问sql查询以查找表中特定事件的最大小时

EN

SELECT  period, max(nb)
FROM (
        SELECT extract(hour from crime_time) as period, count(*) as nb
        FROM crimes    
        WHERE crime_string = 'Robbery'
        GROUP BY extract(hour from crime_time)   
    ) as  subquery_alias
GROUP BY period

SELECT max(nb), period
FROM (
    SELECT count(*) as nb, period
    FROM (
        SELECT crime_string, extract(hour from crime_time) as period
        FROM crimes
        WHERE crime_string = 'Robbery'
    )
    GROUP BY period
);