I have a table consisting of groups of, for example, five rows each. Each row in each group possesses a date value unique to that group.
What I want to do in my query, is go through the table, and increment a user variable (#count) when this date value changes. That's to say, #count should equal the group count, rather than the row count.
My current query looks like this, in case you're wondering:
SELECT #row := #row +1 AS rownum, date
FROM ( SELECT #row := 0 ) r, stats
Thanks a lot.
What about something like this?
SELECT
(CASE WHEN #date <> date THEN #row := #row +1 ELSE #row END) AS rownum,
#date:= date,
date
FROM ( SELECT #row := 0, #date := NOW() ) r, stats
You don't need a user variable to answer the query that you are doing. Is there a reason you want to use the user variable (for example, to emulate a ranking function?)
If not:
-- how many groups are there?
select count(distinct date) distinct_groups from table;
-- each group and count of rows in the group
select date, count(*) from table group by date;
-- if you want the Nth row from each group, assuming you have an auto_increment called id:
select *
from table
join ( select date, max(id) id
from table
group by date
) sq
on table.id = sq.id
Related
I'm trying in MySql to count the number of users created each day and then get an accumulative figure on a row by row basis. I have followed other suggestions on here, but I cannot seem to get the accumulation to be correct.
The problem is that it keeps counting from the base number of 200 and not taking account of previous rows.
Where was I would expect it to return
My Sql is as follows;
SELECT day(created_at), count(*), (#something := #something+count(*)) as value
FROM myTable
CROSS JOIN (SELECT #something := 200) r
GROUP BY day(created_at);
To create the table and populate it you can use;
CREATE TABLE myTable (
id INT AUTO_INCREMENT,
created_at DATETIME,
PRIMARY KEY (id)
);
INSERT INTO myTable (created_at)
VALUES ('2018-04-01'),
('2018-04-01'),
('2018-04-01'),
('2018-04-01'),
('2018-04-02'),
('2018-04-02'),
('2018-04-02'),
('2018-04-03'),
('2018-04-03');
You can view this on SqlFiddle.
Use a subquery:
SELECT day, cnt, (#s := #s + cnt)
FROM (SELECT day(created_at) as day, count(*) as cnt
FROM myTable
GROUP BY day(created_at)
) d CROSS JOIN
(SELECT #s := 0) r;
GROUP BY and variables have not worked together for a long time. In more recent versions, ORDER BY also needs a subquery.
I have the following table on my DataBase:
TimeRank(user-id, name, time)
I would like to order the table by time and get the position of an specific ID on the table, for example:
The user nº 68 is on the 3rd position.
I only need to do a query that returns the position of the user.
MySQL don't have the function row_number, so I don't know how to do it.
SELECT x.user-id,
x.name,
x.time,
x.position
FROM (SELECT t.user-id,
t.name,
t.time,
#rownum := #rownum + 1 AS position
FROM TABLE TimeRank t
JOIN (SELECT #rownum := 0) r
ORDER BY t.time) x
WHERE x.user-id = 123
Alternative:
SELECT user-id,
(SELECT COUNT(*) FROM TimeRank WHERE time <= (SELECT time FROM TimeRank WHERE user-id = 123)) AS position,
time,
name
FROM TimeRank
WHERE user-id = 123
You can generate a position column with a variable
set #pos=0;
select pos,user_id
from (select #pos:=#pos+1 pos,user_id from TimeRank order by time) s
where user_id=68;
If indexing is a concern, you can add a column to your table and update it with
set #pos=0;
update TimeRank set position=(#pos:=#pos+1) order by time;
SELECT * FROM ( SELECT #row := #row +1 AS rownum, [column name] FROM ( SELECT * FROM [table name] ) WHERE rownum % 5 = 1
This does indeed return every 5th row, but in ascending order. What I want is that it first gets all the data, put them in descending order and THEN apply the filter.
If you filter it first and then put it in descending order, it will not start with the latest data added (4/5th of the time).
I would like to know how one should do this.
Thanks in advance
Edit: For people with the same problem, this is what I used:
SELECT * FROM
(SELECT rank, id, Temperature FROM
(SELECT *, #rownum := #rownum + 1 AS rank FROM temperature_room1,
(SELECT #rownum := 0) r) AS T ORDER BY id DESC) AS J WHERE rank % 5 = 1
Select everything from:
Select rank, id and Temperature from:
Select everything and rownumber as rank from the table as t ordered by ID in descending order
Finally, only output the row numbers which can be divided by 5 and the remainder is 1
Don't quote me on this, I'm a big noob regarding SQL stuff. It works for me, so I'm happy.
seems like you just need an order by dec on the desired column in one of the three queries. I think the second one as order by applies to the select at the same level. ans since you want your rownum ordered desc... seems like that's the place...
SELECT *
FROM ( SELECT #row := #row +1 AS rownum, [column name]
FROM ( SELECT * FROM [table name] )
ORDER BY [column name] desc
)
WHERE rownum % 5 = 1
I have a transactions table which has shows various transactions made by several accounts. Some make only one, others more than that. At the moment the SQL I have prints out the first purchase of each account but i need it to print out the second made by each account
SELECT account_id
, purchase_date as second_purchase
, amount as second_purchase_amount
FROM Transactions t
WHERE purchase_date NOT IN (SELECT MIN(purchase_date)
FROM Transactions m
)
GROUP BY account_id
HAVING purchase_date = MIN(purchase_date);
What needs to change that the second purchase date and amount are chosen? I tried adding in a count for the account_id but it was giving me the wrong value.
You can use variables to assign row numbers and get the 2nd purchase.
SELECT account_id,purchase_Date,amount
FROM (
SELECT account_id
,purchase_date
,amount
--, #rn:=IF(account_id=#a_id and #pdate <> purchase_date,#rn+1,1) as rnum
,case when account_id=#a_id and #pdate <> purchase_date then #rn:=#rn+1
when account_id=#a_id and #pdate=purchase_date then #rn:=#rn
else #rn:=1 end as rnum
, #pdate:=purchase_date
, #a_id:=account_id
FROM Transactions t
CROSS JOIN (SELECT #rn:=0,#a_id:=-1,#pdate:='') r
ORDER BY account_id, purchase_date
) x
WHERE rnum=2
Explanation of how it works:
#rn:=0,#a_id:=-1,#pdate:='' - Declare 3 variables and initialize them, #rn for assigning the row numbers, #a_id to hold the account_id and #pdate to hold the purchase_date.
For the first row (ordered by account_id and purchase_date), account_id and #a_id, #pdate and purchase_date will be compared. As they wouldn't be equal, the when conditions fail and the else part would assign #rn=1. Also, the variable assignment happens after this. #aid and #pdate would be updated to current row's values. For the second row, if they are the same account and on a different date the first when condition will be executed and the #rn will be incremented by 1. If there are ties the second when condition would be executed and the #rn remains the same. You can run the inner query to check how the variables are assigned.
Number the rows and choose RowNumber = 2
select *
from (
select
#rn := case when #account_id = account_id then #rn + 1 else #rn := 1 end as RowNumber,
#account_id := account_id as account_id,
purchase_date
from
(select #rn := 1) x,
(select #acount_id :=account_id as account_id, purchase_date
from Transactions
order by account_id, purchase_date) y
) z
where RowNumber = 2;
I have a table containing date, id, and value, with about 1000 id rows per date. I need to calculate the percentile rank of each row, by date. I am using the following code for percentile rank for a single date, but with over 10 years of daily data this is very inefficient to run date-by-date. Seems that it should be able to be formulated in MySQL but I've not been able to make it work.
Date ID Value
date1 01 -7.2
date1 02 0.6
date2 01 1.2
date2 02 3.8
SELECT c.id, c.value, ROUND( (
(#rank - rank) / #rank ) *100, 2) AS rank
FROM (
SELECT * , #prev := #curr , #curr := a.value,
#nxtRnk := #nxtRnk + 1,
#rank := IF( #prev = #curr , #rank , #nxtRnk ) AS rank
FROM (
SELECT id, value
FROM temp
WHERE date = '2013-06-28'
) AS a, (
SELECT #curr := NULL , #prev := NULL , #rank :=0, #nxtRnk :=0
) AS b
ORDER BY value DESC
) AS c
So basically I want to SELECT DISTINCT(date), and then for each date perform the above SELECT, which is preceeded by INSERT INTO table2( ... ) to write the results to table2.
Thanks for any help,
Hugh
I finally developed an acceptable solution by using a temporary table. Maybe not the optimum solution, but it works in about 5 sec on a million + record table.
My temporary table (t1) contains date and the count of rows for date.
The third select above is changed to
SELECT t1.date, t1.cnt, id, value FROM t1 LEFT JOIN temp ON(t1.date = temp.date)
Also, the calculations in the first SELECT above were changed to use c.cnt rather than #rank, and an #prevDate variable was created to reset the rank count on date changes.
Thanks to anyone who looked at this and tried to work up a solution.
I was trying to solve this for quite some time and then I found the following answer. Honestly brilliant. Also quite fast even for big tables (the table where I used it contained approx 5 mil records and needed a couple of seconds).
SELECT
CAST(SUBSTRING_INDEX(SUBSTRING_INDEX( GROUP_CONCAT(field_name ORDER BY
field_name SEPARATOR ','), ',', 95/100 * COUNT(*) + 1), ',', -1) AS DECIMAL)
AS 95th Per
FROM table_name;
As you can imagine just replace table_name and field_name with your table's and column's names.
For further information check Roland Bouman's original post