I do not understand the following (returns numbers of comments for articles with the newest ones dates):
SELECT `id_comment`,COUNT(*) AS `number`, MAX(`date`) AS `newest`
FROM `page_comments`
WHERE TO_DAYS( NOW() )-TO_DAYS(`date`) < 90
GROUP BY `id_comment`
ORDER BY `count` DESC,`newest` DESC
I dont understand how come that the MAX function will not return the MAX value of all the page_comments table? That it automatically takes only the max for the given group. When using MAX, I would expect it to return the highest value of the column. I dont understand how it works together with groupig.
You described the behavior yourself quite correctly already: it automatically takes only the max for the given group.
If you group, you do it (per usual) on every column in the result set, that is not aggregated (not using COUNT, SUM, MIN, MAX...)
That way you get distinct values for all non aggregated columns and the aggregated ones will yield a result that only takes the 'current' group into account.
I am just explaining it to the ground.
MAX() - An aggregate function(Works over the group of data).
If ""group by"" clause is NOT specified, the database implicitly groups the data(column specified) considering the entire result set as group.
If specified, it just groups the data(column) in the group logic specified.
It all boils down to analysis order:
FROM
ON
OUTER
WHERE
GROUP BY
CUBE | ROLLUP
HAVING
SELECT
DISTINCT
10 ORDER BY
TOP
so you first have the from clause
Then you cut the relevant rows via where ( so here your sentence : *I don't understand how come that the MAX function will not return the MAX value of all the page_comments* --fails)
then group it
Then you select it.
The max and aggregate functions apply on the data which is already filtered!
Related
I am trying to use an aggregate function to get the average(count( of a row in SQL Server. However, I continue to get this message:
"Cannot perform an aggregate function on an expression containing an aggregate or a subquery."
In the first picture is the table, in the second picture is the table with the counts for each officer_id, I am trying to find the average amount of calls per officer and cannot seem to find the right SQL query to do it.
The query I thought may work is:
SELECT AVG(COUNT(officer_id))
FROM crime_officers
ORDER BY officer_id;
But this is where I get the aggregate error. Does anyone have any recommendations?
UPDATED table with this query
SELECT officer_id, COUNT(crime_id)
FROM crime_officers
GROUP BY officer_id;
Original table: crime_officers
If I understand correctly, this query provides the average number of crimes per officer. A single value, which is equal to the total number of crimes divided between all officers.
SELECT COUNT(*)*1.0/COUNT(distinct officer_id) as 'Average Crimes per Officer'
FROM crime_officers;
I know it wasn't allowed in SQL-92. But since then it may have changed, particularly when there's a window applied. Can you explain the changes and give the version (or versions if there were more) in which they were introduced?
Examples
Is SUM(COUNT(votes.option_id)) OVER() valid syntax per standard SQL:2016 (or earlier)?
This is my comment (unanswered, an probably unlikely to in such an old question) in Why can you nest aggregate functions when using a window function in PostgreSQL?.
The Calculating Running Total (SQL) kata at Codewars has as its most upvoted solution (using PostgreSQL 13.0, a highly standard compliant engine, so the code is likely to be standard) this one:
SELECT
CREATED_AT::DATE AS DATE,
COUNT(CREATED_AT) AS COUNT,
SUM(COUNT(CREATED_AT)) OVER (ORDER BY CREATED_AT::DATE ROWS UNBOUNDED PRECEDING)::INT AS TOTAL
FROM
POSTS
GROUP BY
CREATED_AT::DATE
(Which could be simplified to:
SELECT
created_at::DATE date,
COUNT(*) COUNT,
SUM(COUNT(*)) OVER (ORDER BY created_at::DATE)::INT total
FROM posts
GROUP BY created_at::DATE
I assume the ::s are a new syntax for casting I didn't know of. And that casting from TIMESTAMP to DATE is now allowed (in SQL-92 it wasn't).)
As this SO answer explains, Oracle Database allows it even without a window, pulling in the GROUP BY from context. I don't know if the standard allows it.
You already noticed the difference yourself: It's all about the window. COUNT(*) without an OVER clause for instance is an aggregation function. COUNT(*) with an OVER clause is a window function.
By using aggregation functions you condense the original rows you get after the FROM clause and WHERE clause are applied to either the specified group in GROUP BY or to one row in the absence of a GROUP BY clause.
Window functions, aka analytic functions, are applied afterwards. They don't change the number of result rows, but merely add information by looking at all or some rows (the window) of the selected data.
In
SELECT
options.id,
options.option_text,
COUNT(votes.option_id) as vote_count,
COUNT(votes.option_id) / SUM(COUNT(votes.option_id)) OVER() * 100.0 as vote_percentage
FROM options
LEFT JOIN votes on options.id = votes.option_id
GROUP BY options.id;
we first join votes to options and then count the votes per option by aggregating the joined rows down to one result row per option (GROUP BY options.id). We count on a non-nullable column in the votes table (COUNT(votes.option_id), so we get a zero count in case there are no votes, because in an outer joined row this column is set to null.
After aggregating all rows and getting thus one row per option we apply a window function (SUM() OVER) on this result set. We apply the analytic SUM on the vote count (SUM(COUNT(votes.option_id)) by looking at the whole result set (empty OVER clause), thus getting the same total vote count in every row. We use this value for a calculation: option's vote count diveded by total vote count times 100, which is the option's percentage of total votes.
The PostgreSQL query is very similar. We select the number of posts per date (COUNT(created_at) is nothing else than a mere COUNT(*)) along with a running total of these counts (by using a window that looks at all rows up to the current row).
So, while this looks like we are nesting two aggregate functions, this is not really the case, because SUM OVER is not considered an agregation function but an analytic/window function.
Oracle does allow applying an aggregate function directly on another, thus invoking a final aggregation on a previous grouped by aggregation. This allows us to get one result row of, say, the average of sums without having to write a subquery for this. This is not compliant with the SQL standard, however, and very unpopular even among Oracle developers at that.
With COUNT DISTINCT, I often make use of a threshold to make it more precise. E.g. COUNT(DISTINCT users, 100000).
If I am using a WINDOW function though I get an error when trying to use a threshold COUNT_DISTINCT must have at most 1 argument(s), found 2. E.g. here's a made-up query that demonstrates the problem:
SELECT
day,
COUNT(DISTINCT state, 100000) OVER (PARTITION BY year, month, day)
FROM [publicdata:samples.natality]
LIMIT 1000
Is this by design? Is there a workaround?
COUNT(DISTINCT) is documented as approximation when used as aggregation function, but when it is used as analytic function - it is actually the exact implementation, so you don't need extra parameter - you will get the exact result without it.
I am using a local Access Database connected to Visual Basic. My query is
SELECT RebateReceived, DatePart('yyyy',[RebateMailedDate]) AS MailedDate, Sum(RebateValue) as MoneyReceived
FROM RebateInfoStorage
where RebateReceived='Received'
group by RebateReceived
having DatePart('yyyy',[RebateMailedDate])
I am trying to get the columns that have the same year and the word(s) that have 'received' to identify the records that need to be summed (Added) together. I am not very familiar with the Group By and Having keywords or the Sum() and DatePart() functions.
So the DBMS will go into the RebateInfoStorage and grab all the rows where RebateReceived = 'Received'. Then, it'll group those records, where each group contains records where the expression DatePart('yyyy', RebateMailedDate) evaluates to the same value (i.e. they have the same year). Then for each group, it'll return a single result row with the year, and the sum of all the RebateValues in that group. Operations happen in that order.
HAVING is like WHERE, but happens after the GROUP BY and is a condition placed on a group of records, whereas WHERE is a condition on a record.
SELECT
YEAR(RebateMailedDate) AS MailedDate,
SUM(RebateValue) as MoneyReceived
FROM
RebateInfoStorage
WHERE
RebateReceived = 'Received'
GROUP BY
YEAR(RebateMailedDate);
EDIT: It would appear that YEAR(x) is a more appropriate function!
You should group by DatePart having RebateReceived='Received'. For more information about the syntax of Having you may refer to http://www.w3schools.com/sql/sql_having.asp
Group by means your output table will be grouped according to unique elements in that column. For example, if there are multiple entry with 2014 as year, they will all be grouped together, and their RebateValue will be added up together. If you are grouping with RebateReceived, all the entry will be added and you will end up with a single sum.
I am pretty new to SQL and am working with a (what I expected to be easy) little bidding tool.
I am trying to compute average lows and highs from the same column. I have managed to figure out how to use SQL's MIN, MAX, AVG functions, but how would I go about averaging MIN and MAX?
This is the query I am using:
$query = $pdo->prepare("SELECT AVG(bid),MIN(bid),MAX(bid) FROM bidding WHERE bid_id=:bid_id GROUP BY bid_id");
Try the following query to accomplish task
SELECT ((max(bid)+min(bid))/2) as average FROM bidding WHERE bid_id=:bid_id GROUP BY bid_id
Because the predefined avg function takes only one argument that may be column from table or single value. So you have to find the average of the min and max value of bid like above
As You are saying you need to find out the Min of Avg and Max of Avg,
Now what you are doing is group on one column this means Avg(Bid) will return only one value. And the thing that you are doing will make sense only if it is done with two column,
For example you wanna know the min of Averages per day. You need to identify one more column on which base you want to find out max and min of Avg. See in my example i am using Date as second column. the query will go like.
Select Max(MAx_Bid),Min(Min_Bid),Min(Avg_Bid),Max(Avg_Bid) FROM
(SELECT AVG(bid) Avg_Bid,MIN(bid) Mix_Bid,MAX(bid) max_bid FROM bidding WHERE bid_id=:bid_id GROUP BY bid_id,Days_Date(Dummy column))A