SQL query - Selecting records based on count condition

Question

The title may not seem very clear - was not sure how to explain the problem in one line.

I have 3 tables

TOPIC
VIDEO 
TOPIC_VIDEO

A topic can have one or two videos. The videos are either sample videos or not.

Sample data in the tables with relevant column names

TOPIC

TOPIC_ID |         NAME          | COURSE_ID
    1    | Excel - Add/Subtract  |     1
    2    | Excel - sort          |     1
    3    | Excel - filter        |     1
    4    | Excel - formulas      |     1


VIDEO

VIDEO_ID | VIDEO_URL
    10   | www.youtube.com?v=123
    12   | www.youtube.com?v=345
    13   | www.youtube.com?v=567
    14   | www.youtube.com?v=879
    15   | www.youtube.com?v=443


TOPIC_VIDEO

TOPIC_VIDEO_ID | TOPIC_ID | VIDEO_ID | IS_SAMPLE    
      1        |    1    |    10    |    Y
      2        |    2    |    12    |    N
      3        |    3    |    13    |    N
      4        |    3    |    14    |    Y
      5        |    4    |    15    |    N

So what I am trying to do is for a given course select all topics and their corresponding videos. Now if a topic has more than one video - I want to select the video with is_sample as "N".

Like in the above example - topic_id 3 has two videos with video_id 13 and 14 - so the video_id 13 should be selected.

This is the query I have so far

select topic.*,count(topic.topic_id),video.video_id,topic_video.is_sample
from topic
left join topic_video ON topic_video.topic_id = topic.topic_id
left join video ON video.video_id = topic_video.video_id
where course_id=1
group by topic.topic_id

So now if the count is 2 - I want to select the record with is_sample='N' Is this possible and how can I accomplish this. Thanks a lot for your time

Answer 1

One way to solve this is to

• Join to an inline view that calculates the count by topic.
• Use the count to create a case statement that only joins to IS_SAMPLE = N

SELECT * 
FROM   topic 
       LEFT JOIN (SELECT topic_id, 
                         Count(topic_id) t_count 
                  FROM   TOPIC_VIDEO 
                  GROUP  BY topic_id) t 
              ON topic.topic_id = t.topic_id 
       LEFT JOIN topic_video 
              ON topic_video.topic_id = topic.topic_id 
                 AND TOPIC_VIDEO.IS_SAMPLE = CASE 
                                               WHEN t.t_count = 2 THEN 'N' 
                                               ELSE TOPIC_VIDEO.IS_SAMPLE 
                                             END 
       LEFT JOIN video 
              ON video.video_id = topic_video.video_id 

DEMO

Output

| TOPIC_ID |                 NAME | COURSE_ID | T_COUNT | TOPIC_VIDEO_ID | VIDEO_ID | IS_SAMPLE |             VIDEO_URL |
-------------------------------------------------------------------------------------------------------------------------
|        1 | Excel - Add/Subtract |         1 |       1 |              1 |       10 |         Y | www.youtube.com?v=123 |
|        2 |         Excel - sort |         1 |       1 |              2 |       12 |         N | www.youtube.com?v=345 |
|        3 |       Excel - filter |         1 |       2 |              3 |       13 |         N | www.youtube.com?v=567 |
|        4 |     Excel - formulas |         1 |       1 |              5 |       15 |         N | www.youtube.com?v=443 |