Question

I have two tables, items & categories, sample data below:

Items:
Title    category_id
Item A   1
Item B   2
Item C   3
Item D   2
Item E   3
Item F   2

Categories
category_id   category
1             wood
2             plastic
3             metal

What I need to do is to count the total number of items, then list how many are in each category and what % that is of the total

I know I can count each item and total e.g.

select 
  count(*) as total, 
  sum(category_id=1) as cat_1, 
  sum(category_id=2
.... etc etc

But is there a way to do it all without counting each (there maybe new categories added and would like this to stay working) and then joining with categories table to produce the name?

Ideally this is what I’d like to return:

Category    how many    % of total
wood        1           17%
plastic     3           50%
metal       2           33%

Total       6           100%

(17% is 1/6th => 16.666666667% rounded).

Answer 1

select ifnull(c.category, 'TOTAL') as Category, 
    count(i.category_id) AS `how many`, 
    round(count(i.category_id) / (select count(*) from Items) * 100) as `% of total`
from Categories c
left outer join Items i on c.category_id = i.category_id
where c.category is not null
group by c.category
with rollup

Note, this will also correctly handle empty categories.

SQL Fiddle Example

Output:

| CATEGORY | HOW MANY | % OF TOTAL |
------------------------------------
|    glass |        0 |          0 |
|    metal |        2 |         33 |
|  plastic |        3 |         50 |
|     wood |        1 |         17 |
|    TOTAL |        6 |        100 |