Spiting first part and last part from string - tsql

Question

I am looking for separating string into first part and last part. Basically, last word is considered as last part and the remaning part is called first part. Below is what I have tried so far
DECLARE #Col TABLE
(
ColumnName VARCHAR(100)
)
INSERT INTO #Col(ColumnName)
VALUES
('ABC, DEF GHI,'), -- Expeted output: FirstPart: ABC, DEF LastPart: GHI
('A,'), -- Expected Output: FirstPart: NULL LastPart: A
(' '), -- Expected Output: FirstPart: NULL LastPart: NULL
(''), -- Expected Output: FirstPart: NULL LastPart: NULL
('A , '),-- Expected Output: FirstPart: NULL LastPart: A
('ABC DEF G.HI, ') -- Expected Output: FirstPart: ABC DEF LastPart: G.HI
SELECT '' AS FirstPart, -- I don't know how to get the FirstPart of the string
SUBSTRING(ColumnName, 1, LEN(ColumnName) - CHARINDEX(' ', REVERSE(ColumnName))) AS LastPart FROM #Col

Answer 1

Here's one way to do it - using a couple of common table expressions and relying on the fact that trailing spaces are ignored in string comparison I came up with this (I know it looks cumbersome and perhaps it's the time (it's almost 1 A.M here), but it gives the correct results):
WITH CTE1 AS
(
SELECT ColumnName,
CHARINDEX(' ', REVERSE(ColumnName), PATINDEX('%[A-Z]%', REVERSE(ColumnName))) As LastSpaceBeforeLastWord
FROM #Col
), CTELastPart AS
(
SELECT ColumnName,
LastSpaceBeforeLastWord,
TRIM(
CASE
-- LEN(' ') also returns 0...
WHEN LEN(ColumnName) = 0 THEN NULL
WHEN LastSpaceBeforeLastWord = 0 THEN ColumnName
ELSE RIGHT(ColumnName, LastSpaceBeforeLastWord)
END
) As LastPart
FROM CTE1
)
-- ' ' equals ''...
SELECT ColumnName, TRIM(NULLIF(REPLACE(ColumnName, LastPart, ''), '')) As FirstPart, LastPart
FROM CteLastPart
Results:
ColumnName FirstPart LastPart
ABC, DEF GHI, ABC, DEF GHI,
A, NULL A,
NULL NULL
NULL NULL
A , NULL A ,
ABC DEF G.HI, ABC DEF G.HI,
Online demo on DB<>Fiddle

Answer 2

Here's how I'd do it:
SELECT
c.ColumnName,
FirstPart = SUBSTRING(clean.string,0,NULLIF(f.X,1)),
LastPart = LTRIM(SUBSTRING(clean.string, f.X, 8000))
FROM #Col AS c
CROSS APPLY (VALUES(REPLACE(c.ColumnName,',',''))) AS clean(string)
CROSS APPLY (VALUES(
NULLIF(LEN(clean.string),0)+SIGN(LEN(clean.string)-1)-
CHARINDEX(' ',RTRIM(LTRIM(REVERSE(clean.string)))))) AS f(X);
Returns:
ColumnName FirstPart LastPart
-------------------- ----------- ----------
ABC, DEF GHI, ABC DEF GHI
A, NULL A
NULL NULL
NULL NULL
A , NULL A
ABC DEF G.HI, ABC DEF G.HI