get max, min, count and mode (occurrence) - mysql

Question

I have an items table in my database that i want my query to process the values and give me the data of the max price, min price, most recurrent max price in that specific item category and no of items (and ignore the ones that are null), so here is my items table:
id
category
min_price
max_price
1
kids
10
100
2
adult
20
200
3
both
null
null
4
adult
20
100
5
adult
50
100
6
adult
50
200
7
kids
20
100
8
both
20
100
9
kids
null
null
10
adult
10
500
11
misc
null
null
I want the query to return this result:
category
min_price
max_price
price_mode
no_items
kids
10
100
100
3
adult
20
500
200
5
both
20
100
100
2
misc
null
null
null
1
so just to further explain the adult lowest price in 20 and highest is 500 and the 100 and 200 max_price has 2 occurrences both i want to take the highest as the price_mode which is 200 in this case and the no_items is just the count of how many times adult is shown in the table.
am struggling to get the mode honestly and grouping it correctly to get the output I want.
Below is the commands to create table and feed it with data. Tried to put it in SqlFiddle but that's not working for me i don't know why.
CREATE TABLE IF NOT EXISTS `items` (
`id` int(6) unsigned NOT NULL,
`category` TEXT NOT NULL,
`min_price` FLOAT DEFAULT NULL,
`max_price` FLOAT DEFAULT NULL,
PRIMARY KEY (`id`)
);
INSERT INTO `items` (`id`, `category`, `min_price`, `max_price`) VALUES
('kids', 10, 100),
('adult', 20, 200),
('both', null, null),
('adult', 20, 100),
('adult', 50, 100),
('adult', 50, 200),
('kids', 20, 100),
('both', 20, 100),
('kids', null, null),
('adult', 10, 500),
('misc', null, null);

Answer 1

Your create table + insert data syntax doesn't work in fiddle because your data VALUES are for just 3 columns whereby you define 4 columns in the INSERT:
INSERT INTO `items` (`id`, `category`, `min_price`, `max_price`) VALUES
('kids' , 10 , 100),
/*where's the value for `id`?*/
...
If you remove id from the INSERT syntax, it won't work as well because you've set it as PRIMARY KEY so it can't be empty. What you can do in addition to removing id from INSERT is to define AUTO_INCREMENT on the id column:
CREATE TABLE IF NOT EXISTS `items` (
`id` int(6) unsigned NOT NULL AUTO_INCREMENT,
....
Now, to get the expected result on your price_mode, you may want to try using GROUP_CONCAT() with ORDER and define which of the data in there that you want to return. Let's say you do GROUP_CONCAT(max_price ORDER BY max_price DESC) to return the set with max_price in descending order like this:
SELECT category,
MIN(min_price),
MAX(max_price),
GROUP_CONCAT(max_price ORDER BY max_price DESC),
COUNT(*)
FROM items
GROUP BY category;
Then you'll get a result like this:
category
MIN(min_price)
MAX(max_price)
GROUP_CONCAT(max_price ORDER BY max_price DESC)
COUNT(*)
adult
10
500
500,200,200,100,100
5
both
20
100
100
2
kids
10
100
100,100
3
misc
NULL
NULL
NULL
1
So, there's a consistent pattern in the GROUP_CONCAT() result that you probably can work out with. Assuming that you want the second largest value in the set, you can apply SUBSTRING_INDEX() twice to get it like this:
SELECT category,
MIN(min_price) AS min_price,
MAX(max_price) AS max_price,
SUBSTRING_INDEX(
SUBSTRING_INDEX(
GROUP_CONCAT(max_price ORDER BY max_price DESC),',',2),',',-1)
AS price_mode,
COUNT(*) AS no_items
FROM items
GROUP BY category;
This return the following result:
category
min_price
max_price
price_mode
no_items
adult
10
500
200
5
both
20
100
100
2
kids
10
100
100
3
misc
NULL
NULL
NULL
1
Demo fiddle
The following is an updated suggestion after getting further clarification:
SELECT i.category,
MIN(i.min_price),
MAX(i.max_price),
v2.mp AS price_mode,
COUNT(DISTINCT i.id)
FROM items i
LEFT JOIN
(SELECT cat,
mp,
cnt,
CASE WHEN cat = #cat
THEN #rownum := #rownum + 1
ELSE #rownum:=1 END AS rownum,
#cat := cat
FROM
(SELECT category cat,
max_price mp,
COUNT(*) cnt
FROM items
GROUP BY category,
max_price) v1
CROSS JOIN (SELECT #rownum := 1,
#cat := NULL) seq
WHERE mp IS NOT NULL
ORDER BY cat, cnt DESC, mp DESC) v2
ON i.category=v2.cat
AND v2.rownum=1
GROUP BY i.category, v2.mp;
The query starts with getting the COUNT(*) value of category and max_price combination. Then generating a custom row numbering on it with a WHERE condition that doesn't return max_price with NULL after the first operation. Probably the crucial part here is the ORDER BY cat, cnt DESC, mp DESC since the row numberings are assigned based on it. Otherwise, the row numbering will mess up. Finally, LEFT JOIN the items table with it with ON i.category=v2.cat AND v2.rownum=1 condition. It's important to make sure the v2.rownum=1 is placed at ON condition instead of WHERE in order to return the last row value of misc; since the subqueries will not have the value with the present sample data.
Here's an updated fiddle for reference, including the sample of 3 adult=NULL.

Answer 2

Maybe this query will help
with maximumvaluecounts
as ( select
count(max_price) as c, category, max_price
from yourtable
group by category
),
maximumcountpercategory
as ( select category,max(c) as c
from maximumvaluecounts
group by category
),
modes as ( select category, max_price as modevalue
from maximumcountpercategory m1
join maximumvaluecounts m2
on m1.category=m2.category
and m1.c=m2.c
)
, others as (
select
category,
min(min_price) as min_price,
max(max_price) as max_price,
count(max_price) as no_items
from yourtable
group by category
)
select o.*, m.modevalue as price_mode
from others o join
modes m on o.category=m.category