SQL Server 2019 (Linux)

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CREATE TABLE table_name ( qty VARCHAR(20) );
INSERT INTO table_name ( qty )
SELECT '10*4*2' UNION ALL
SELECT '4*3*1' UNION ALL
SELECT '5*1*1' UNION ALL
SELECT '7';
4 rows affected

WITH multiplied_values ( qty, value, start_pos, end_pos ) AS (
  SELECT qty,
         1,
         1,
         CHARINDEX( '*', qty, 1 )
  FROM   table_name
UNION ALL
  SELECT qty,
         value * CAST( SUBSTRING( qty, start_pos, end_pos - start_pos ) AS INT ),
         end_pos + 1,
         CHARINDEX( '*', qty, end_pos + 1 )
  FROM   multiplied_values
  WHERE  end_pos > 0
)
SELECT qty,
       value * CAST( SUBSTRING( qty, start_pos, LEN( qty ) - start_pos + 1 ) AS INT )
         AS value
FROM   multiplied_values
WHERE  end_pos = 0;


qty
value


7
7

5*1*1
5

4*3*1
12

10*4*2
80