SQLCount distinct of one column based on another column - sql

Question

I need to count the sessions which visited a particular page once and sessions which visited the same page once or more than once. For example: consider these sessions between 1st to 4th April:
Session_id| Date
----+---------
1| 01/04/2016
1| 02/04/2016
2| 01/04/2016
3| 01/04/2016
4| 01/04/2016
4| 03/04/2016
4| 04/04/2016
I can not do it using a sub query as there are millions of session. Hence a query like this won't work for me:
select case when no_of_visits=1 then 'single_visit'
when no_of_visits>1 then 'multiple_visits' end as visit,
count(distinct session_id) as sessions
FROM (
select session_id,
count(distinct date) as no_of_visits
from my_table
group by session_id
) a
group by case when no_of_visits=1 then 'single_visit'
when no_of_visits>1 then 'multiple_visits' end
The answer should be like this:
Visit|Sessions
single_visit|2
multiple_visit|2
Is there any way where i can do something like this:
count(distinct session_id) where no_of_visits=1 and count(distinct session_id) where no_of_visits>=1 without subquery or self join?
Any help would be deeply appreciated.

Answer 1

You can't avoid subqueries to get this result, but you can get rid of the count(distinct) (which is probably the most expensive part):
select no_of_visits,
count(*) as sessions
FROM
(
select session_id,
case when min(date) <> max(date) -- at least two different dates
then 'multiple_visits'
else 'single_visit'
end as no_of_visits
from table_name
group by session_id
) a
group by no_of_visits

Answer 2

It seems you want something like this:
select session_id, count(distinct dt) as no_of_visits,
case when count(distinct dt) = 1 then "Single visit"
else "Multiple visits" end as visit
from my_table
group by session_id;
Note: I used dt rather than "date" as a column name; date is an Oracle key word, and such words should not be used as schema, table or column names.

Answer 3

Oracle Setup:
CREATE TABLE table_name ( Session_id, "Date" ) AS
SELECT 1, DATE'2016-04-01' FROM DUAL UNION ALL
SELECT 1, DATE'2016-04-02' FROM DUAL UNION ALL
SELECT 2, DATE'2016-04-01' FROM DUAL UNION ALL
SELECT 3, DATE'2016-04-01' FROM DUAL UNION ALL
SELECT 4, DATE'2016-04-01' FROM DUAL UNION ALL
SELECT 4, DATE'2016-04-03' FROM DUAL UNION ALL
SELECT 4, DATE'2016-04-04' FROM DUAL;
Query 1:
SELECT visit,
COUNT(1) AS Sessions
FROM (
SELECT session_id,
CASE WHEN COUNT(1) > 1
THEN 'Multiple Visits'
ELSE 'Single Visit'
END AS visit
FROM table_name
GROUP BY session_id
)
GROUP BY visit;
Output:
VISIT SESSIONS
--------------- ----------
Multiple Visits 2
Single Visit 2
Query 2:
SELECT first_visit - multiple_visits AS single_visit,
multiple_visits
FROM (
SELECT COUNT( CASE rn WHEN 1 THEN 1 END ) AS first_visit,
COUNT( CASE rn WHEN 2 THEN 1 END ) AS multiple_visits
FROM (
SELECT rn --
FROM ( --
SELECT ROW_NUMBER() OVER ( PARTITION BY session_id ORDER BY ROWNUM ) AS rn
FROM table_name
) --
WHERE rn <= 2 -- These lines are optional
)
);
Output:
SINGLE_VISIT MULTIPLE_VISITS
------------ ---------------
2 2