select t1.concatenate, t2.concatenate 
from table1 t1
left join table2 t2
on t1.concatenate = t2.concatenate

-- make some sample data
DROP TABLE #Sample
CREATE TABLE #Sample (Record INT, [Concatenate] CHAR(12))
INSERT INTO #Sample (Record, [Concatenate])
SELECT 1, '201201010510' UNION ALL
SELECT 2, '201201010511' UNION ALL
SELECT 3, '201201010514' UNION ALL
SELECT 4, '201201010515' UNION ALL
SELECT 5, '201201010517';

CREATE UNIQUE CLUSTERED INDEX ucx_Everything ON #Sample ([Concatenate], Record);


--------------------------
-- construct an inline tally table (Jeff Moden et al). This source of rows will be used
-- to fill in the missing rows in the data
;WITH E1(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
                ),                          --10E+1 or 10 rows
       E2(N) AS (SELECT 1 FROM E1 a, E1 b), --10E+2 or 100 rows
   iTally(N) AS (SELECT 1 FROM E2 a, E2 b), --10E+4 or 10,000 rows max
--------------------------
-- Find the rows at the beginning and end of each gap
GapBounds AS (
	SELECT 
		  StartDate = CAST(s.[Concatenate] AS BIGINT),
		  EndDate = x.iConcatenate
	FROM #Sample s -- CI scan
	CROSS APPLY ( -- CI seek
		SELECT TOP 1 
			iConcatenate = CAST(i.[Concatenate] AS BIGINT)  
		FROM #Sample i 
		WHERE i.[Concatenate] > s.[Concatenate]
		ORDER BY i.[Concatenate]) x
	WHERE x.iConcatenate - CAST(s.[Concatenate] AS BIGINT)  > 1
)
---------------------------
-- Construct the missing rows
-- Start and End date from GapBounds tells us how many rows to take from iTally
-- Add 0-based sequence to Startdate to obtain the date for the row 
SELECT CAST(StartDate + gaprows.rn AS CHAR(12))
FROM GapBounds g
CROSS APPLY (
	SELECT TOP (EndDate+1-StartDate)  
		rn = ROW_NUMBER() OVER(ORDER BY StartDate)-1
	FROM iTally 
) gaprows