how to do math function (divide) for results of a group by function - sql

Question

I have a table of 6 digit event numbers;
i am trying to count one event number (let's say 600189) that has occurred during a specific time period, and divide it by a count of a second event number (lets say 600122) during the same time period.
the below query gets my two counts but I want to modify the query to do the calculation (count of 600189 / count of 600122) * 100 :
select count (messageno)
from event
where timestamp > '2019-03-14' and timestamp < '2019-03-15' and
messageno in ('600122','600189')
group by messageno

Answer 1

I would simply use conditional aggregation:
select sum(case when messageno = '600122' then 1 else 0 end) as cnt_1,
sum(case when messageno = '600189' then 1 end) as cnt_2,
( sum(case when messageno = '600122' then 1 else 0 end) /
sum(case when messageno = '600189' then 1 end)
) as ratio
from event
where timestamp > '2019-03-14' and
timestamp < '2019-03-15' and
messageno in ('600122', '600189');
Note that the else in the denominator is removed. This is intentional. If there are no messages with that number, then this returns NULL instead of a divide-by-zero error.

Answer 2

There are a couple of query patterns.
One approach is conditional aggregation. We can return a value of 1 if a condition is true, otherwise return 0, and add up the 1s and 0s with a SUM aggregate to get a count. Consider:
For MySQL, we can do something like this:
SELECT SUM( IF(e.messageno='600122',1,0) ) AS cnt_600122
, SUM( IF(e.messageno='600189',1,0) ) AS cnt_600189
, ( 100.0
* SUM( IF(e.messageno='600122',1,0) )
/ SUM( IF(e.messageno='600189',1,0) )
) AS pct
FROM event e
WHERE e.timestamp > '2019-03-14'
AND e.timestamp < '2019-03-15'
AND e.messageno IN ('600122','600189')
More portable ANSI standards compliant equivalent will work for Microsoft SQL Server et al.:
SELECT SUM( CASE e.messageno WHEN '600122' THEN 1 ELSE 0 END ) AS cnt_600122
, SUM( CASE e.messageno WHEN '600189' THEN 1 ELSE 0 END ) AS cnt_600189
, ( 100.0
* SUM( CASE e.messageno WHEN '600122' THEN 1 ELSE 0 END )
/ SUM( CASE e.messageno WHEN '600122' THEN 1 ELSE 0 END )
) AS pct
FROM event e
WHERE e.timestamp > '2019-03-14'
AND e.timestamp < '2019-03-15'
AND e.messageno IN ('600122','600189')
Other approaches would be to do aggregation in an inline view(s), or use subqueries in the SELECT list:
SELECT 100.0
* ( SELECT SUM(1)
FROM event e
WHERE e.timestamp > '2019-03-14'
AND e.timestamp < '2019-03-15'
AND e.messageno IN ('600122')
)
/ ( SELECT SUM(1)
FROM event e
WHERE e.timestamp > '2019-03-14'
AND e.timestamp < '2019-03-15'
AND e.messageno IN ('600189')
)
AS pct