I'm trying to fix my problem on using AVG, I found some similar/relevant solutions and have attempted to apply it on my query however I still couldn't get the correct solution.
Here's my query:
SELECT unt_id, sum(enteredCode) / count(DISTINCT unt_id) AS 'avg' FROM tbl_code GROUP BY unt_id
Where enteredCode has the value of 1 and unt_id could be Chemistry, Physics, etc.
I think I'm missing something as it only shows the sum not the average.
You should just use the AVG function:
Select unt_id, avg(enteredCode) from tbl_code group by unt_id
Try thinking of GROUP BY as taking a big pile of rows and putting them into smaller piles, where each smaller pile is identified with a unique value among all groups.
In your query:
SELECT unt_id
, sum(enteredCode) / count(DISTINCT unt_id) AS 'avg'
FROM tbl_code
GROUP BY unt_id
you have asked to have all the rows grouped by unt_id and if unt_id has 6 distinct values like 1, 3, 9, 12, 18, 24 (across however many rows there are in the table), you'll have 6 groups where the first group consists of all rows where unt_id = 1, the second group where all rows have unt_id = 2, and so on.
Now what? Group functions like SUM(), COUNT(), AVG(), MAX(), MIN() will look at the individual groups of rows and give you one result per group.
Going back to your query, asking for COUNT(DISTINCT unt_id) for, say, the group where unt_id = 3, you're just going to get 1; the database put all rows where unt_id = 3 in that one group so there's only one distinct unt_id value. Same goes for groups 1, 9, 12, 18, and 24; they only have one distinct unt_id in their groups as well. (As an aside, note that using this grouping, MAX(unt_id) = MIN(unt_id) = unt_id for each group.)
If you just consider:
SELECT unt_id
, count(DISTINCT unt_id) as dist_count
FROM tbl_code
GROUP BY unt_id
you'll get one row returned for each unique unt_id value in your data, and the dist_count will be 1 for each of them. Using this count in your own average calculation (SUM(enteredCode) / COUNT(DISTINCT unt_id)) you're basically just doing SUM(enteredCode) / 1 so you're only seeing the sum.
When you said "enteredCode has the value of 1" it sounds like 1 is the only value and if that was the case, the average isn't going to be all that exciting - it'll just be 1, too. Assuming enteredCode has other values, as #bitfiddler said, you can just used the AVG function and call it done, assuming that you're looking to find the average enteredCode for all of the rows in each unt_id group. (If you wanted to use the explicit SUM / COUNT calculation for the average, just remember that AVG() = SUM() / COUNT() and rows where is null will not be considered.)
Hopefully this provides some insight as to why you're not getting the result you were intending.
Related
I have a table (tblExam) showing exam data score designed as follow:
Exam Name: String
Score: number(pecent)
Basically I am trying to pull the records by Exam name where the score are less than a specific amount (0.695 in my case).
I am using the following statement to get the results:
SELECT DISTINCTROW tblExam.name, Count(tblExam.name) AS CountOfName
FROM tblExam WHERE (((tblExam.Score)<0.695))
GROUP BY tblExam.name;
This works fine but does not display the exam that have 0 records more than 0.695; in other words I am getting this:
Exam Name count
firstExam 2
secondExam 1
thirdExam 3
The count of 0 and any exams with score above 0.695 do not show up. What I would like is something like this:
Exam Name count
firstExam 2
secondExam 1
thirdExam 3
fourthExam 0
fifthExam 0
sixthExam 2
.
..
.etc...
I hope that I am making sense here. I think that I need somekind of LEFT JOIN to display all of the exam name but I can not come up with the proper syntax.
It seems you want to display all name groups and, within each group, the count of Score < 0.695. So I think you should move < 0.695 from the WHERE to the Count() expression --- actually remove the WHERE clause.
SELECT
e.name,
Count(IIf(e.Score < 0.695, 1, Null)) AS CountOfName
FROM tblExam AS e
GROUP BY e.name;
That works because Count() counts only non-Null values. You could use Sum() instead of Count() if that seems clearer:
Sum(IIf(e.Score < 0.695, 1, 0)) AS CountOfName
Note DISTINCTROW is not useful in a GROUP BY query, because the grouping makes the rows unique without it. So I removed DISTINCTROW from the query.
Do I detect a contradiction? The query calls for results <0.695 but your text says you are also looking for results >0.695. Perhaps I don't understand. Does this give you what you are looking for:
SELECT DISTINCTROW tblExam.ExamName, Count(tblExam.ExamName) AS CountOfExamName
FROM tblExam
WHERE (((tblExam.Score)<0.695 Or (tblExam.Score)>0.695))
GROUP BY tblExam.ExamName;
I have my SQL table like this:
**CLIENTS:**
id
country
I want to echo a table with all countries I have with percentage fo each.
For example, if I have 2 Canadians and 1 French in my table, I want:
1 - Canada - 66%
2 - France - 33%
What I tried:
SELECT country FROM `mytable` GROUP BY `Country`;
It works, but how to have the percentage for each ?
Thanks.
You can use subquery:
SELECT
country,
COUNT(id) * 100 / (SELECT COUNT(id) FROM `mytable`) AS `something`
FROM
`mytable`
GROUP BY
`Country`;
You don't specify a falvor of SQL, but years ago microsoft posted their suggested solution:
select au_id
,(convert(numeric(5,2),count(title_id))
/(Select convert(numeric(5,2),count(title_id)) from titleauthor)) * 100
AS "Percentage Of Total Titles"
from titleauthor group by au_id
To calculate the percentage of total records contained within a group
is a simple result that you can compute. Divide the number of records
aggregated in the group by the total number of records in the table,
and then multiply the result by 100. This is exactly what the
preceding query does. These points explain the query in greater
detail:
The inner nested query returns the total number of records in the
TitleAuthor table: [ Select convert(numeric(5,2),count(title_id)) from
titleauthor ]
The value returned by the COUNT(title_id) in the outer
GROUP BY query returns the number of titles written by a specific
author.
The value returned in step 2 is divided by the value returned
in step 1, and the result is multiplied by 100 to compute and display
the percentage of the total number of titles written by each author.
The nested SELECT is executed once for each row returned by the outer
GROUP BY query
The CONVERT function is used to cast the values
returned by the COUNT aggregate function to the numeric data type with
a precision of 5 and a scale of 3 to achieve the required level of
precision.
I'm stumped with how to do the following purely in MySQL, and I've resorted to taking my result set and manipulating it in ruby afterwards, which doesn't seem ideal.
Here's the question. With a dataset of 'items' like:
id state_id price issue_date listed
1 5 450 2011 1
1 5 455 2011 1
1 5 490 2011 1
1 5 510 2012 0
1 5 525 2012 1
...
I'm trying to get something like:
SELECT * FROM items
WHERE ([some conditions], e.g. issue_date >= 2011 and listed=1)
AND state_id = 5
GROUP BY id
HAVING AVG(price) <= 500
ORDER BY price DESC
LIMIT 25
Essentially I want to grab a "group" of items whose average price fall under a certain threshold. I know that my above example "group by" and "having" are not correct since it's just going to give the AVG(price) of that one item, which doesn't really make sense. I'm just trying to illustrate my desired result.
The important thing here is I want all of the individual items in my result set, I don't just want to see one row with the average price, total, etc.
Currently I'm just doing the above query without the HAVING AVG(price) and adding up the individual items one-by-one (in ruby) until I reach the desired average. It would be really great if I could figure out how to do this in SQL. Using subqueries or something clever like joining the table onto itself are certainly acceptable solutions if they work well! Thanks!
UPDATE: In response to Tudor's answer below, here are some clarifications. There is always going to be a target quantity in addition to the target average. And we would always sort the results by price low to high, and by date.
So if we did have 10 items that were all priced at $5 and we wanted to find 5 items with an average < $6, we'd simply return the first 5 items. We wouldn't return the first one only, and we wouldn't return the first 3 grouped with the last 2. That's essentially how my code in ruby is working right now.
I would do almost an inverse of what Jasper provided... Start your query with your criteria to explicitly limit the few items that MAY qualify instead of getting all items and running a sub-select on each entry. Could pose as a larger performance hit... could be wrong, but here's my offering..
select
i2.*
from
( SELECT i.id
FROM items i
WHERE
i.issue_date > 2011
AND i.listed = 1
AND i.state_id = 5
GROUP BY
i.id
HAVING
AVG( i.price) <= 500 ) PreQualify
JOIN items i2
on PreQualify.id = i2.id
AND i2.issue_date > 2011
AND i2.listed = 1
AND i2.state_id = 5
order by
i2.price desc
limit
25
Not sure of the order by, especially if you wanted grouping by item... In addition, I would ensure an index on (state_id, Listed, id, issue_date)
CLARIFICATION per comments
I think I AM correct on it. Don't confuse "HAVING" clause with "WHERE". WHERE says DO or DONT include based on certain conditions. HAVING means after all the where clauses and grouping is done, the result set will "POTENTIALLY" accept the answer. THEN the HAVING is checked, and if IT STILL qualifies, includes in the result set, otherwise throws it out. Try the following from the INNER query alone... Do once WITHOUT the HAVING clause, then again WITH the HAVING clause...
SELECT i.id, avg( i.price )
FROM items i
WHERE i.issue_date > 2011
AND i.listed = 1
AND i.state_id = 5
GROUP BY
i.id
HAVING
AVG( i.price) <= 500
As you get more into writing queries, try the parts individually to see what you are getting vs what you are thinking... You'll find how / why certain things work. In addition, you are now talking in your updated question about getting multiple IDs and prices at apparent low and high range... yet you are also applying a limit. If you had 20 items, and each had 10 qualifying records, your limit of 25 would show all of the first item and 5 into the second... which is NOT what I think you want... you may want 25 of each qualified "id". That would wrap this query into yet another level...
What MySQL does makes perfectly sense. What you want to do does not make sense:
if you have let's say 4 items, each with price of 5 and you put HAVING AVERAGE <= 7 what you say is that the query should return ALL the permutations, like:
{1} - since item with id 1, can be a group by itself
{1,2}
{1,3}
{1,4}
{1,2,3}
{1,2,4}
...
and so on?
Your algorithm of computing the average in ruby is also not valid, if you have items with values 5, 1, 7, 10 - and seek for an average value of less than 7, element with value 10 can be returned just in a group with element of value 1. But, by your algorithm (if I understood correctly), element with value 1 is returned in the first group.
Update
What you want is something like the Knapsack problem and your approach is using some kind of Greedy Algorithm to solve it. I don't think there are straight, easy and correct ways to implement that in SQL.
After a google search, I found this article which tries to solve the knapsack problem with AI written in SQL.
By considering your item price as a weight, having the number of items and the desired average, you could compute the maximum value that can be entered in the 'knapsack' by multiplying desired_cost with number_of_items
I'm not entirely sure from your question, but I think this is a solution to your problem:
SELECT * FROM items
WHERE (some "conditions", e.g. issue_date > 2011 and listed=1)
AND state_id = 5
AND id IN (SELECT id
FROM items
GROUP BY id
HAVING AVG(price) <= 500)
ORDER BY price DESC
LIMIT 25
note: This is off the top of my head and I haven't done complex SQL in a while, so it might be wrong. I think this or something like it should work, though.
This is a summary version of the problems I am encountering, but hits the nub of my problem. The real problem involves huge UNION groups of monthly data tables, but the SQL would be huge and add nothing. So:
SELECT entity_id,
sum(day_call_time) as day_call_time
from (
SELECT entity_id,
sum(answered_day_call_time) as day_call_time
FROM XCDRDNCSum201108
where (day_of_the_month >= 10 AND day_of_the_month<=24)
and LPAD(core_range,4,"0")="0987"
and LPAD(subrange,3,"0")="654"
and SUBSTR(LPAD(core_number,7,"0"),4,7)="3210"
) as summary
is the problem: when the table in the subquery XCDRDNCSum201108 returns no rows, because it is a sum, the column values contain null. And entity_id is part of the primary key, and cannot be null.
If I take out the sum, and just query entity_id, the subquery contains no rows, and thus the outer query does not fail, but when I use sum, I get error 1048 Column 'entity_id' cannot be null
how do I work around this problem ? Sometimes there is no data.
You are completely overworking the query... pre-summing inside, then summing again outside. In addition, I understand you are not a DBA, but if you are ever doing an aggregation, you TYPICALLY need the criteria that its grouped by. In the case presented here, you are getting sum of calls for all entity IDs. So you must have a group by any non-aggregates. However, if all you care about is the Grand total WITHOUT respect to the entity_ID, then you could skip the group by, but would also NOT include the actual entity ID...
If you want inclusive to show actual time per specific entity ID...
SELECT
entity_id,
sum(answered_day_call_time) as day_call_time,
count(*) number_of_calls
FROM
XCDRDNCSum201108
where
(day_of_the_month >= 10 AND day_of_the_month<=24)
and LPAD(core_range,4,"0")="0987"
and LPAD(subrange,3,"0")="654"
and SUBSTR(LPAD(core_number,7,"0"),4,7)="3210"
group by
entity_id
This would result in something like (fictitious data)
Entity_ID Day_Call_Time Number_Of_Calls
1 10 3
2 45 4
3 27 2
If all you cared about were the total call times
SELECT
sum(answered_day_call_time) as day_call_time,
count(*) number_of_calls
FROM
XCDRDNCSum201108
where
(day_of_the_month >= 10 AND day_of_the_month<=24)
and LPAD(core_range,4,"0")="0987"
and LPAD(subrange,3,"0")="654"
and SUBSTR(LPAD(core_number,7,"0"),4,7)="3210"
This would result in something like (fictitious data)
Day_Call_Time Number_Of_Calls
82 9
Would:
sum(answered_day_call_time) as day_call_time
changed to
ifnull(sum(answered_day_call_time),0) as day_call_time
work? I'm assuming mysql here but the coalesce function would/should work too.
I've got a database table called servers with three columns 'id', 'name', and 'votes'.
How can I select the position of column id 5 by votes?
Example, I want to check which position server 3 is in by votes in my competition?
If I've interpreted your question correctly, you are asking how to find the rank of the row with id 5 in a list of servers sorted by votes. There is a complex solution, which requires sorting, but the easier solution which can be done in O(log(n)) space and O(n) time is to simply measure the number of votes for id = 5
select votes from servers where id = 5;
and then walk through the database and add one for every server encountered that has smaller number of votes. Alternatively, you can do something like:
select count(*) from servers where votes <= %votes
It is excessive to sort this (O(nlog(n) time) when you can simple iterate through the entire list once and gather all the information you need.
Use LIMIT:
SELECT id, name, votes FROM servers ORDER BY votes DESC LIMIT 2,1;
LIMIT a, b means "give me b rows, starting at row a", and a is zero-based.
OK, I misunderstood. Now. Suppose your server has 27 votes.
SELECT COUNT(*) FROM servers WHERE votes < 27;
Your server's rank will be 1 plus the result; ties are possible (i.e. ranks will be like 1, 2, 3, 3, 3, 6, 7, 7, 9 etc.).