MySQL 8.0

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CREATE TABLE test (id INT AUTO_INCREMENT PRIMARY KEY,  -- PK, value will not be provided
                   single_arg INT,                     -- value will be provided as separate
                   csv_arg INT);                       -- value will be provided as CSV
-- Provided parameters values
SET @single_arg := 123;
SET @csv_arg := '321,654,987';
-- Check how recursive CTE parses and prepares the data for the insertion
WITH RECURSIVE
cte AS ( SELECT @single_arg single_arg,
                SUBSTRING_INDEX(@csv_arg, ',', 1) single_from_csv,
                TRIM(LEADING ',' FROM TRIM(LEADING SUBSTRING_INDEX(@csv_arg, ',', 1) FROM @csv_arg)) slack_from_csv
         UNION ALL
         SELECT single_arg,
                SUBSTRING_INDEX(slack_from_csv, ',', 1),
                TRIM(LEADING ',' FROM TRIM(LEADING SUBSTRING_INDEX(slack_from_csv, ',', 1) FROM slack_from_csv))
         FROM cte
         WHERE TRIM(slack_from_csv) <> '')
SELECT *
FROM cte;


single_arg
single_from_csv
slack_from_csv


123
321
654,987

123
654
987

123
987



INSERT INTO test (single_arg, csv_arg)
WITH RECURSIVE
-- intermediate CTE which allows to provide source data only once
source_data AS ( SELECT @single_arg single_arg,
                        @csv_arg csv_arg ),
-- CTE which parses the data
cte AS ( SELECT single_arg,
                SUBSTRING_INDEX(csv_arg, ',', 1) single_from_csv,
                TRIM(LEADING ',' FROM TRIM(LEADING SUBSTRING_INDEX(csv_arg, ',', 1) FROM csv_arg)) slack_from_csv
         FROM source_data
         UNION ALL
         SELECT single_arg,
                SUBSTRING_INDEX(slack_from_csv, ',', 1),
                TRIM(LEADING ',' FROM TRIM(LEADING SUBSTRING_INDEX(slack_from_csv, ',', 1) FROM slack_from_csv))
         FROM cte
         WHERE TRIM(slack_from_csv) <> '' )
-- штывуке parsed data into the table
SELECT single_arg, 
       single_from_csv
FROM cte;

Records: 3  Duplicates: 0  Warnings: 0


SELECT * FROM test;


id
single_arg
csv_arg


1
123
321

2
123
654

3
123
987