I'm writing an online math testing program, and currently working on the scripts to calculate the rank that each user got. The following code works, but I cringe every time I see it.
get_set() puts the result of the query into $users
function rank_users_in_test($tid){
$GLOBALS['DB']->get_set($users,"select user,test from user_results where test=$tid order by points desc,time");
// $users are already in order by rank thanks to ORDER BY
$rank = 1;
foreach ($users as $u){
$GLOBALS['DB']->query("update user_results set world_rank=$rank where user={$u['user']} and test={$u['test']}");
$rank++;
}
}
The query in the loop makes me cry a bit. My question is, is there a way that MySQL can automatically update each user's rank based on the order they appeared in the result on the first query? There is a related question here, but it does not use UPDATE.
I'm using MySQL 5.
Thanks to ring0 above, the following reduced the running time from minutes to mere seconds :D
create table temp (
rank int auto_increment,
user int,
test int,
primary key(rank)
);
insert into temp(user,test) (select user,test from user_results where test=$tid order by points desc,time);
update user_results ur, temp t set ur.world_rank=t.rank where ur.user=t.user and ur.test=t.test;
drop table temp;
Related
I have a table name "chat_details" I wanna access only data with green underline according to time, I use the following query
//suppose $user_id = 1;
"SELECT *
FROM chat_details WHERE from_user_id='$user_id' OR to_user_id='$user_id' ORDER BY time DESC"
It fetch all the rows because all rows contain user_id = 1 in one of the column, but i need only green underline rows as compare to red one because green one are latest according to time(column), how can i fetch only these green underlines?
This is one posible query, that should solve your question.
SELECT *
FROM chat_details c
WHERE (c.from_user_id='$user_id' OR c.to_user_id='$user_id')
AND NOT EXISTS (
SELECT 1 FROM chat_details d
WHERE d.from_user_id = c.from_user_id
AND d.to_user_id = c.to_user_id
AND d.time > c.time)
ORDER BY c.time DESC"
Actually I could not test, hope I haven't made a mistake.
The query selects all data as in your query, but only thoose which haven't a newer chat between the two users.
For the EXISTS keyword see http://www.mysqltutorial.org/mysql-exists/
I hava three tables called t_asset,t_device and t_asset_device.The relationship between t_asset and the t_device is multiple pairs.Each table column is :
t_asset :id , asset_name,asset_code,create_time,creator
t_device:id, device_name,device_code,latitude,longitude,create_time,creator
t_assets_device:id,asset_id,device_id,create_time,creator
Now I want to get all the t_asset and the latitude,longitude of the first device,So I write the code and function like these:
fun_getLatitudeByAssetId(`assetId` varchar(50)){
BEGIN
declare v_latituede DECIMAL(10,5) DEFAULT(-1) ;
select latitude into v_latituede
from t_device tDevice
inner join t_assets_device tAssetsDevice
on tAssetsDevice.asset_id=assetId and
tDevice.id=tAssetsDevice.device_id
and tDevice.latitude!=-1
ORDER BY tDevice.id desc
limit 0,1;
return v_latituede;
END
}
fun_getLongititueByAssetId(`assetId` varchar(50)){
BEGIN
declare v_longititue DECIMAL(10,5) DEFAULT(-1) ;
select longititueinto v_longititue
from t_device tDevice
inner join t_assets_device tAssetsDevice
on tAssetsDevice.asset_id=assetId and
tDevice.id=tAssetsDevice.device_id
and tDevice.latitude!=-1
ORDER BY tDevice.id desc
limit 0,1;
return v_longititue ;
END
}
The final query sql is:
select tAsset.*,fun_getLatitudeByAssetId(tAsset.id) latitude,
fun_getLongititueByAssetId(tAsset.id) longititue from t_asset tAsset
It seems that I have query the latitude and longititue two times,If I want to get the other field from the t_device,I do not want to write another function
like fun_getDeviceCodeByAssetId, How can I optimized my code?
I don't think a function or procedure is the way to go - why not just define a view that has asset_id + all the other fields you want? Then just join to it on asset_id rather than calling functions. In addition to just being cleaner, I'd be concerned about performance with row rather than set processing with the function approach (this is total speculation, I don't have deep enough knowledge of MySQL to know how it's handled)
Is it really necessary to do it with functions?
You can do it with views, for example:
create view latitudeLongitude as
select latitude,longitude,asset_id
from t_device tDevice
inner join t_assets_device tAssetsDevice
on tDevice.id=tAssetsDevice.device_id
and tDevice.latitude!=-1;
Finally your last select should look like this:
select tAsset.*,latitudeLongitude.latitude,
latitudeLongitude.longititue
from t_assettAsset inner join latitudeLongitude
on t_assettAsset.id = latitudeLongitude.asset_id
If you're trying to return several values at once then you should rather declare a stored procedure, not a function. Then you'll be able to write select latitude, longitude from ... inside your procedure and then call it with a command like call getLatAndLong(...)
Im making a table generator as a school project.
In MySQL I have 3 tables namely process,operation,score. Everything looked fine until i tested out my "ADD column" button in the web app.
Previous saved data should be read properly but also include the new column in the format, problem is the previous data queried does not include any values for the new table, so I intended it to return a score of 0 if no records were found, tried IFNULL & COALESCE but nothing happens(maybe im just using it wrong)
process - processID, processName
operation - operationID, operationName
score - scoreID, score, processID, operationID, scoreType (score
types are SELF,GL,FINAL)
ps = (PreparedStatement)dbconn.prepareStatement("SELECT score FROM score WHERE processID=? and operationID=? and type=?ORDER BY processid");
here's a pic of a small sample http://i50.tinypic.com/2yv3rf9.jpg
The reason that IFNULL doesn't work is that it only has an effect on values. A result set with no rows has no values, so it does nothing.
First, it's probably better to do this on the client than on the server. But if you have to do it on the server, there's a couple of approaches I can think of.
Try this:
SELECT IFNULL(SUM(score), 0) AS score
FROM score
WHERE processID=? and operationID=? and type=?
ORDER BY processid
The SUM ensures that exactly one row will be returned.
If you need to return multiple rows when the table contains multiple matching rows then you can use this (omitting the ORDER BY for simplicity):
SELECT score
FROM score
WHERE processID = ? and operationID = ? and type = ?
UNION ALL
SELECT 0
FROM (SELECT 0) T1
WHERE NOT EXISTS
(
SELECT *
FROM score
WHERE processID = ? and operationID = ? and type = ?
)
Is there way to realize this algorithm with mysql without 100500 queries and lots of resources?
if (exists %name% in table.name) {
num = 2;
while(exists %newname%+(num) in table.name) num++;
%name% = newname+(num);
}
Thanks
I don't know how much better you can do with a stored procedure in MySql, but you can definitely do better than 100500 queries:
SELECT name FROM table WHERE name LIKE 'somename%' ORDER BY name DESC LIMIT 1
At that point, you know that you can increment the number at the end of name and the result will be unused.
I 'm glossing over some fine print (this approach will never find and fill any "holes" in the naming scheme that may exist, and it's still not guaranteed that the name will be available due to race conditions), but in practice it can be made to work quite easily.
The simpliest way I can see of doing it is to create a table of sequential numbers
then cross join on to it....
SELECT a.name,b.id
FROM table a
WHERE a.name = 'somename'
CROSS JOIN atableofsequentialnumbers b
WHERE NOT EXISTS (SELECT 1 FROM table x WHERE x.name = CONCAT(a.name,b.id))
LIMIT 10
This will return the first 10 available numbers/names
Im trying to make an operation of creating user network based on call detail records in my CDR table.
To make things simple lets say Ive got CDR table :
CDRid
UserAId
UserBId
there is more than 100 mln records so table is quite big.
I reated user2user table:
UserAId
UserBId
NumberOfConnections
then using curos I iterate through each row in the table, then I make select statement:
if in user2user table there is record which has UserAId = UserAId from CDR record and UserBId = UserBId from CDR record then increase NumberOfConnections.
otherwise insert such a row which NumebrOfConnections = 1.
Quite simple task and it works as I said using cursor but it is very bad in performance (estimated time at my computer ~60 h).
I heard about Sql Server Integration Services that it has got better performance when we are talking about such big tables.
Problem is that I have no idea how to customize SSIS package for creating such task.
If anyone has got any idea how to help me, any good resources etc I would be really thankful.
Maybe there is any other good solution to make it work faster. I used indexes and variable tables and so on and performance is still pure.
thanks for help,
P.S.
This is script which I wrote and execution of this takes sth like 40 - 50 h.
DECLARE CDR_cursor CURSOR FOR
SELECT CDRId, SubscriberAId, BNumber
FROM dbo.CDR
OPEN CDR_cursor;
FETCH NEXT FROM CDR_cursor
INTO #CdrId, #SubscriberAId, #BNumber;
WHILE ##FETCH_STATUS = 0
BEGIN
--here I check if there is a user with this number (Cause in CDR i only have SubscriberAId --and BNumber so that I need to check which one user is this (I only have users from
--network so that each time I cant find this user I add one which is outide network)
SELECT #UserBId = (Select UserID from dbo.Number where Number = #BNumber)
IF (#UserBId is NULL)
BEGIN
INSERT INTO dbo.[User] (ID, Marked, InNetwork)
VALUES (#OutUserId, 0, 0);
INSERT into dbo.[Number](Number, UserId) values (#BNumber, #OutUserId);
INSERT INTO dbo.User2User
VALUES (#SubscriberAId, #OutUserId, 1)
SET #OutUserId = #OutUserId - 1;
END
else
BEGIN
UPDATE dbo.User2User
SET NumberOfConnections = NumberOfConnections + 1
WHERE User1ID = #SubscriberAId AND User2ID = #UserBId
-- Insert the row if the UPDATE statement failed.
if(##ROWCOUNT = 0)
BEGIN
INSERT INTO dbo.User2User
VALUES (#SubscriberAId, #UserBId, 1)
END
END
SET #Counter = #Counter + 1;
if((#Counter % 100000) = 0)
BEGIN
PRINT Cast (#Counter as NVarchar(12));
END
FETCH NEXT FROM CDR_cursor
INTO #CdrId, #SubscriberAId, #BNumber;
END
CLOSE CDR_cursor;
DEALLOCATE CDR_cursor;
The thing about SSIS is that it probably won't be much faster than a cursor. It's pretty much doing the same thing: reading the table record by record, processing the record and then moving to the next one. There are some advanced techniques in SSIS like sharding the data input that will help if you have heavy duty hardware, but without that it's going to be pretty slow.
A better solution would be to write an INSERT and an UPDATE statement that will give you what you want. With that you'll be better able to take advantage of indices on the database. They would look something like:
WITH SummaryCDR AS (UserAId, UserBId, Conns) AS
(
SELECT UserAId, UserBId, COUNT(1) FROM CDR
GROUP BY UserAId, UserBId)
UPDATE user2user
SET NumberOfConnections = NumberOfConnections + SummaryCDR.Conns
FROM SummaryCDR
WHERE SummaryCDR.UserAId = user2user.UserAId
AND SummaryCDR.UserBId = user2user.UserBId
INSERT INTO user2user (UserAId, UserBId, NumberOfConnections)
SELECT CDR.UserAId, CDR.UserBId, Count(1)
FROM CDR
LEFT OUTER JOIN user2user
ON user2user.UserAId = CDR.UserAId
AND user2user.UserBId = CDR.UserBId
WHERE user2user.UserAId IS NULL
GROUP BY CDR.UserAId, CDR.UserBId
(NB: I don't have time to test this code, you'll have to debug it yourself)
is this what you need?
select
UserAId, UserBId, count(CDRid) as count_connections
from cdr
group by UserAId, UserBId
Could you break the conditional update/insert into two separate statements and get rid of the cursor?
Do the INSERT for all the NULL rows and the UPDATE for all the NOT NULL rows.
Why are you even considering doing row-by-row processing on a table that size? You know you can use the merge statment and insert or update and it will be faster. Or you could write an update to insert all rows that need updating in one set-based stament and an insert to insert alll rows when the row doesn't exist in one set-based statement.
Stop using the values clause and use an insert with joins instead. Same thing with updates. If you need extra complexity the case stamenet will probably give you all you need.
In general stop thinking of row-by-row processing. If you can write a select for the cursor, you can write a set-based statement to do the work 99.9% of the time.
You may still want a cursor with a table this large but one to process batches of data (for instance a 1000 records at time) not one to run ro-by-row.