Truncate and concatenate mysql results based on number of results - mysql

Question

I would like to concatenate all the item names of an order; however, if the total number of unique item names exceeds a certain number, then I want to to truncate each name before concatenating the names. Below are the conditions:
If the total number of unique item names in the order is less than 5, then use the full-length item name and concatenate the names; else if the total number of unique item names is greater than 5, then truncate each item name to 20 characters and concatenate the truncated names. For example, below is my table:
order_id | item_name | item_name_len
---------|-------------------------------------|--------------
1 | "pampers diapers ultra sensitive" | 31
1 | "cabbage salad pure organic greens" | 33
1 | "milky way" | 9
1 | "sea salt" | 8
1 | "cool waters fruit juice" | 23
| |
2 | "pure clear glass crystals" | 25
2 | "simple sugar edible paper" | 25
I want the following results:
order_id | all_item_names
---------|-----------------------------------------------------------
1 | "pampers diapers ultr ; cabbage salad pure o ; milky way ;
| sea salt ; cool waters fruit ju"
|
2 | "pure clear glass crystals ; simple sugar edible paper"
For Order #1, since there are 5 unique item names in the order, we truncate each of the item names to 20 characters and concatenate the truncated names. For Order #2, since there are only 2 unique item names in the order, we take the full-length of the name and concatenate them. (I've included the item name strlen in the table above for illustration.)
I'm trying to use a ternary condition, but it's not working.
IF( COUNT(DISTINCT item_name) < 5, item_name, SUBSTRING(item_name, 1, 20) )
See query below. I get Error code: 1111. Invalid use of group function
SELECT
w.order_id,
(SELECT GROUP_CONCAT( IF ( COUNT(DISTINCT o.item_name) < 5 ,
o.item_name, SUBSTRING(o.item_name, 1, 20) ) ) separator ' ; ' )
FROM order_items o WHERE o.order_id = w.order_id)AS all_item_names
FROM order_items w
GROUP BY order_id

Answer 1

You can do this with one aggregation and no join:
select oi.order_id,
(case when count(*) < 5
then group_concat(oi.item_name separator '; ')
else group_concat(left(oi.item_name, 20) separator ';')
end) as all_item_names
from order_items oi
group by oi.order_id

Answer 2

Group once to get the number of items and join to the table for the final group_concat:
select
o.order_id,
group_concat(
case
when counter < 5 then item_name
else left(item_name, 20)
end SEPARATOR ' ; '
) all_item_names
from order_items o inner join (
select
order_id, count(*) counter
from order_items
group by order_id
) g on g.order_id = o.order_id
group by o.order_id
See the demo
Results:
| order_id | all_item_names |
| -------- | ----------------------------------------------------------------------------------------- |
| 1 | pampers diapers ultr ; cabbage salad pure o ; milky way ; sea salt ; cool waters fruit ju |
| 2 | pure clear glass crystals ; simple sugar edible paper |