Calculate the difference in the average number of vehicular hijackings and aggravated vehicular hijackings in the districts.
"VEHICULAR HIJACKING" and "AGGRAVATED VEHICULAR HIJACKING" are both secondary_description; difference in average = Subtract (Divide(Count(secondary_description = 'VEHICULAR HIJACKING'), Count(district_name)), Divide(Count(secondary_description = "AGGRAVATED VEHICULAR HIJACKING"), Count(district_name)))
SELECT ROUND(CAST(COUNT(CASE WHEN T1.secondary_description = 'VEHICULAR HIJACKING' THEN T1.iucr_no END) AS REAL) / CAST(COUNT(DISTINCT CASE WHEN T1.secondary_description = 'VEHICULAR HIJACKING' THEN T3.district_name END) AS REAL) - CAST(COUNT(CASE WHEN T1.secondary_description = 'AGGRAVATED VEHICULAR HIJACKING' THEN T1.iucr_no END) AS REAL) / CAST(COUNT(DISTINCT CASE WHEN T1.secondary_description = 'AGGRAVATED VEHICULAR HIJACKING' THEN T3.district_name END) AS REAL), 4) AS "difference" FROM IUCR AS T1 INNER JOIN Crime AS T2 ON T2.iucr_no = T1.iucr_no INNER JOIN District AS T3 ON T3.district_no = T2.district_no