IF OBJECT_ID( 'tempdb..#t' ) IS NOT NULL
  DROP TABLE #t
;
CREATE TABLE #t ( [ID] INT, [MainID] INT, [DocTypeID] INT, [DocNum] VARCHAR(50) )
;
INSERT INTO #t
VALUES
( 1, 1, 7, 'N 1024' ),
( 2, 1, 8, 'N 1025' ),
( 3, 1, 8, 'N 1026' ),
( 4, 2, 7, 'N 1027' )
;
SELECT
  [MainID]
FROM
  #t
WHERE
  [DocTypeID] IN ( 7,8 )
GROUP BY
  [MainID]
HAVING
  COUNT( DISTINCT [DocTypeID] ) = 2
;
IF OBJECT_ID( 'tempdb..#t' ) IS NOT NULL
  DROP TABLE #t
;

IF OBJECT_ID( 'tempdb..#t' ) IS NOT NULL
  DROP TABLE #t
;
IF OBJECT_ID( 'tempdb..#g' ) IS NOT NULL
  DROP TABLE #g
;
CREATE TABLE #g ( [DocTypeID] INT )
INSERT INTO #g
VALUES
  ( 7 ),
  ( 8 )
;
CREATE TABLE #t ( [ID] INT, [MainID] INT, [DocTypeID] INT, [DocNum] VARCHAR(50) )
;
INSERT INTO #t
VALUES
( 1, 1, 7, 'N 1024' ),
( 2, 1, 8, 'N 1025' ),
( 3, 1, 8, 'N 1026' ),
( 4, 2, 7, 'N 1027' )
;
/* Ex1 */
WITH
g AS (
  SELECT
    [count] = COUNT(*)
  FROM
    #g
),
t AS (
  SELECT
    t.[MainID],
    [count] = COUNT( DISTINCT t.[DocTypeID] )
  FROM
    #g g
    INNER JOIN #t t ON (
          t.[DocTypeID] = g.[DocTypeID] )
  GROUP BY
    t.[MainID]
)
SELECT
  t.[MainID]
FROM
  g
  INNER JOIN t ON (
        t.[count] = g.[count] )
;
/* Ex2 */
DECLARE @cnt INT = ( SELECT COUNT(*) FROM #g )
;
SELECT
  t.[MainID]
FROM
  #g g
  INNER JOIN #t t ON (
        t.[DocTypeID] = g.[DocTypeID] )
GROUP BY
  t.[MainID]
HAVING
  COUNT( DISTINCT t.[DocTypeID] ) = @cnt
;
IF OBJECT_ID( 'tempdb..#t' ) IS NOT NULL
  DROP TABLE #t
;
IF OBJECT_ID( 'tempdb..#g' ) IS NOT NULL
  DROP TABLE #g
;