Is there a way to do a query that orders by a field after a certain element id. I am trying to implement pagination based on the last returned element, and want to be able to both order elements a property and return the next paged based on the last element of the previous page.
For example a user may ask for 25 elements after element with id = 10 sorted on cost.
Imagine you have:
id | name | price
1 | Fish | 5
2 | Burger | 2
3 | Veggies | 6
If we want to get after id=2 sorted by price it should return
2 | Burger | 2
1 | Fish | 5
3 | Veggies | 6
If we want to get after id=1 sorted by price it should return
1 | Fish | 5
3 | Veggies | 6
You can do:
SELECT *
FROM YOURTABLE
WHERE id >= 10
ORDER BY cost ASC
LIMIT 25;
EDIT:
According to your new information, you can do that with:
SELECT *
FROM YOURTABLE
WHERE price >= (SELECT price from Table1 WHERE id = 2)
ORDER BY price ASC
LIMIT 25;
sql fiddle demo
Related
Following is my scenario
I have tables named
Products
id | name | count | Price
-------------------------
1 | meat | 1 | 10
Users
id | name | balance
-----------------
1 | Tim | 10
2 | Joe | 10
Work flow
select products if count >= 1,
reduce user's balance and count = count - 1
if no_balance or count < 1 throw error
Let's say if both users placing an order for 1 product at exact same time, products table count updates to -1, means query executes for both users.
Products
id | name | count | Price
-------------------------
1 | meat | -1 | 10
During placeing of an order,I have used the below query to select matching products
Select * from products where count >= 1 and price >= 10
Also, if users place orders with even little time difference, the expecting output gathered.
Is there any solution to this ?
You should consider use lock for each row, for example.
Select * from products where count >= 1 and price >= 10 FOR UPDATE.
But in your scenario, I advice you use Redis to do that.
How to design a second kill system for online shop
We have a DB called transaction. It has user_id, date, value and so on. I use pagination in my query also. I have thousands of record in my table which has user_id equal to 2 or other value. put the user_id = 2 at the very last page.
I want to sort the result like this:
sort the results by date but if the user_id= 2 , put all results associated with the user_id= 2 at the end.
to be more clear, I show you what I want in the below.
-------------------------------------
| ID | user_id | date | ......
-------------------------------------
| 1 | 10 | 2018-10-20 |
-------------------------------------
| 2 | 11 | 2018-10-21 |
-------------------------------------
| 3 | 2 | 2018-10-22 |
-------------------------------------
| 4 | 2 | 2018-10-23 |
the results have to be like this:
first: ID = 2, second: ID = 1, third: ID = 4, last: ID = 3
tip *:
I use field function but unfortunately in vain.
ORDER BY FIELD(user_id, 2) DESC, date DESC
You may try using a CASE expression in your ORDER BY clause:
SELECT *
FROM yourTable
ORDER BY
CASE WHEN user_id = 2 THEN 1 ELSE 0 END,
date DESC;
I'm not sure if you want each group sorted by date ascending or descending. If you want ascending date order, then remove the DESC keyword at the end of my query.
I have table where I have id and time.
ID | Time
1 | 8.35
1 | 8.40
3 | 8.43
4 | 8.45
1 | 8.50
2 | 8.52
3 | 8.54
4 | 8.55
1 | 8.57
2 | 9.01
3 | 9.05
5 | 9.06
Required Result
ID | Time
5 | 9.06
3 | 9.05
3 | 8.54
3 | 8.43
2 | 9.01
2 | 8.52
1 | 8.57
1 | 8.50
1 | 8.40
1 | 8.35
4 | 8.55
4 | 8.45
Currently I am doing it by Select * from table group by ID order by Time DESC and get
Result One:
ID | Time
5 | 9.06
3 | 9.05
2 | 9.01
1 | 8.57
4 | 8.55
then writing second query and storing data in list.
foreach value in Result one:
Select * from Table where ID = value
Instead of writing a loop, I will like to have only one query.
Basic problem is I want to group IDs and top group should be the item that occured recently. As in example 1 occurs many time but I will consider only the latest time while grouping.
Can I write only one query to get result?
SELECT ID, Time FROM Table ORDER BY ID, Time
Grouping combines matching rows, so you do not want to group, ordering puts them in order, and that's what you want, you want all the IDs in order then all the times in order within those ids, so you want to order by ID then order by time.
UPDATE due to question edit
This can be done with a join to a sub select
SELECT t.ID. t.Time FROM Table t
JOIN (SELECT ID, Max(Time) as Time FROM Table GROUP BY ID) ss
ON t.ID = ss.ID
ORDER BY ss.Time DESC, t.ID DESC, t.Time DESC
The sub select (ss) does the first query you have there, and joins it to the main table, letting you order by the highest(max) time for each ID, then by the ID and the Time for the row itself. Note that all the ordering is done on the final query, ordering in the sub select is useless, since the join will reorder it anyways.
I'm building a little quiz game in PHP/MySQL. After asking questions I want a screen to display how many of the last round were answered correctly.
Im storing whether they were answered correctly or not in a table that looks like this:
rowID | questionid | playerid | answercorrect |
1 | 1 | 1 | 1 |
2 | 2 | 1 | 1 |
3 | 3 | 1 | 1 |
4 | 4 | 1 | 1 |
5 | 5 | 1 | 0 |
6 | 6 | 1 | 1 |
7 | 7 | 1 | 1 |
I want to see how many of the last x (usually 5) questions were answered correctly.
I thought this would be simple. I'm trying this:
SELECT sum( answercorrect ) FROM `answersgiven` ORDER BY id DESC LIMIT 5
I thought this would sum the answercorrect column for the last 5 rows, giving me an answer of 4, but it's giving me 7, which is the result for ALL of the rows.
I feel like I'm missing something obvious. It seems like a simple thing to want to do.
Any ideas?
Try this:
SELECT sum(answercorrect)
FROM (SELECT answercorrect FROM `answersgiven` ORDER BY rowID DESC LIMIT 5) t1
Example Fiddle
In your query, the LIMIT clause affects the overall result: So first all are summed up (which results in one row - the sum over all rows) and then the first 5, if available, are taken (which again is just the one row).
The easiest way of achieving your target is to first select just the first 5 rows (in the subselect) and then sum up afterwards.
try this query
SELECT sum(col)
FROM (SELECT col FROM `table` ORDER BY id DESC LIMIT 5) t1
You're almost there. Just sum up those top 5 answers now:
SELECT SUM('top5')
FROM
(SELECT answercorrect AS 'top5'
FROM `answersgiven`
ORDER BY id DESC LIMIT 5) 'x'
I am fetching all stations which belong to a station group from my database. SELECT * FROM stations WHERE station_group_id = 1.
Now, from all the fetched results, I want certain ones to appear first (e.g. the stations which have line_id = 2 to appear first). For example, if this is my stations table:
id | station_group_id | line_id
-------------------------------
1 | 1 | 1
2 | 1 | 2
3 | 1 | 3
I would like the output to be:
id | station_group_id | line_id
-------------------------------
1 | 1 | 2
2 | 1 | 1
3 | 1 | 3
So that line_id = 2 is the first record in the output.
I thought about using ORDER BY, but it isn't quite an order issue, it is more a "preference" one.
So, is it possible to place some records on top of the output, based on a condition, preferably in one query? Thanks!
Try Below:
SELECT * FROM stations
WHERE station_group_id = 1
ORDER BY if(line_id in('2','X','Y','Z'),0,1)
SELECT * FROM stations WHERE station_group_id = 1 and line_id = 2
union
SELECT * FROM stations WHERE station_group_id = 1 and
line_id != 2 order by line_id asc
As you are saying, it is actually a preference, so you should either model it as an extra field on the table (e.g. ordinal, or order, or preferredOrder), or you keep sorting by line_id, and do the "special sort" in code. (find element with id=2, move to top)