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'),  
('lmn','blat'),
('pqr','fg/456');

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

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

WITH src AS 
  (
    SELECT col1, pf = LEFT(col2, 
           NULLIF(charindex('/', col2)-1,-1))
    FROM dbo.t1
  ),
  agg AS 
  (
    SELECT col1, agg = STRING_AGG(pf, ',')
    FROM (SELECT col1, pf FROM src GROUP BY col1, pf) AS x
    GROUP BY col1
  )
  UPDATE t2 SET t2.col2 = agg.agg
    FROM dbo.t2
    INNER JOIN agg 
    ON t2.col1 = agg.col1;
3 rows affected

SELECT * FROM dbo.t2;


col1
col2


xyz
ab,fg

pqr
fg,uv

lmn
null