Row to column conversion in large result set

Question

I need to get the differnt levels of alerts for each user, with the total alert counts and counts for each type of alert.
SELECT COUNT(*) TOTAL,
        u.name,
        d.deptname,
        COUNT(CASE WHEN al.severity = 5 THEN 1 END) INDETERMINATE,
        COUNT(CASE WHEN al.severity = 4 THEN 1 END) WARNING,
        COUNT(CASE WHEN al.severity = 3 THEN 1 END) MINOR,
        COUNT(CASE WHEN al.severity = 2 THEN 1 END) MAJOR,
        COUNT(CASE WHEN al.severity = 1 THEN 1 END) CRITICAL
FROM alerts al, user u, dept d
WHERE al.userid = u.userid
  AND u.deptid = d.deptid
  AND al.time = sysdate - 4/24
  AND al.time <= sysdate
GROUP BY u.name, d.deptname  This might be inefficient especially when the amount of data is huge and potentially you could be looking at millions of rows in a given interval. So I was thinking of grouping by severity, which would eliminate the CASE statements, but then the problem on group by is that I get it as a rows instead of columns.
Would anyone have a suggestion as to do it effeciently especially one that has millions of rows? Thanks.

Answer 1

I guess eventually we are probably going to move towards that, but using a staright forward query is the immediate solution and any help on that would be great. Thanks.