Oracle 21c

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CREATE TABLE employee (id NUMBER);
with t (key, vals) as (
    SELECT 'k1', 'a,b'   FROM DUAL UNION ALL
    SELECT 'k2', 'c,d,e' FROM DUAL
)
SELECT key, val
FROM   t
       LEFT OUTER JOIN LATERAL (
         SELECT regexp_substr(vals, '[^,]+', 1, level) AS val
         FROM   DUAL
         CONNECT BY LEVEL <= REGEXP_COUNT(vals, '[^,]+')
       )
       ON (1 = 1)


KEY
VAL


k1
a

k1
b

k2
c

k2
d

k2
e


with t (key, vals) as (
    SELECT 'k1', 'a,b'   FROM DUAL UNION ALL
    SELECT 'k2', 'c,d,e' FROM DUAL
)
SELECT key,
       regexp_substr(vals, '[^,]+', 1, level) AS val
FROM   t
CONNECT BY LEVEL <= REGEXP_COUNT(vals, '[^,]+')
AND PRIOR key = key
AND PRIOR SYS_GUID() IS NOT NULL;


KEY
VAL


k1
a

k1
b

k2
c

k2
d

k2
e


with t (key, vals) as (
    SELECT 'k1', 'a,b'   FROM DUAL UNION ALL
    SELECT 'k2', 'c,d,e' FROM DUAL
),
bounds (key, vals, spos, epos) AS (
  SELECT key, vals, 1, INSTR(vals, ',', 1)
  FROM t
UNION ALL
  SELECT key, vals, epos + 1, INSTR(vals, ',', epos + 1)
  FROM bounds
  WHERE  epos > 0
)
SEARCH DEPTH FIRST BY key SET key_order
SELECT key,
       CASE epos
       WHEN 0
       THEN SUBSTR(vals, spos)
       ELSE SUBSTR(vals, spos, epos - spos)
       END AS val
FROM   bounds;


KEY
VAL


k1
a

k1
b

k2
c

k2
d

k2
e