Ignore the database duplicates when setting LIMIT with PDO - php

Question

I want to ignore the duplicates in my database when I will set my "LIMIT 0, 50", then "LIMIT 50, 50" then LIMIT..... I will need to scan the duplicates on only 1 column of my table, not all the columns at once. I can't merge the duplicates because they are different in a way : these duplicates have different prices.
more precisely, I will need to show a list of these items, but to show their different prices at their right.
I need a precise number (50) per pages, so I cant load less then go to the next page. I could therefore load more from the beginning (changing the max and previous offsets if i'm on a far page) in a way that if i ignore the duplicates, I will got exactly 50 per pages and I will get the good number of pages shown at the end.
I'm a bit beginner with PHP and I have no idea about how to do that. Maybe pre-scan all the table and then start writing my code, by being flexible with my scan's variables of LIMIT and everything ? what functions I need ? how ?
Else, do something pre-programmed or a function of php that I don't know it exists can solve this problem ? Or I really need to get an headhache xD

Answer 1

I am not entirely certain of what you are asking, but I think you might want to do a aggregate statement along these lines:
select
itemID,
group_concat(itemPrice)
from
yourTable
group by
itemID
limit 50
This will bring back a list of 50 items and a second column where all the prices are grouped together. Then in your PHP code, you can either explode() that second column keep it as is.
Edit: If you select every field, you can't then use an aggregate function. If you want to select other columns that won't be different, add them to both the select and the group by sections like this:
select
itemID,
itemName,
itemSomething,
itemSomethingElse,
group_concat(itemPrice)
from
yourTable
group by
itemID,
itemName,
itemSomething,
itemSomethingElse
limit 50

Answer 2

Probably you can group by item, and use GROUP_CONCAT to show different prices list? In this way you can still use LIMIT 50. If the price column is numeric, cast it to VCHAR.

Answer 3

I admit I borrowed the group_concat() function from the other answers :)
After reading this paragraph from the docs:
The default behavior for UNION is that duplicate rows are removed from the result.
The optional DISTINCT keyword has no effect other than the default because it also
specifies duplicate-row removal. With the optional ALL keyword, duplicate-row removal
does not occur and the result includes all matching rows from all the SELECT statements.
Assume the following table (testdb.test):
ID Name Price
1 Item-A 10
2 Item-A 15
3 Item-A 9.5
4 Item-B 5
5 Item-B 4
6 Item-B 4.5
7 Item-C 50
8 Item-C 55
9 Item-C 40
You can page this table rows (9 rows) or groups (3 groups, based on the item's name).
If you would like to page your items based on the item groups, this should help:
SELECT
name, group_concat(price)
FROM
testdb.test
GROUP BY name
LIMIT 1 , 3
UNION SELECT
name, group_concat(price)
FROM
testdb.test
GROUP BY name
LIMIT 0 , 3; -- Constant 0, range is the same as the first limit's
If you would like to page your items based on all the items (I don't think that's what you were asking for, but just in case it helps someone else), this should help:
SELECT
name, price
FROM
testdb.test
LIMIT 1 , 5
UNION SELECT
name, price
FROM
testdb.test
LIMIT 0 , 5; -- Constant 0, range is the same as the first limit's
A very important thing to note is how you'll have to modify the limits. The first limit is your key, you can start from any limit you'd like as long as it's <= count(*) but you will have to have the same range as the second limit (i.e 3 in the first example and 5 in the second example). And the second limit will always start from 0 as shown.
I enjoyed working on this, hope this helps.