SQL Server 2022

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CREATE TABLE dbo.t1(col1 varchar(32), col2 varchar(32));

INSERT dbo.t1 VALUES
('xyz','ab/cde'), 
('pqr','uv/wxy'),
('xyz','fg/hij'),
('xyz','blat'),
('lmn','rtg'),
('pqr','12/456');

CREATE TABLE dbo.t2(col1 varchar(32), col2 varchar(32));

INSERT dbo.t2 VALUES
('xyz', null),
('pqr', null);   
8 rows affected

UPDATE t2 SET t2.col2 = agg.agg
  FROM dbo.t2
  INNER JOIN (
SELECT col1, agg = STRING_AGG(SUBSTRING(col2, 1, 
   NULLIF(charindex('/', col2)-1,-1)), ',')
FROM dbo.t1
GROUP BY col1) agg
  ON t2.col1 = agg.col1;
2 rows affected

SELECT * FROM dbo.t2;


col1
col2


xyz
fg,ab

pqr
12,uv


UPDATE t2 SET t2.col2 = agg.agg
  FROM dbo.t2
  INNER JOIN (
SELECT col1, agg = STRING_AGG(SUBSTRING(col2, 1, 
   COALESCE(NULLIF(charindex('/', col2)-1,-1), 255)), ',')
FROM dbo.t1
GROUP BY col1) agg
  ON t2.col1 = agg.col1;
2 rows affected

SELECT * FROM dbo.t2;


col1
col2


xyz
fg,blat,ab

pqr
12,uv