How to include NULL values in a query with Outer Join and Group By

Question

I have two tables with the following sample data:

Table 1: `item_name`
| item_id | item_desc |
| 1       | apple     |
| 2       | orange    |
| 3       | banana    |
| 4       | grape     |
| 5       | mango     |

Table 2: `user_items`
| user_id | item_id |
| 127     | 1       |
| 127     | 2       |
| 127     | 4       |
| 128     | 1       |
| 128     | 5       |

I'm trying to select a total of each item_id both user_id 127 and 128 have, along with the corresponding item_desc using the following query:

SELECT IFNULL(COUNT(ui.user_id), 0) AS total, in.item_desc 
FROM user_items AS ui 
RIGHT OUTER JOIN item_name AS in 
    ON ui.item_id = in.item_id 
WHERE ui.user_id IN (127, 128) 
GROUP BY ui.item_id
ORDER BY total DESC

The result of the above query is:

| total | item_desc |
| 2     | apple     |
| 1     | orange    |
| 1     | grape     |
| 1     | mango     |

but it didn't include item_id 3, banana, which I wanted to retrieve with RIGHT OUTER JOIN. I was hoping to get a result that looked like this:

| total | item_desc |
| 2     | apple     |
| 1     | orange    |
| 1     | grape     |
| 1     | mango     |
| 0     | banana    |

Is there any way to modify the query to end up with the intended result above? Thank you for your time.

Nicola Cossu · Accepted Answer · 2011-12-29 00:38:01Z

6

There was a little error in your query using count. This works.

select count(ui.item_id) as total, in.item_desc
from   item_name `in`
       left join user_items ui on ui.item_id = in.item_id
                                        and ui.user_id in (127, 128)
group by
       in.item_desc
order by total desc

answered Dec 29, 2011 at 0:38

Nicola Cossu

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2 Comments

Lieven Keersmaekers Over a year ago

I don't see the error? afaik, COUNT(*) is valid SQL with any GROUP BY arrangement (at least in SQL Server).

Nicola Cossu Over a year ago

If you test your query you will see that you'll have banana 1. ;)

Lieven Keersmaekers · Accepted Answer · 2011-12-29 00:37:33Z

4

Your WHERE clause as it is is essentially removing all records from item_name that don't relate to user_id's 127 & 128.

In order to fix that, the easiest solution would be to LEFT JOIN from the item_name table to the user_items table and apply the selection of user_id's on the JOIN.

SELECT COUNT(*), itn.item_desc
FROM   item_name AS itn
       LEFT OUTER JOIN user_items AS ui ON ui.item_id = itn.item_id
                                           AND ui.user_id IN (127, 128)
GROUP BY
       itn.item_desc

note that while this can be written as a RIGHT OUTER JOIN, I find that type of joins to be counterintuïtive and would advise on only using them when absolutely necessary.

edited Dec 29, 2011 at 0:37

answered Dec 29, 2011 at 0:30

Lieven Keersmaekers

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