I think you need to use a recursive CTE in this case, so that all possible relations are created through joining to the data itself .
if you are not familiar with recursive CTE's Dwain Camps has a decent article here: http://www.sqlservercentral.com/articles/T-SQL/90955/
your example has only a single parent-child pair, but this would go deeper, if the rest of the data is more hierarchical that the example.


/*--Results
Bold_ID  Code    Parentphase
5144471  1000317_EV  1
5144535  100317_SUPPORT 5144471
*/
IF OBJECT_ID('[dbo].[zzzTest]') IS NOT NULL
DROP TABLE [dbo].[zzzTest]
GO
CREATE TABLE zzzTest(
  Bold_ID int,Code varchar(255),Parentphase int)

INSERT INTO zzzTest
values ('5144471', '1000317_EV', '1');
insert into zzztest
values ('5144535', '100317_SUPPORT', '5144471');

--get the magically created hierarchy
;WITH cte
AS (
SELECT Bold_ID, Code,Parentphase FROM zzzTest WHERE Parentphase = 1
UNION ALL
SELECT t1.Bold_ID, t1.Code,t1.Parentphase
FROM zzztest t1
INNER JOIN cte c1
  ON c1.Bold_ID = t1.Parentphase
)
 
SELECT *
FROM cte
ORDER BY Parentphase