如果满足条件，则SQL查询返回1列，否则返回0列

Question

I've created a nice query to watch all the unique IPs entering my site. The schema is as follows:

Page_Statistic - Contains data on each page hit (one record per hit).

• page_statistic_id.
• ip - ip address of visitor.
• tag - used to record special things, like "demo" if they tried the demo.
• url - of the page they visited.
• requested_on - time of visit.

Ip_Location_Info - Contains location data on each visitor (one record per ip).

• ip
• country
• region
• city
• zip

I want to get all the unique IP addresses, with locations, number of visits, time range, and whether they tried the demo on my website - and this last part is what I'm having trouble with. I want a 1 if they've tried the demo, and 0 if they haven't. An IP has tried the demo iff a tupple exists in Page_Statistic with tag set to 'demo'.

I've created the following query which solves this:

WITH uniqueips(frst, lst, ip, visits) AS
(
    SELECT MIN(requested_on), MAX(requested_on), ip, count(*) AS visits
    FROM Page_Statistic p
    GROUP BY ip
),
tried_demo(ip, tried_demo) AS
(
    SELECT DISTINCT ip.ip, ISNULL(p.page_statistic_id - p.page_statistic_id + 1, 0) 
    FROM uniqueips ip
    LEFT JOIN Page_Statistic p ON p.ip = ip.ip AND p.tag = 'demo'
)
SELECT 
i.*, ip.frst AS first_appeared_on, 
    ip.lst AS last_appeared_on, 
    ip.visits,
    d.tried_demo
FROM Ip_Location_Info i
JOIN uniqueips ip ON ip.ip = i.ip
JOIN tried_demo d ON ip.ip = d.ip
ORDER BY ip.frst

But the ugly part is where I set the 1 or 0 for tried_demo.

ISNULL(p.page_statistic_id - p.page_statistic_id + 1, 0)

This works because if p.page_statistic_id is null, adding 1 to null is null so isnull will return the second item in the list of params (0). It's such a hack! What is the correct way to do this?

Answer 1

The way you have it works, so that's good, but if you wanted to change it, you could change it to

case when p.page_statistic_id is null then 0 else 1 end

OR

change your tried_demo CTE to this

tried_demo(ip, tried_demo) AS
(
    SELECT DISTINCT p.ip, cast(1 as bit) 
    FROM Page_Statistic 
    WHERE p.tag = 'demo'
)

then your select can be like this

SELECT 
i.*, ip.frst AS first_appeared_on, 
    ip.lst AS last_appeared_on, 
    ip.visits,
    coalesce(d.tried_demo,0) as tried_demo
FROM Ip_Location_Info i
JOIN uniqueips ip ON ip.ip = i.ip
LEFT JOIN tried_demo d ON ip.ip = d.ip
ORDER BY ip.frst