Question

I have three table in the Database –

1. Activity table with activity_id, activity_type
2. Category table with category_id, category_name
3. Link table with mapping between activity_id and category_id

I need to write a select statement to get the following data:

activity_id, activity_type, Category_name.

The issue is some of the activity_id have no entry in the link table.

If I write:

select a.activity_id, a.activity_type, c.category_name 
from activity a, category c, link l 
where a.activity_id = l.activity_id and c.category_id = l.category_id

then I do not get the data for the activity_ids that are not present in the link table.
I need to get data for all the activities with empty or null value as category_name for those not having any linking for category_id.

Please help me with it.

PS. I am using MS SQL Server DB

Answer 1

I believe you’re looking for a LEFT OUTER JOIN for your activity table to return all rows.

SELECT
a.activity_id, a.activity_type, c.category_name 
FROM activity a
LEFT OUTER JOIN link l 
ON a.activity_id = l.activity_id
LEFT OUTER JOIN category c
ON c.category_id = l.category_id;