Question

I have a database in which there are multiple posts. Each post can be associated with one or more location. Each location is then associated with one country.

This is handled through four tables:

• post (with the id and post_title)
• postlocation (with the fields post_id and location_id – to allow a one to many relationship)
• location (with the fields id, location_title and country_id)
• country (with the fields, id and country_title)

I want to perform a simple, effective select to retrieve a list of posts and each one’s associated locations and each of those locations’ country.

I’m at a loss as to how to best achieve this, and any help would be most welcome. Thank you.

Answer 1

Use LEFT JOIN:

SELECT post.*, location.*, country.*
FROM post
LEFT JOIN postlocation ON post.id = postlocation.post_id
LEFT JOIN location ON postlocation.location_id = location.id
LEFT JOIN country ON location.country_id = country.id

Here’s an explanation of how LEFT JOIN works: http://www.mysqltutorial.org/mysql-left-join.aspx

The MySQL LEFT JOIN clause works like this: when a row from the left
table matches a row from the right table based on join_condition, the
row’s content are selected as an output row. When row in the left
table has no match it is still selected for output, but combined with
a “fake” row from the right table that contains NULL in all columns.
In short, the MySQL LEFT JOIN clause allows you to select all rows
from the left table even there is no match for them in the right
table.

When you have multiple LEFT JOINs, then in each case it uses the cumulative results so far as the “left” table and the table your are joining on as the “right” table.

So the query above will return one row for every post/location/country. Note that this means there will be potentially several rows with the same post.* data (one for each postlocation associated with taht post).

And if a post has no postlocations associated with it, that post will still show up in the results (but with null rows for location.* and country.*). Similarly, if a location has no country associated it with it, the post/location rows will still show up in the output (but with null rows for country.*). This is the advantage of LEFT JOIN — you always get all the rows from the table on the “left”.