AS (
         SELECT N,
                RTRIM(LTRIM(SUBSTRING(@Delimiter + @list + @Delimiter,N+1,CHARINDEX(',',@Delimiter + @list + @Delimiter,N+1)-N-1)))
           FROM Tally
          WHERE N < LEN(@Delimiter + @list + @Delimiter)
            AND SUBSTRING(@Delimiter + @list + @Delimiter,N,1) = ','
        )
SELECT t.N
  FROM Tally t
       LEFT JOIN ItemSplit ON t.N = ItemSplit.Item
 WHERE ItemSplit.Item IS NULL
   AND t.N < (select MAX(convert(int,Item)) from ItemSplit)
 ORDER BY t.N

DECLARE @list VARCHAR(50) = '1,2,4,5,7,8,11,12,13,15,17,19,20',
        @Delimiter CHAR(1) = ',';

SELECT  Numbers.n
FROM    dbo.Numbers(REVERSE(LEFT(REVERSE(@list), CHARINDEX(@Delimiter, REVERSE(@list)) - 1))) Numbers
WHERE   CHARINDEX(@Delimiter + CONVERT(VARCHAR(11), Numbers.n) + @Delimiter, @Delimiter + @list + @Delimiter) = 0;

SET ANSI_NULLS ON
SET QUOTED_IDENTIFIER ON
GO
CREATE   FUNCTION [dbo].[Numbers]
        (
        @Count  BIGINT
        )
RETURNS TABLE
WITH    SCHEMABINDING
AS      RETURN
        WITH
        E1      AS      
                (
                SELECT  n = 1 UNION ALL 
                SELECT  2 UNION ALL 
                SELECT  3 UNION ALL 
                SELECT  4 UNION ALL 
                SELECT  5 UNION ALL 
                SELECT  6 UNION ALL 
                SELECT  7 UNION ALL 
                SELECT  8 UNION ALL 
                SELECT  9 UNION ALL 
                SELECT  10
                ),                                          -- 1E1 rows
        E2      AS (SELECT X.n FROM E1 X CROSS JOIN E1),    -- 1E2 rows
        E4      AS (SELECT X.n FROM E2 X CROSS JOIN E2),    -- 1E4 rows
        E8      AS (SELECT X.n FROM E4 X CROSS JOIN E4),    -- 1E8 rows
        Numbers AS (SELECT n = ROW_NUMBER() OVER (ORDER BY (SELECT 0)) FROM E8)
        SELECT  TOP (@Count)
                N.n
        FROM    Numbers N
        ORDER   BY 
                N.n ASC;
GO

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

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

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

;with wcte as (
Select Top(@vMax) ROW_NUMBER() over(order by a.N) NAll
from @MissingNumbers a, @MissingNumbers 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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DECLARE @MissingNumbers TABLE (N INT)
Declare @vMax int

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

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

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

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.N) NAll
from @MissingNumbers a, @MissingNumbers 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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