Write SQL query to solve given problem: What is the percentage of severe cases that are related to sexual assault?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT CAST(SUM(CASE WHEN primary_description = 'CRIM SEXUAL ASSAULT' THEN 1 ELSE 0 END) AS REAL) * 100 / COUNT(*) FROM IUCR WHERE index_code = 'I'