Make UNIQUE KEY relational to query - mysql

Question

That might sound weird, I know. This is an explanation:
1- I have the following table - items (which gets updated because users can update the amount of items as well as the content inside of them):
| id | content | item_id | Order (Unique Index)|
|:-----------|------------:|:------------:|:--------------------:
| 1 | This | 1 | 1
| 2 | is | 1 | 2
| 3 | content | 1 | 3
| 4 | Some | 2 | 1
| 5 | More | 2 | 2
| 6 | More! | 3 | 1
2- On my server, I am running a query that will iterate through my POSTed content check every item based on its item_id as well as to check if the order in that row is set. If order is set, then update the content, else insert new content
Lets say that my content is POSTing 4 items and the item_id = 1. Preferably, what I would want it to do would be this:
| id | content | item_id | Order (Unique Index)|
|:-----------|------------:|:------------:|:--------------------:
| 1 | This | 1 | 1
| 2 | updated | 1 | 2
| 3 | content | 1 | 3
| 4 | Some | 2 | 1
| 5 | More | 2 | 2
| 6 | More! | 3 | 1
> 7 | added | 1 | 4
Note that what happened was, it added a new row because my POSTed content had four items in it. It iterated though every single one, checked if the order existed, if the order existed, then update the value, else create a new row and insert the value as well as the order (key). The order is pretty much the key. That's how I am setting it in there.
It doesn't work when I do this:
// Start loop - for (key in content) {
INSERT INTO items (item_id, content, content_order) VALUES (?, content[key], ?) WHERE item_id = ? ON DUPLICATE KEY UPDATE content = ?
// End loop
What the loop is doing, it is iterating through every content POSTed and inserting it into the database, and if there is a duplicate key (in this case, the Unique Index is the Order column) then only update the content inside of it.
The problem with this is, it will only work on the first three items. Why? Because the first three items are the first ones with those unique indexes. If I was to update the item in which the item_id is 2, then it would give me an error because I cannot update items that have the same unique key. I cannot even INSERT anything because it violates the Unique Index constraints!
So how can I do this?
Is there a way to make the Unique Index absolute to the query - meaning that it will only keep in mind the Unique Indexes based on the queries' specified item_id? (Doubt it)
How can I make it so that it checks if the order is set and update the content or insert a new row without using unique keys?
Is there an alternate way to write this?
If elaboration is needed, please let me know. Thanks.

Answer 1

A straightforward design for your needs would probably have no problems. Although your question is unclear, especially about new content.
Order is not a key of items. Because column order is not unique. The key you want is (item_id,order).
Do you need items id? I'm going to ignore it. I'm going to treat new content as if it were in a table. You probably should build a constant subquery from it. Do all new content item_ids appear in items?
1. No NULLs.
A simple design is to have a version of items called content that holds the rows that make the following fill-in-the-blanks statement true. I'll assume items order is consecutive within item_id.
// item [item_id]'s [order]th content is [content]
content(item_id,order,content)
primary key (item_id,order)
I will guess at your new content format and effect. I'll assume order is consecutive from 1 within item_id. I'll replace all content info for a new content item_id.
// item [item_id]'s [order]th content is [content]
more_content(item_id,order,content)
primary key (item_id,order)
delete from content
where item_id in (select item_id from more_content)
insert into content
select * from more_content
2. NULLs
If NULL order indicates that there is no content then you can instead have content NULL and order=1. (You can also have another table and no NULLs.) If NULL order indicates that there is an unchanged default then just have another table:
// item [item_id]'s content is a default
has_default(item_id)
primary key (item_id)
delete from has_default
where item_id in (select item_id from more_content)
delete from content
where item_id in (select item_id from more_content)
insert into content
select * from more_content
If you want items readable the way it is, make a view:
// [order] is null AND item [item_id]'s default content is [content]
// OR [order] is not null AND item [item_id]'s [order]th content is [content]
create view items as (
select c.item_id,c.content,if(d.item_id is null,c.ord,NULL) as ord
from content c left join has_default d on c.item_id=d.item_id
)
It's hard to make out much more about your design.
It may be difficult to implement constraints for any design for your needs in SQL. But you should start with a straightforward design.