Richard is correct about the 12, and Keith's solution should work just fine.
This is an alteration to the CTE that should also work. No idea which is better - Perhaps speed test the two and pick the faster one.
The difference here is simply a well-placed modulo 12.
```
WITH HR(EventDate, EventTime, EventHour) AS (
SELECT EventDate, EventTime,
CAST(substr(EventTime,0,instr(EventTime,':')) AS INT) % 12 +
CASE WHEN EventTime LIKE '%PM' THEN 12 ELSE 0 END
FROM trafficdata
)
SELECT EventDate, EventHour, COUNT(*) AS AccidentsInHour
FROM HR
GROUP BY EventDate, EventHour
;
```