Parent   Child
A1        B1
A1        B2
A1        B3
B1        C1
B1        C2
B1        C3
C1        D1
C1        D2
D1        E1
D1        E2

A2        B4
A2        B5
B5        C4

Result
B1
B2
B3
C1
C2
C3
D1
D2
E1
E2

DECLARE @Table  Table(
  Parent varchar(2),
  Child  varchar(2) )
INSERT @Table 
SELECT 'A1','B1' UNION ALL
SELECT 'A1','B2' UNION ALL
SELECT 'A1','B3' UNION ALL
SELECT 'B1','C1' UNION ALL
SELECT 'B1','C2' UNION ALL
SELECT 'B1','C3' UNION ALL
SELECT 'C1','D1' UNION ALL
SELECT 'C1','D2' UNION ALL
SELECT 'D1','E1' UNION ALL
SELECT 'D1','E2' UNION ALL
SELECT 'A2','B4' UNION ALL
SELECT 'A2','B5' UNION ALL
SELECT 'B5','C4';

WITH Dependencies AS(
  SELECT Parent, Child
  FROM @Table
  WHERE Parent = 'A1'
  UNION ALL
  SELECT t.Parent, t.Child
  FROM @Table t
  JOIN Dependencies d ON t.Parent = d.Child
)
SELECT Child
FROM Dependencies

WITH Dependencies AS(
  SELECT Parent, Child
  FROM @Table
  WHERE Parent = 'CAR'
  UNION ALL
  SELECT t.Parent, t.Child
  FROM @Table t
  JOIN Dependencies d ON t.Parent = d.Child
)
SELECT Parent,Child
FROM Dependencies