Fazendo uma junção à esquerda e fazendo com que todas as correspondências incluam uma linha nula extra

Eu vejo 3 maneiras de fazer isso, mas todas envolvem UNION ALL:

• sua versão`:

  SELECT
      T1.C1, ....., T1.CN,
      T2.C1, ..., T2.CM
  FROM
      Table1 T1 JOIN Table2 T2
          ON T1.Key1 = T2.Key1
  
  UNION ALL
  
  SELECT
      T1.C1, ..., T1.CN,
      NULL, ... NULL
  FROM
      Table1 T1 ;
  

• mudando ligeiramente a segunda parte:

  SELECT
      T1.C1, ....., T1.CN,
      T2.C1, ..., T2.CM
  FROM
      Table1 T1 JOIN Table2 T2
          ON T1.Key1 = T2.Key1
  
  UNION ALL
  
  SELECT
      T1.C1, ....., T1.CN,
      T2.C1, ..., T2.CM
  FROM
      Table1 T1 LEFT JOIN Table2 T2
          ON 0 = 1 ;                   -- FALSE
  

• primeiro a UNION, depois junte-se:

  SELECT
      T1.C1, ....., T1.CN,
      T2.C1, ..., T2.CM
  FROM
      Table1 T1 JOIN  
        ( SELECT * FROM T2
          UNION ALL
          SELECT NULL, ..., NULL
         ) AS T2
          ON T1.Key1 = T2.Key1 
          OR T2.Key1 IS NULL ;
  

Não acho que haverá muita diferença nos planos de execução e eficiência, mas o primeiro parece mais simples.

Aqui está outra opção que leva apenas uma única passagem pelos dados, realizando a junção à esquerda e, em seguida, para cada linha resultante, acrescentando a linha NULL, se necessário.

SELECT Results.*
FROM #Table1 T1
LEFT OUTER JOIN #Table2 T2
    ON T1.Key1 = T2.Key1
CROSS APPLY (
    SELECT T1.C1, T1.C2, T2.C1 AS T2_C1, T2.C2 AS T2_C2
    UNION ALL
    SELECT T1.C1, T1.C2, NULL, NULL
    WHERE T2.Key1 IS NOT NULL
) Results

Se usarmos o script de teste a seguir, podemos ver que obtemos os resultados corretos e que o plano de execução passa uma única vez por #Table1 e #Table2. Pelo menos neste conjunto de dados de 10.000 linhas, o desempenho em termos de tempo de execução e IO é melhorado quando comparado com as soluções anteriores que foram apresentadas.

-- Create fake data with 10,000 rows each, 5,000 of which match
CREATE TABLE #Table1 (Key1 INT NOT NULL IDENTITY(1,1) PRIMARY KEY, C1 INT NOT NULL, C2 INT NOT NULL)
GO
CREATE TABLE #Table2 (Key1 INT NOT NULL IDENTITY(1,2) PRIMARY KEY, C1 INT NOT NULL, C2 INT NOT NULL)
GO
INSERT INTO #Table1 VALUES (1,2)
GO 10000
INSERT INTO #Table2 VALUES (3,4)
GO 10000

SET STATISTICS TIME, IO ON 

-- The proposed solution
-- CPU time = 16 ms,  elapsed time = 113 ms.
--Table '#Table1'. Scan count 1, logical reads 28, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
--Table '#Table2'. Scan count 1, logical reads 15, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
SELECT Results.*
FROM #Table1 T1
LEFT OUTER JOIN #Table2 T2
    ON T1.Key1 = T2.Key1
CROSS APPLY (
    SELECT T1.C1, T1.C2, T2.C1 AS T2_C1, T2.C2 AS T2_C2
    UNION ALL
    -- Add the extra row iff the LEFT JOIN matched, but don't require
    -- another read of the #Table1 table in order to do so
    SELECT T1.C1, T1.C2, NULL, NULL
    WHERE T2.Key1 IS NOT NULL
) Results
GO

-- ypercube #1: Basically the same performance, but twice as many reads on #Table1
-- CPU time = 16 ms,  elapsed time = 115 ms.
--Table '#Table1'. Scan count 2, logical reads 56, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
--Table '#Table2'. Scan count 1, logical reads 15, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
SELECT T1.C1, T1.C2, T2.C1, T2.C2 
FROM
    #Table1 T1 JOIN #Table2 T2
        ON T1.Key1 = T2.Key1
UNION ALL
SELECT T1.C1, T1.C2, NULL, NULL
FROM #Table1 T1 ;
GO

-- ypercube #3: We end up with 10,000 seeks into #Table2, a much less efficient plan
-- Note that SQL Server chooses the same plan (but with scan instead of seek) if there is no primary key as well
-- This results in over 30,000ms of CPU time for the query.  SQL does not seem to be processing this query very well!
-- CPU time = CPU time = 94 ms,  elapsed time = 140 ms.
--Table '#Table1'. Scan count 2, logical reads 56, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
--Table '#Table2'. Scan count 10000, logical reads 20024, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.  --Table 'Worktable'. Scan count 8, logical reads 531141, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
SELECT T1.C1, T1.C2, T2.C1, T2.C2 
FROM #Table1 T1 JOIN  
      ( SELECT * FROM #Table2
        UNION ALL
        SELECT NULL, NULL, NULL
       ) AS T2
        ON T1.Key1 = T2.Key1 
        OR T2.Key1 IS NULL;
GO

Aqui está o plano: