How many simple assaults happened on 2018/9/8?
simple assault refers to primary_description = 'ASSAULT'AND secondary_description = 'SIMPLE'; on 2018/9/8 refers to date like '%9/8/2018%'
SELECT SUM(CASE WHEN T2.secondary_description = 'SIMPLE' THEN 1 ELSE 0 END) FROM Crime AS T1 INNER JOIN IUCR AS T2 ON T1.iucr_no = T2.iucr_no WHERE T1.date LIKE '%9/8/2018%' AND T2.primary_description = 'ASSAULT'